DESCRIPTION
CHARGING METHOD, BATTERY PACK, AND ITS CHARGER
TECHNICAL FIELD
[0001] The present invention relates to a charging method, a
battery pack and a charger for the battery pack, and more
particularly relates to a technique for reducing a charging
time.
BACKGROUND ART
[0002] FIG. 7 is a graph showing a typical conventional
method for controlling a charge voltage and a charge current,
which realizes a shorter charging time. FIG. 7 shows the case
of a lithium ion battery, wherein αl indicates changes in charge
voltage of a secondary battery and α2 indicates changes in
charge current to be supplied to the secondary battery.
[0003] Firstly, changes in charge voltage are explained. A
trickle charging area, wherein a small constant current I1, e.g.
a charge current of 50 mA is supplied, starts from the beginning
of the charging and ends when a cell voltage of one cell, or
cell voltages of all the plurality of cells have reached the
same end voltage Vm for the trickle charging e.g. 2.5 V.
[0004] When the cell voltage reaches the end voltage Vm, a
transition is made from the trickle charging area to a constant
current (CC) charging area, wherein the end voltage Vf is being
1

applied across the charge terminals of a battery pack until the
terminal voltage across the charge terminals reaches a
predetermined end voltage Vf (4.2 V per cell i.e., 12.6 V in the
case of three cells connected in series). In this constant
current (CC) charging area, applied is a charge current, which
is obtained by multiplying 70 % of 1 C by the number P of the
cells connected in parallel, provided that a current value with
which a nominal capacity NC is discharged in an hour by carrying
out the constant-current discharging is 1 C.
[0005] When the terminal voltage across the charge terminals
reaches the end voltage Vf, a transition is made from the
constant-current charging area to a constant-voltage (CV)
charging area wherein a charge current is supplied while
reducing a charge current value so as not to exceed the end
voltage Vf until the charge current value is decreased to a
current value I3 as set based on temperatures. In this state,
it is determined that the charge current has been supplied to
the full charge, and the supply of the charge current is
stopped. With this structure, the shorter charging time can be
realized by increasing the current to be supplied to the
constant current (CC) charging area. An amount of electric
charges injected within the same period of time can be increased
not only by increasing the charge current, but also by
increasing the charge voltage. For example, according to Patent
Document 1, the residual capacity is detected before carrying

out the constant-current charging with an overvoltage, and the
charging is carried out only with respect to those with small
residual capacities, thereby preventing overcharging.
[0006] However, the conventional technology disclosed in
Patent Document 1 has such problem that the residual capacity
needs to be measured before carrying out the charging. Besides,
an overvoltage is liable to be applied to the secondary battery
although influences are not significant.
Patent Document 1:
Japanese Unexamined Patent Publication No. Tokukaihei 6-
78471/1994
DISCLOSURE OF THE INVENTION
[0007] It is therefore an object of the present invention to
provide a charging method, a battery pack and a charger for the
battery pack, which permits the time required for charging to be
reduced without applying an overvoltage to a secondary battery.
[0008] A charging method according to one aspect of the
present invention includes: a constant-current charging step
wherein a constant charge current is supplied to a secondary
battery to be charged to a predetermined end voltage; and a
constant-voltage charging step wherein the predetermined end
voltage is maintained by reducing the charge current after the
secondary battery is charged to the end voltage, wherein the
constant-current charging step includes a charging step to be

carried out with the end voltage set to an open circuit voltage
(OCV) which is a voltage when no current is flowing, and with a
voltage across charge terminals of the battery pack set to an
overvoltage above the OCV, and the constant-voltage charging
step includes a step of reducing the voltage across the charge
terminals to the OCV after the voltage across the charge
terminals is increased to the overvoltage or after the charge
current across the charge terminals is reduced to or below a
predetermined current level.
[0009] According to the foregoing method for charging the
secondary battery such as a lithium ion battery, the constant-
current (CC) charging is performed wherein a constant charge
current is supplied to the secondary battery to be charged to a
predetermined end voltage (e.g. 4.2 V in the case of the lithium
ion battery) as a target voltage, subsequent to the trickle
charging to be carried out with a small current in the initial
stage of the charging process. Then, after the secondary
battery is charged to the end voltage, a constant-voltage
charging is performed wherein the predetermined end voltage is
maintained by reducing the charge current. In the constant-
voltage charging step, the end voltage is set to an open circuit
voltage (OCV) which is a voltage when no current is flowing, and
in the constant-current charging step, the voltage across the
charge terminals of the battery pack is set to an overvoltage
above the OCV. After the voltage across the charge terminals

reaches the overvoltage and a transition is made to the
constant-voltage charging, or after the charge current of the
charge terminal is reduced to or below a predetermined current
level, the voltage across the charge terminals is reduced to the
end voltage.
[0010] As described, according to the foregoing charging
method of the present invention, although a voltage above the
end voltage is applied across the charge terminals, such voltage
is not applied to the respective cells in the constant-current
(CC) charging. Moreover, a difference in voltage between the
voltage across the terminals and the cell voltage can be
consumed by a voltage drop caused by switches and current
detection resistances provided for safety control and the
charge/discharge control. With this arrangement, since the
charge current in the constant current (CC) charging period can
be reduced in a short period of time, a transition can be made
immediately to the constant-voltage (CV) charging period even
for almost fully charged battery packs. The foregoing charging
method of the present embodiment is therefore applicable to
battery packs in any state, and the charge voltage to be applied
in the constant-voltage (CV) charging period can be increased,
which in turn increases an amount of charges to be injected
while surely preventing an application of an overvoltage to the
respective cells, and thereby preventing an overcharge of the
respective cells. Additionally, by setting a charge voltage and

a reduction in current to be detected to the same level as those
of the conventional method, as a final full charge condition,
the time required for an overall charging process can be
reduced, while maintaining the full charge capacity at the same
level.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a block diagram showing the electrical
structure of a charging system employing a charging method
according to a first embodiment of the invention,
FIG. 2 is a graph showing a method for controlling a charge
voltage and a charge current by the charging method according to
the first embodiment of the invention,
FIG. 3 is a block diagram showing another example of a
trickle charge circuit,
FIG. 4 is a block diagram showing still another example of
the trickle charge circuit,
FIG. 5 is a graph showing another method for controlling a
charge voltage and a charge current by the charging method
according to the first embodiment of the invention,
FIG. 6 is a block diagram showing the electrical structure
of a charging system employing a charging method according to a
second embodiment of the invention, and
FIG. 7 is a graph showing another method for controlling a
charge voltage and a charge current according to a typical
conventional technology.

BEST MODE FOR CARRYING OUT THE INVENTION
[0012] Hereinafter, embodiments of the present invention are
described with reference to the accompanying drawings. In the
following description, elements having the same
structures/functions are designated by the same or similar
reference numerals and are not repeatedly described in some
cases.
[0013] (First Embodiment)
FIG. 1 is a block diagram showing the electrical structure
of a charging system employing a charging method according to a
first embodiment of the present invention. As shown in FIG. 1,
the charging system includes a battery pack 1 and a charger 2
for charging the battery pack 1. The charging system of the
present invention is not limited to the foregoing structure, and
may further include a load equipment (not shown), to which power
is supplied from the battery pack 1. In the case of the
charging system of FIG. 1, the battery pack 1 is charged by the
charger 2; however, in the case of the above example of the
charging system provided with the load equipment, the battery
pack 1 may be mounted on the load equipment to be charged via
the load equipment. The battery pack 1 and the charger 2 are
interconnected by high voltage direct current terminals T11, T21
for power supply, terminals T12, T22 for communication signals,
and GND terminals T13, T23 for power supply and communication

signals. For the charging system provided with the load
equipment, terminals are provided in the same manner as the case
of FIG. 1.
[0014] In the battery pack 1, FETs 12, 13 having different
conduction modes for charging/discharging are provided in a high
voltage direct current charge path 11 extending from the
terminal T11, and the charge path 11 is connected to a high
voltage terminal of an assembled battery 14. A low voltage
terminal of the assembled battery 14 is connected to the GND
terminal T13 via a low voltage direct current charge path 15,
and a current sensing resistor 16 as a current detector for
converting a charge current and a discharge current into current
values is provided in this charge path 15.
[0015] The assembled battery 14 includes a plurality of
secondary battery cells connected in series and the temperatures
of the cells are detected by a temperature sensor 17 and are
inputted to an analog/digital converter 19 in a control IC 18.
A voltage across terminals of each cell is detected by a voltage
detection circuit 20 and is inputted to the analog/digital
converter 19 in the control IC 18. The current values detected
by the current sensing resistor 16 are also inputted to the
analog/digital converter 19 in the control IC 18. The
analog/digital converter 19 converts the respective input values
into digital values to be outputted to a charge control judging
section 21.

[0016] The charge control judging section 21 includes a
microcomputer and its peripheral circuits, calculates a voltage
value, a. current value and a pulse width (duty) of a charge
current required to be outputted from the charger 2 in response
to the respective input values from the analog/digital converter
19 and transmits them to the charger 2 via the terminals T12,
T22; T13, T23 from a communicator 22. The charge control
judging section 21 also performs a protection operation of, for
example, cutting the FETs 12, 13 off if abnormality outside the
battery pack 1 such as a short circuit between the terminals T11
and T13 or an abnormal current from the charger 2 or an abnormal
temperature increase of the assembled battery 14 is detected
based on inputs from the analog/digital converter 19.
[0017] The charge control judging section 21 constitutes a
charge controller together with the FETs 12, 13, and switches ON
the FETs 12, 13 to enable charging/discharging when a
charging/discharging is being performed properly while switching
them OFF to disable charging/discharging when an abnormality is
detected.
[0018] In the charger 2, a request from the charge control
judging section 21 is received by a communicator 32 of a control
IC 30, and a charge controller 31 controls a charge current
supply circuit 33 to supply a charge current of the above
voltage value, a current value and a pulse width as requested.
The charge current supply circuit 33 includes an AC-DC converter

and a DC-DC converter, and converts an input voltage into a
voltage value, current value and a pulse width as instructed by
the charge controller 31, to be supplied to the charge paths 11,
15 via the terminals T21, T11; T23, T13. The charge controller
31 and the charge current supply circuit 33 constitute the
charge controller of the present embodiment. Residual capacity
data obtained through communication from the battery pack 1 is
displayed on a display panel 34.
[0019] In the battery pack 1, a trickle charge circuit 25 is
provided in parallel with the FET 12 for normal (quick) charge
in the high voltage direct-current charge path 11. This trickle
charge circuit 2 5 includes a series circuit made up of a
current-limiting resistor 26 and a FET 27. The charge control
judging section 21 performs a trickle charging in an initial
state of the charging process and in the last stage close to the
full charge by switching OFF the FET 12 for the quick-charging
and switching ON the FET 27 for quick-charging while keeping the
FET 13 for discharging in the ON state. The charge control
judging section 21 performs a trickle charging in the normal
charging/discharging by switching ON the FET 12 for the quick-
charging and switching OFF the FET 27 for quick-charging while
keeing FET 13 for discharging in the ON state.
[0020] The present embodiment has the following essential
feature. That is, the trickle charge circuit 25 has another
series circuit which is made up of a current-limiting resistor

28 and a FET 29, and which is connected in parallel to the
series circuit of the current-limiting resistor 26 and the FET
27. The charge control judging section 21 divides a trickle
charging area into a first half and a second half. In the first
half, the charge control judging section 21 carries out a
trickle charging in the same manner as the conventional trickle
charging method using the current-limiting resistor 26, wherein
the FET 27 is switched ON and the FET 29 is switched OFF. In
the second half, the charge control judging section 21 carries
out a trickle charging with a supply current larger than the
conventional trickle charge current using the current-limiting
resistor 28 having a smaller resistance value than the current-
limiting resistor 26 wherein the FET 29 is switched ON and the
FET 27 is switched OFF. Another essential feature of the
present embodiment lies in the following. That is, the charge
control judging section 21, which performs the constant-current
and constant-voltage charging, carries out the constant-current
charging with the end voltage set to the OCV, and carries out
the constant-voltage charging with the voltage across the charge
terminals T11 and T13 set to an overvoltage above the end
voltage, wherein when a transition is made from the constant-
current charging to the constant-voltage charging, and the
charge current is reduced to or below a predetermined current
level, the voltage across the charge terminals T11 and T13 is
reduced to the end voltage.

[0021] FIG. 2 is a graph showing a method of controlling the
charge voltage and charge current according to the above
described present embodiment. FIG. 2 also shows the case of a
lithium ion battery similar to FIG. 7 showing the conventional
technology described above, wherein all indicates changes in
voltage relating to each cell of the battery pack 1 and the
assembled battery 14, and al2 indicates changes in charge
current to be supplied to the secondary battery 1.
[0022] Firstly, changes in voltage are explained. A trickle
charging area starts from the beginning of the charging as in
the case of conventional method. The charge control judging
section 21 first requests the charge control section 31 via the
communicators 22 and 32 for a trickle charge current, and
switches ON the FET 13 for discharging and switches OFF the FET
12 for charging, and in the meantime, switches ON the FET 27 and
switches OFF the FET 29. The charge control judging section 21
then starts carrying out the trickle charging using the current-
limiting resistance 26 with a small constant current I11, e.g. a
charge current of 50 mA as a trickle charge current as in the
conventional method. The trickle charging is continued until
the voltage detection circuit 20 detects that a cell voltage of
a cell or cell voltages of all the plurality of cells reach a
switch voltage Vma newly set in this embodiment, e.g. 1.0 V.
[0023] When the respective cell voltages of all the
plurality of cells have reached the switch voltage Vma, a

middle-speed current charging area in the trickle charging area
starts where the charge control judging section 21 performs the
charging with a larger current I12 than the conventional trickle
charge current using the current-limiting resistor 28 having a
smaller resistance value than the current-limiting resistor 26
as described earlier by switching ON the FET 29 and switching
OFF the FET 27. In this middle-speed current charging area, a
charge current I12 is applied, which is obtained by multiplying
5 to 20 % of an 1C by the number P of the cells connected in
parallel, provided that a current value with which a nominal
capacity NC is discharged in an hour by carrying out the
constant current discharging is a level 1 C (e.g. 200 mA at 5 %
when NC = 2000 mAh and two cells are connected in parallel).
Thereafter, the trickle-charging process continues until the
voltage detection circuit 20 detects that a cell voltage of one
cell, or cell voltages of all the plurality of cells have
reached the same end voltage Vm as that of the conventional
trickle charging method, e.g. 2.5 V.
[0024] Specifically, the trickle charging in the present
embodiment is performed in the following manner. That is, the
trickle charging process is performed by dividing into two
areas, i.e., the first half trickle charging area wherein a
conventional trickle charging is performed with a current value
I11 adopted in the conventional trickle charging method, and the
second half trickle charging area wherein a trickle charging is

performed with a current value I12 larger than the current value
I11, wherein the trickle charging by the conventional current
value I11 is performed in a shorter period of time than the
conventional current value I11, and in the second half of the
trickle charging period (area) which is defined as the middle
speed current charging area, the charging is performed with a
larger current value I12 than the current value I11.
[0025] The current values I11, I12 of the trickle charging
are determined based on a difference between the voltage applied
across the terminals T11, T13 and the voltage across the
terminals of the assembled battery 14, the resistance values of
the current-limiting resistors 26, 28, the FETs 27, 29 and the
like. For such charge current supply circuit 33 of the charger
2 capable of supplying a current value I12 which is larger than
the current value I11 adopted in the conventional trickle
charging, the same current may be requested in the trickle
charging area and the middle-speed charging area. However, it
is possible to reduce losses caused by the current-limiting
resistor 26 and the like in the trickle charging by requesting
different current values respectively for the trickle charging
area and the middle-speed charging area.
[0026] When the cell voltages reach the end voltage Vm, a
transition is made to a super quick charging area wherein the
charging is performed with a constant current (CC), and the
charge control judging section 21 requests the charge controller

31 via the communicators 22, 32, for a large charge current I13,
e.g. 1C and an overvoltage Vfal newly set in this embodiment,
e.g. 4.3 V per cell and in the meantime switching ON the FET13
for discharging and ON the FET12 for charging, and switching OFF
the FETs 27 and 29 of the trickle charge circuit 25, thereby
starting the super quick charging period (area).
[0027] Thereafter, when the voltage across the terminals T11
and T13 is increased, and the current sensing resistor 16
detects a decrease in charge current to or below a predetermined
current level I14, e.g. 0.9 C, which is smaller than the charge
current I13, the charge control judging section 21 determines
that a transition is made to the constant-voltage (CV) charging
area, and requests the charge controller 31 via the
communicators 22, 32, for a current equal to or above the
current level I14 and an overvoltage Vfa2, e.g. 4.25 V per cell
to continue the quick charge.
[0028] Even if the charge current is reduced in such a
manner, the charge control judging section 21 requests the
charge controller 31 via the communicators 22, 32a for a current
equal to or above a current level 115 and the end voltage Vf,
e.g. 4.2 V per cell as in the conventional constant-voltage (CV)
charge when the voltage across the terminals Til and T13 is
raised again and the current sensing resistor 16 detects a
reduction in charge current to or below the level 115, e.g.
0.8C.

[0029] When the current sensing resistor 16 detects a
reduction in current to or below a charge current 116, e.g. 0.1C
as in the conventional method with a charge voltage as a final
full charge condition set to 4.2 V, the charge control judging
section 21 judges the full charge and requests the charge
controller 31 via the communicators 22, 32a for a charge current
of 0A and a charge voltage of OV to stop the supply of the
charge current.
[0030] The current value I13 can be set, for example, in a
range of 1C to 4C; the current value I14, for example, in a
range of 0.9 C to 1.5 C; the current value 115, for example, to
0.7C; and the current value 116, for example, in a range of 0.15
C to 0.03 C. These current values may be suitably selected
according to temperatures and the like. The overvoltage Vfa may
be further segmented.
[0031] As described, according to the battery pack 1 and the
charger 2 of the present embodiment, the trickle charge circuit
25 is capable of varying the charge current by providing another
series circuit which is made up of the current limiting resistor
28 and the FET 29, and which is connected in parallel to the
conventional series circuit of the current limiting resistor 26
and the FET 27. Then, the charge control judging section 21
controls the trickle charge circuit 25 to increase the charge
current when the voltage detection circuit 20 detects that the
cell voltage reaches the predetermined switch voltage Vma set

lower than the end voltage Vm of the trickle charging.
Furthermore, the current value quickly increases if the residual
capacity of the secondary battery is not reduced significantly.
If the cell voltages of the assembled battery 14 are lower than
the switch voltage Vma and the residual capacity is almost null,
the charging is performed at low speed with the conventional
trickle charge current I11 to increase the cell voltages. Then,
after the cell voltages are increased to the predetermined
level, the charging is performed with the current I12 which is
larger than the conventional trickle charge current. As a
result, the time required for the trickle charging can be
reduced, thereby reducing an overall time required for the
charging.
[0032] Further, according to the battery pack 1 and the
charger 2 in accordance with the present embodiment, the end
voltage Vf is set to the OCV, and the charge control judging
section 21 requests in the constant-current charging, the charge
controller 31 via the communicators 22, 32 for such charge
voltage that the voltage across the charge terminals T11 and T13
of the battery pack 1 becomes the overvoltages Vfal, Vfa2 which
is above the end voltage Vf for the constant-current (CC)
charging. When the current sensing resistor 16 detects that the
charge current I13 is reduced to or below the predetermined
level I14, the charge control judging section 21 determines that
a transition is made to the constant-voltage (CV) charging, and

requests the charge controller 31 via the communicators 22, 32
for a charge voltage with which the voltage across the charge
terminals T11 and T13 can be reduced to the end voltage Vf and
for a charge current 115 with which the charge voltage as
reduced can be maintained. With this structure, although the
overvoltages Vfal, Vfa2 above the end voltage Vf are applied
across the charge terminals T11, T13 in the constant-current
(CC) charging period, such voltages above the end voltage Vf are
not applied to the respective cells. Moreover, a difference in
voltage between the voltage Vfal, Vfa2 and the cell voltage of
the assembled cell 14 can be consumed by a voltage drop caused
by the ON-resistance of the FETs 12, 13, the current sensing
resistor 16, the wiring resistance of the charge paths 11, 15
and the like. With this arrangement, since the charge current
in the constant current (CC) charging period can be reduced in a
short period of time, a transition can be made immediately to
the constant-voltage (CV) charging period even for almost fully
charged battery packs. The foregoing charging method of the
present embodiment is therefore applicable to battery packs in
any state, and the charge voltage to be applied in the constant-
voltage (CV) charging period can be increased, which in turn
increases an amount of charges to be injected while surely
preventing an application of an overvoltage to the respective
cells, and thereby preventing an overcharge of the respective
cells. Additionally, by setting a charge voltage and a

reduction in current to be detected to the same level as those
of the conventional method, as a final full charge condition,
the time required for an overall charging process can be
reduced, while maintaining the full charge capacity at the same
level.
[0033] Further, according to the battery pack 1 and the
charger 2 in accordance with the present embodiment, voltages
above the end voltage Vf are not applied to the respective cells
in the constant current (CC) charging period, regardless of the
state of the battery pack as described above. It is therefore
possible to prevent overcharging. The charge current supply
circuit 33 performs the super quick charging by setting the
current value of the charge current I13 in a range of 1C to 4C
which is higher than the charge current 0. 7C adopted in the
conventional method. It is therefore possible to further reduce
the charging time. Here, the lower limit for the current value
in the super quick charging area is not particularly limited, as
long as larger than the current value adopted in the
conventional method, and any current 0.8C or larger may be
adopted in the present embodiment.
[0034] As one example structure of the present embodiment,
the foregoing trickle charge circuit 25 includes the series
circuit of the current limiting resistor 26 and FET 27 and the
series circuit of the current limiting resistor 28 and FET 29,
which are connected in parallel, the current limiting resistors

26 and 28 having mutually different resistance values, and the
charge control judging section 21 switches ON the FET 27
corresponding to the current limiting resistor 26 having the
higher resistance value in the initial stage of the charging,
and switches ON the FET 29 corresponding to the current limiting
resistor 28 having the lower resistance value when a cell
voltage of a cell or cell voltages of all the plurality of cells
reach a switch voltage Vma. The present embodiment is not
limited to the foregoing structure, and for example, the trickle
charge circuit 25a shown in FIG. 3, or the trickle charge
circuit 25b shown in FIG. 4 or the like may be adopted.
[0035] In the trickle charge circuit 25, the use of the
resistor 28 and the FET 29 may be stopped and a pulse control
(PWM control) by ON/OFF controlling the FET 27 may be performed.
In this case, the pulse control for the trickle charge circuit
25 is performed to obtain a trickle charge current having an
average current value as requested.
[0036] As shown in FIG. 3, the trickle charge circuit 25a
includes the series circuit of the current limiting resistor 26a
and FET 27 and the series circuit of the current limiting
resistor 28a and FET 29, which are connected in parallel, the
current limiting resistors 26a and 28a having the same
resistance values, and the charge control judging section 21
switches ON only either one of the FETs 27 and 29, for example
the FET 27 corresponding to the current limiting resistor 26a so

as to have a high resistance in the initial stage of the
charging, and switches ON both of the FETs 27 and 29
corresponding to the current limiting resistors 26a and 28a so
as to have a low resistance value when a cell voltage of a cell
or cell voltages of all the plurality of cells reach the switch
voltage Vma, thereby increasing the trickle charge current.
[0037] As shown in FIG. 4, the trickle charge circuit 25b
includes two current limiting resistors 26b, 28b and one FET 27
which are connected in series, and another FET 29 is provided
for bypassing the current limiting resistors 28b, and the charge
control judging section 21 switches ON only the FET 27 so as to
have a high resistance value in the initial stage of the
charging, and switches ON only the FET 29 for bypassing the
current limiting resistors 28b so as to have a low resistance
value when a cell voltage of a cell or cell voltages of all the
plurality of cells reach the switch voltage Vma, thereby
increasing the trickle charge current. Other than the foregoing
circuit structures, any circuit structures for the current
limiting resistances and FETs may be adopted as long as a larger
current I12 than the conventional trickle charge current I11 can
be supplied.
[0038] According to the foregoing examples, it is determined
on the side of the battery pack 1 that a transition has been
made to the constant-voltage charging area based on a reduction
in current to the current I14, and an overvoltage Vfa2 and

current are requested to the charger 2. However, the present
embodiment is not intended to be limited to the above structure,
and it may be arranged to transit to the constant-voltage
charging area based on a reduction in current on the side of the
charger 2 in the similar manner, and to output the predetermined
voltage and current.
[0039] It may be also arranged on the side of the charger 2
such that a transition is made to the constant voltage (CV)
charging when the voltage across the terminals T21 and T23 is
increased to the overvoltage lVfal, and to output the
predetermined voltage and current. The method for controlling
the charge voltage and current in this example is as shown in
FIG. 5. When comparing FIG. 5 with FIG. 2, the charging time
with the overvoltage Vfal is slightly longer in FIG. 2, and the
residual capacity up to the full charge therefore decreases more
in the case of FIG. 2 by the difference in charging time,
thereby realizing a shorter charging time in the case of FIG. 2.
However, when comparing FIG. 5 with the conventional method
shown in FIG. 7, the method shown in FIG. 5 of the present
embodiment wherein it is determined that a transition is made to
the constant-voltage charging area based on the voltage across
the terminals T21 and T23, and the voltage is reduced from the
overvoltage Vfal to the overvoltage Vfa2, realizes a shorter
constant-current charging period (area), thereby realizing a
reduction in time required for an overall charging process as

compared to the conventional method shown in FIG. 7.
[0040] In the case of constructing an electronic device
system including a load device to have power supplied from the
battery pack 1 in addition to the battery pack 1 and the charger
2 as described above, a current may be decreased due to the
operation of the load device even during the charging. In this
case, a judgment error can be prevented by making the judgment
on the transition to the constant-voltage (CV) charging area at
or above a predetermined voltage. Specifically, since the
voltage across the terminals T21 and T23 decreases due to the
operation of the load device, the judgment on the current drop
may not be made when the voltage is decreased below the
predetermined voltage.
[0041] (Second Embodiment)
FIG. 6 is a block diagram showing the electrical structure
of a charging system employing a charging method according to a
second embodiment of the present invention. This charging
system is similar to the one shown in FIG. 1 and corresponding
parts are identified by the same reference numerals and not
described. An essential feature of the charging system in
accordance with the present embodiment lies in that only the
conventional series circuit of the current limiting resistor 26
and FET 27 is provided in a trickle charge circuit 25c of a
battery pack la and, instead, a charge current supply circuit
33a of a charger 2a can supply a current I12 in the middle-speed

current charging area.
[0042] Thus, a charge control judging section 21a of a
control IC 18a performs the trickle charging in a similar manner
to the conventional method by switching ON the FETs 13, 27 and
using the current limiting resistor 26 in the initial stage of
the charging process as described above. The charge control
judging section 21a then requests a charge controller 31a of a
control IC 30a of the charger 2a via the communicators 22, 32,
for a charge current of the current value I12 which is larger
than the current value I11 in the trickle charging and is
smaller than the constant current value I13 in the constant-
current /constant-voltage charging when the cell voltage reaches
a switch voltage Vma, and controls the trickle charge circuit
25c to switch OFF the FET 27 and switch ON the FET 12 for
charging, so that the charge current from the charger 2a is
directly outputted to the assembled battery 14. The charge
controller 31a controls the charge current supply circuit 33a to
supply a charge current of the current value I12 in response to
the request. When the cell voltage reaches the end voltage Vm
for the trickle charging, a transition is made to the super
quick constant current/constant-voltage charging. The charge
control judging section 21a then requests for a charge current
of the constant current value I13, and the charge controller 31a
controls the charge current supply circuit 33a to supply a
charge current of the current value I13 in response to the

request.
[0043] With the foregoing structure, the trickle charging
time can be reduced, thereby reducing a time required for an
overall charging process.
[0044] As described, according to the foregoing charging
method of the present invention, although a voltage above the
end voltage is applied across the charge terminals, such voltage
is not applied to the respective cells in the constant-current
(CC) charging. Moreover, a difference in voltage between the
voltage across the terminals and the cell voltage can be
consumed by a voltage drop caused by switches and current
detection resistances provided for safety control and the
charge/discharge control. With this arrangement, since the
charge current in the constant current (CC) charging period can
be reduced in a short period of time, a transition can be made
immediately to the constant-voltage (CV) charging period even
for almost fully charged battery packs. The foregoing charging
method of the present embodiment is therefore applicable to
battery packs in any state, and the charge voltage to be applied
in the constant-voltage (CV) charging period can be increased,
which in turn increases an amount of charges to be injected
while surely preventing an application of an overvoltage to the
respective cells, and thereby preventing an overcharge of the
respective cells. Additionally, by setting a charge voltage and
a reduction in current to be detected to the same level as those

of the conventional method, as a final full charge condition,
the time required for an overall charging process can be
reduced, while maintaining the full charge capacity at the same
level.
[0045] According to the charging method of the present
invention, when the residual capacity of the secondary battery
is not reduced significantly, a current value is increased
immediately, and if the cell voltages of the secondary battery
are lower than the switch voltage and the residual capacity is
almost null, the charging is performed at low speed with the
conventional trickle charge current to increase the cell
voltages. When the cell voltages are increased to the
predetermined level, the charging is performed with a current
larger than the conventional trickle charge current. As a
result, the time required for the trickle charging can be
reduced, thereby reducing a time required for an overall
charging process.
[0046] Furthermore, according to the charging method of the
present invention, it is possible to realize a shorter charging
time in the trickle charging as describe above and at the same
time to realize a shorter charging time in the constant
current/constant-voltage charging, thereby realizing a further
reduction in time required for an overall charging.
[0047] As described, according to the foregoing charging
method of the present invention, although a voltage above the

end voltage is applied across the charge terminals, such voltage
is not applied to the respective cells in the constant-current
(CC) charging. Moreover, a difference in voltage between the
voltage across the terminals and the cell voltage can be
consumed by a voltage drop caused by switches and current
detection resistances provided for safety control and the
charge/discharge control. With this arrangement, since the
charge current in the constant current (CC) charging period can
be reduced in a short period of time, a transition can be made
immediately to the constant-voltage (CV) charging period even
for almost fully charged battery packs. The foregoing charging
method of the present embodiment is therefore applicable to
battery packs in any state, and the charge voltage to be applied
in the constant-voltage (CV) charging period can be increased,
which in turn increases an amount of charges to be injected
while surely preventing an application of an overvoltage to the
respective cells, and thereby preventing an overcharge of the
respective cells. Additionally, by setting a charge voltage and
a reduction in current to be detected to the same level as those
of the conventional method, as a final full charge condition,
the time required for an overall charging process can be
reduced, while maintaining the full charge capacity at the same
level.
[0048] Accordingly to the battery pack of the present
invention, when the residual capacity of the secondary battery

is not reduced significantly, a current value is increased
immediately; on the other hand, when the cell voltages of the
secondary battery are lower than the switch voltage and the
residual capacity is almost null, the charging is performed at
low speed with the conventional trickle charge current to
increase the cell voltages. When the cell voltages are
increased to the predetermined level, the charging is performed
with a current larger than the conventional trickle charge
current. As a result, the time required for the trickle
charging can be reduced, thereby reducing a time required for an
overall charging process.
[0049] Accordingly to the battery pack of the present
invention, when the residual capacity of the secondary battery
is not reduced significantly, a current value is increased
immediately; on the other hand, when the cell voltages of the
secondary battery are lower than the switch voltage and the
residual capacity is almost null, the charging is performed at
low speed with the conventional trickle charge current to
increase the cell voltages. When the cell voltages are
increased to the predetermined level, the charging is performed
with a current larger than the conventional trickle charge
current. As a result, the time required for the trickle
charging can be reduced, thereby reducing a time required for an
overall charging process.
[0050] Furthermore, according to the charging method of the

present invention, it is possible to realize a shorter charging
time in the trickle charging as describe above and at the same
time to realize a shorter charging time in the constant
current/constant-voltage charging, thereby realizing a further
reduction in time required for an overall charging.
[0051] As described, according to the foregoing charger of
the present invention, although a voltage above the end voltage
is applied across the charge terminals, such voltage is not
applied to the respective cells in the constant-current (CC)
charging. Moreover, a difference in voltage between the voltage
across the terminals and the cell voltage can be consumed by a
voltage drop caused by switches and current detection
resistances provided for safety control and the charge/discharge
control. With this arrangement, since the charge current in the
constant current (CC) charging period can be reduced in a short
period of time, a transition can be made immediately to the
constant-voltage (CV) charging period even for almost fully
charged battery packs. The foregoing charging method of the
present embodiment is therefore applicable to battery packs in
any state, and the charge voltage to be applied in the constant-
voltage (CV) charging period can be increased, which in turn
increases an amount of charges to be injected while surely
preventing an application of an overvoltage to the respective
cells, and thereby preventing an overcharge of the respective
cells. Additioanlly, by setting the charge voltage and the

detected drop in current equal to the conventional levels, as a
final full charge condition, it is possible to reduce the time
required for charging while maintaining the full charge capacity
at the same level.
[0052] Accordingly to the charger of the present invention,
when the residual capacity of the secondary battery is not
reduced significantly, a current value is increased immediately,
and if the cell voltages of the secondary battery are lower than
the switch voltage and the residual capacity is almost null, the
charging is performed at low speed with the conventional trickle
charge current to increase the cell voltages. When the cell
voltages are increased to the predetermined level, the charging
is performed with a current larger than the conventional trickle
charge current. As a result, the time required for the trickle
charging can be reduced, thereby reducing a time required for an
overall charging process.
[0053] In view of the foregoing embodiments, the present
invention can be summarized as follows. Specifically, a
charging method in accordance with one aspect of the present
invention includes: a constant-current charging step wherein a
constant charge current is supplied to a secondary battery to be
charged to a predetermined end voltage; and a constant-voltage
charging step wherein the predetermined end voltage is
maintained by reducing the charge current after the secondary
battery is charged to the end voltage, wherein the constant-

current charging step includes a charging step to be carried out
with the end voltage set to an open circuit voltage (OCV) which
is a voltage when no current is flowing, and with a voltage
across charge terminals of the battery pack set to an
overvoltage above the OCV, and the constant-voltage charging
step includes a step of reducing the voltage across the charge
terminals to the OCV after the voltage across the charge
terminals is increased to the overvoltage or after the charge
current across the charge terminals is reduced to or below a
predetermined current level.
[0054] According to the foregoing method of charging the
secondary battery such as a lithium ion battery, the constant
current (CC) charging wherein a constant charge current is
supplied to the secondary battery to be charged to a
predetermined end voltage (e.g. 4.2 V in the case of the lithium
ion battery) as a target voltage, subsequent to the trickle
charging to be carried out in the initial stage of the charging
process wherein a small current is applied. Then, after the
secondary battery is charged to the end voltage, a constant-
voltage charging is performed wherein the predetermined end
voltage is maintained by reducing the charge current. In the
constant-voltage charging step, the end voltage is set to an
open circuit voltage (OCV) which is a voltage when no current is
flowing, and in the constant-current charging step, the voltage
of a charge terminal of the battery pack is set to an

overvoltage above the OCV. After the voltage across the charge
terminals is increased to the overvoltage and a transition is
made to the constant-voltage charging, or after the charge
current of the charge terminal is reduced to or below a
predetermined level, the voltage across the charge terminals is
reduced to the end voltage.
[0055] As described, according to the foregoing charging
method of the present invention, although a voltage above the
end voltage is applied across the charge terminals, such voltage
is not applied to the respective cells in the constant-current
(CC) charging. Moreover, a difference in voltage between the
voltage across the terminals and the cell voltage can be
consumed by a voltage drop caused by switches and current
detection resistances provided for safety control and the
charge/discharge control. With this arrangement, since the
charge current in the constant current (CC) charging period can
be reduced in a short period of time, a transition can be made
immediately to the constant-voltage (CV) charging period even
for almost fully charged battery packs. The foregoing charging
method of the present embodiment is therefore applicable to
battery packs in any state without a need of detecting a
residual capacity before the charging process is to be
performed, and the charge voltage to be applied in the constant-
voltage (CV) charging period can be increased, which in turn
increases an amount of charges to be injected while surely

preventing an application of an overvoltage to the respective
cells, and thereby preventing an overcharge of the respective
cells. Additionally, by setting a charge voltage and a
reduction in current to be detected to the same level as those
of the conventional method, as a final full charge condition,
the time required for an overall charging process can be
reduced, while maintaining the full charge capacity at the same
level.
[0056] In the foregoing charging method, the charge current
for the constant-current charging step is set in a range of from
0.8 C to 4 C provided that a current value with which a nominal
capacity of the secondary battery is discharged in an hour by
carrying out constant current discharging, is 1 C.
[0057] According to the foregoing structure, as described
above, no higher voltage than the end voltage is applied to the
secondary battery at the time of the constant-current (CC)
charging and overcharge is reliably prevented regardless of the
state of the secondary battery. Thus, the charge current value
can be set in a range of 0.8C to 4C which is higher as compared
with the conventional value of 0.7C, provided that a current
value with which a nominal capacity NC is discharged in an hour
by carrying out constant current discharging is 1 C.
[0058] According to the foregoing structure, in addition to
the feature that the voltage across the charge terminals is set
higher than the end voltage at the time of the constant-current

(CC) charging, the charge current is increased. It is therefore
possible to inject a still larger amount of charges, thereby
reducing an overall charging time.
[0059] The foregoing charging method further includes: a
trickle charging step to be carried out in an initial stage of a
charging process of the secondary battery, wherein the trickle
charging step includes the steps of: setting a switch voltage to
a voltage below the end voltage for the trickle charging step
and carrying out a trickle charging with a trickle charge
current from a beginning of the charging process, charging with
a current which is larger than the trickle charge current after
the voltage across the charge terminals is increased to the
switch voltage, and terminating the trickle charging step when
the voltage across the charge terminals is increased to the end
voltage for the trickle charging step.
[0060] According to the foregoing method of the trickle
charging to be performed in the initial stage of the charging
process of the secondary battery such as a lithium ion battery,
or the like, the conventional trickle charging period (area) is
divided into a first half and a second half without changing the
end voltage from that adopted in the conventional trickle
charging method. In the first half, the trickle charging is
performed in the same manner as the conventional trickle
charging method. In the second half, the trickle charging is
performed with a trickle charge current larger than that adopted

in the conventional method. With this structure, the switch
voltage is set to a voltage below the end voltage for the
conventional trickle charging. when the charging operation is
started, the foregoing first half where the conventional trickle
charging is performed starts and ends when the cell voltage of
the secondary battery reaches the switch voltage. When the cell
voltage reaches the switch voltage, a transition is made to the
second half where the trickle charging is performed with a
trickle charge current larger than that of the conventional
trickle charging method. Then, when the cell voltage reaches
the end voltage for the conventional trickle charging, the
trickle charging is terminated. According to the foregoing
method, the first half where the trickle charging is carried out
in the conventional manner is performed in a shorter period of
time, and is transited to the second half of the trickle charge
period (area) where the trickle charging is performed with a
larger trickle charge current.
[0061] The switch voltage is set to the lowest limit voltage
provided that a damage on the secondary battery can be avoided,
in connection with the current value of the current larger than
the conventional trickle charge current, and the current value
is set to the largest limit value. After terminating the
trickle charging, a normal charge control such as constant
current/constant-voltage charging is performed.
[0062] Accordingly, when the residual capacity of the

secondary battery is not reduced significantly, a transition is
made immediately to the second half, and if the cell voltages of
the secondary battery are lower than the switch voltage and the
residual capacity is almost null, the charging is performed at
low speed with the conventional trickle charge current to
increase the cell voltages. When the cell voltages are
increased to the predetermined level, the charging is performed
with a current larger than the conventional trickle charge
current. As a result, the time required for the trickle
charging can be reduced, thereby reducing a time required for an
overall charging process.
[0063] According to the foregoing structure, it is possible
to reduce the charging time at the time of the trickle charging
as described above, while reducing the charging time at the time
of the constant current/constant-voltage charging, thereby
reducing an overall time required for charging.
[0064] A battery pack according to the present invention
includes: a secondary battery; a current detector which detects
a charge current of the secondary battery; a communicator which
communicates with a charger; and a charge controller which
carries out a constant-current charging wherein a constant
charge current is supplied to the secondary battery to be
charged to a predetermined end voltage by sending a request for
a charge voltage and a charge current to the charger via the
communicator and which carries out a constant-voltage charging

wherein the end voltage is maintained by reducing the charge
current after the secondary battery is charged to the end
voltage, wherein the charge controller sets the end voltage to
an OCV, which is a voltage when no current is flowing, requests
the charger via the communicator for a charge voltage with which
the voltage across the charge terminals can be increased to an
overvoltage above the OCV when carrying out the constant-current
charging, and requests for a charge voltage with which the
voltage across the charge terminals can be maintained at the OCV
after the voltage across the charge terminals reaches the
overvoltage and the current detector detects that the charge
current is reduced to or below a predetermined current level.
[0065] According to the foregoing structure of the battery
pack which includes the secondary battery such as a lithium ion
battery, and the current detector, the communicator and the
charge controller for charging the secondary battery, the charge
controller sends a request for a charge voltage and a charge
current to the charger via the communicator, to carry out the
constant-current (CC) charging wherein a constant charge current
is supplied to the secondary battery to be charged to a
predetermined end voltage (e.g. 4.2 V in the case of the lithium
ion battery) as a target voltage. Then, after the secondary
battery is charged to the end voltage, a constant-voltage
charging is performed. When the constant-voltage charging is to
be performed, the charge controller sets the end voltage to an

open circuit voltage (OCV) which is a voltage when no current
is flowing, requests the charger via the communicator for such
charge voltage with which the voltage across the charge
terminals of the battery pack is increased an overvoltage above
the end voltage. Then, when the voltage across the charge
terminals reaches the overvoltage, and the current detector
detects that the charge current across the terminals is reduced
to or below a predetermined current level, the charge controller
requests the charger for a charge voltage with which the voltage
across the charge terminals can be reduced to the end voltage
step by step or gradually, and for a charge current with which
the voltage as reduced can be maintained.
[0066] As described, according to the foregoing charging
method of the present invention, although a voltage above the
end voltage is applied across the charge terminals, such voltage
is not applied to the respective cells in the constant-current
(CC) charging. Moreover, a difference in voltage between the
voltage across the terminals and the cell voltage can be
consumed by a voltage drop caused by switches and current
detection resistances provided for safety control and the
charge/discharge control. With this arrangement, since the
charge current in the constant current (CC) charging period can
be reduced in a short period of time, a transition can be made
immediately to the constant-voltage (CV) charging period even
for almost fully charged battery packs. The foregoing charging

method of the present embodiment is therefore applicable to
battery packs in any state without a need of detecting a
residual capacity before the charging process is to be
performed, and the charge voltage to be applied in the constant-
voltage (CV) charging period can be increased, which in turn
increases an amount of charges to be injected while surely
preventing an application of an overvoltage to the respective
cells, and thereby preventing an overcharge of the respective
cells. Additionally, by setting a charge voltage and a
reduction in current to be detected to the same level as those
of the conventional method, as a final full charge condition,
the time required for an overall charging process can be
reduced, while maintaining the full charge capacity at the same
level.
[0067] In the foregoing battery pack, the charge current for
the constant-current charging step is set in a range of from 0.8
C to 4 C provided that a current value with which a nominal
capacity of the secondary battery is discharged in an hour by
carrying out constant current discharging, is 1 C.
[0068] According to the foregoing structure, a voltage above
the end voltage is not applied to the secondary battery in the
constant-current (CC) charging, and overcharge is reliably
prevented regardless of the state of the secondary battery. It
is therefore possible to set the charge current value in a range
of 0.8 C to 4 C, which is higher than the charge current value

(0.7 C) adopted in the conventional structure.
[0069] According to the foregoing structure, in addition to
the feature that the voltage across the charge terminals is set
higher than the end voltage at the time of the constant-current
(CC) charging, the charge current is increased. It is therefore
possible to inject a still larger amount of charges, thereby-
reducing an overall charging time.
[0070] The above battery pack further includes a voltage
detector which detects a cell voltage of the secondary battery;
and a trickle charge circuit capable of varying a charge current
to be supplied to the secondary battery and trickle-charging the
secondary battery while limiting the charge current from the
charger in a period from a begging of the charging process until
the voltage detector detects that the cell voltage of the
secondary battery reaches a predetermined end voltage for the
trickle charging, wherein the charge controller controls the
trickle charge circuit to increase the charge current when the
voltage detector detects that the cell voltage reaches a
predetermined switch voltage set below the end voltage for the
trickle charging, and to terminate the trickle charging when the
cell voltage reaches the end voltage for the trickle charging.
[0071] According to the foregoing structure of the battery
pack which includes the secondary battery such as a lithium ion
battery, and the elements for charging the secondary battery,
i.e., the trickle charge circuit, the voltage detector, the

communicator and the charge controller, wherein when carrying
out the trickle charging, a constant trickle charge current is
supplied from the charger, while a variable charge current can
be supplied to the secondary battery from the trickle charge
circuit, constituted, for example, by a parallel circuit made up
of current limiting resistors for limiting current flowing in
the circuit and a switching element which permits the current to
flow without limiting. The charge control circuit then
increases the charge current to be supplied to the trickle
charge circuit when the voltage detection circuit detects that
the cell voltage reaches the predetermined switch voltage set
lower than the end voltage for the trickle charging, and
terminates the trickle charging when the cell voltage reaches
the end voltage for the trickle charging. According to the
foregoing method, the conventional trickle charging area is
divided into the first half where the trickle charging is
carried out in the conventional manner and the second half where
the trickle charging is performed with a larger trickle charge
current without changing the end voltage for the trickle
charging, and the first half where the trickle charging is
carried out in the conventional manner is performed in a shorter
period of time, and is transited to the second half of the
trickle charge period (area) where the trickle charging is
performed with a larger trickle charge current.
[0072] Accordingly, if the residual capacity of the

secondary battery is not reduced significantly, a transition is
made immediately to the second half, and if the cell voltages of
the secondary battery are lower than the switch voltage and the
residual capacity is almost null, the charging is performed at
low speed with the conventional trickle charge current to
increase the cell voltages. When the cell voltages are
increased to the predetermined level, the charging is performed
with a current larger than the conventional trickle charge
current. As a result, the time required for the trickle
charging can be reduced, thereby reducing a time required for an
overall charging process.
[0073] In the above battery pack, the trickle charge circuit
includes two current limiting resistors and FETs paired with the
two current limiting resistors, and the charge controller
switches a resistance value of the trickle charge circuit by
controlling ON/OFF of the FETs, thereby varying the charge
current to be supplied to the secondary battery.
[0074] According to the foregoing structure, the trickle
charge circuit includes the two current limiting resistors and
the FETS paired with the current limiting resistors in order to
be able to supply a current larger than the conventional trickle
charge current as the trickle charge current. The current
limiting resistors and the FETs may be constructed into an
arbitrary series-parallel circuit. For example, series circuits
of current limiting resistors having different resistance values

and FETs paired with the current limiting resistors are
connected in parallel with each other, and the charge controller
can increase the trickle charge current through a selective
control of turning on the FET corresponding to the current
limiting resistor having the higher resistance value at the
start of charging and turning on the FET corresponding to the
lower current limiting resistor having the lower resistance
value when the switch voltage is reached. Alternatively, series
circuits of current limiting resistors having different or equal
resistance values and FETs paired with the current limiting
resistors are connected in parallel with each other, and the
charge controller can increase the trickle charge current by
turning on only the FET corresponding to one current limiting
resistor to set the higher resistance value at the start of
charging and turning on the FETs corresponding to the both
current limiting resistors to set the lower resistance value
when the switch voltage is reached. Further, two current
limiting resistors and one FET are connected in series, another
FET is provided for bypassing one current limiting resistor, and
the charge controller can increase the trickle charge current by
turning only the FET in series on to set the higher resistance
value at the start of charging and turning the FET for bypass on
to set the lower resistance value when the switch voltage is
reached.
[0075] The foregoing structure provides one example of the

trickle charging circuit.
[0076] In the foregoing battery pack, the charge controller
requests the charger via the communicator for a charge current
which is larger than a charge current for the trickle charging
and is smaller than a constant charge current for the constant-
current charging, and directly outputs the charge current to the
trickle charge circuit when the voltage detector detects that
the cell voltage reaches the predetermined switch voltage set
below the end voltage for the trickle charging, and makes a
transition from the trickle charging to the constant-current
charging and requests the charger for a constant charge current
when the voltage detector detects that the cell voltage reaches
the end voltage for the trickle-charging.
[0077] According to the foregoing structure of the battery
pack which includes the secondary battery such as a lithium ion
battery, and the elements for charging the secondary battery,
i.e., the trickle charge circuit, the voltage detector, the
communicator and the charge controller, wherein the charge
controller controls the trickle charge circuit to carry out the
trickle charging for charging the secondary battery while
limiting the charge current from the charger in the period from
the beginning of the charging period until the voltage detector
detects that the cell voltages of the secondary battery reach
the predetermined end voltage for the trickle charging. When
the cell voltage reaches the end voltage for the trickle

charging, the charge controller controls the trickle charge
circuit to directly output the charge current from the charger
to the trickle charge circuit, and sends a request the charger
via the communicator for a charge voltage and a charge current,
thereby carrying out the constant-current constant-voltage
charging with respect to the secondary battery. In the battery
pack of the foregoing structure, currents of two different
values are requested to the charger for the trickle charging,
i.e. i) the current of the same value as the conventional
current value and ii) the current of a larger value than the
conventional current value and of a smaller value than the
constant current value for the constant current/constant-voltage
charging. The charge controller requests the charger for a
charge current via the communicator which is larger than a
current in the trickle charging and is smaller than a constant
current in the constant-current charging, and directly outputs
the charge current to the trickle charge circuit when the
voltage detector detects that the cell voltage is increased to
the predetermined switch voltage set below the end voltage for
the trickle charging. The charger controller than makes a
transition from the trickle charging to the constant-current
charging and requests the charger for a constant charge current
when the voltage detector detects that the cell voltage reaches
the end voltage for the trickle-charging. According to the
foregoing method, the conventional trickle charging area is

divided into the first half where the trickle charging is
carried out in the conventional manner and the second half where
the trickle charging is performed with a larger trickle charge
current without changing the end voltage for the trickle
charging, and the first half where the trickle charging is
carried out in the conventional manner is performed in a shorter
period of time, and is transited to the second half of the
trickle charge period (area) where the trickle charging is
performed with a larger trickle charge current.
[0078] Accordingly, if the residual capacity of the
secondary battery is not reduced significantly, a transition is
made immediately to the second half, and if the cell voltages of
the secondary battery are lower than the switch voltage and the
residual capacity is almost null, the charging is performed at
low speed with the conventional trickle charge current to
increase the cell voltages. When the cell voltages are
increased to the predetermined level, the charging is performed
with a current larger than the conventional trickle charge
current. As a result, the time required for the trickle
charging can be reduced, thereby reducing a time required for an
overall charging process.
[0079] According to the foregoing structure, it is possible
to reduce the charging time at the time of the trickle charging
as described above, while reducing the charging time at the time
of the constant current/constant-voltage charging, thereby

reducing an overall time required for charging.
[0080] A charger of the present invention includes: a charge
current supply circuit which supplies a charge current to a
battery pack; a communicator which communicates with the battery
pack; and a charge controller which carries out a constant-
current charging wherein a constant charge current is supplied
to a secondary battery of the battery pack to be charged to a
predetermined end voltage by controlling a charge current from
the charge current supply circuit in response to a request from
the battery pack inputted via the communicator and which carries
out a constant-voltage charging by reducing the charge current
so as to maintain the end voltage after the second battery is
charged to the end voltage, wherein when the constant-current
charging is to be carried out, the charge controller sets the
end voltage to an OCV, which is a voltage when no current is
flowing, in response to a request from the battery pack inputted
via the communicator, and controls the charge current supply
circuit so as to output such charge current that the voltage
across the charge terminals of the battery pack becomes higher
than the OCV, and when the voltage across the charge terminals
reaches the overvotlage, and a transition is made from the
constant-current charging to the constant-voltage charging, or
when the charge current across the charge terminals is reduced
to or below a predetermined current level, the charge controller
controls the charge current supply circuit so as to reduce the

voltage across the charge terminals to the OCV and to supply
such a charge current to maintain the voltage across the charge
terminals at the OCV.
[0081] The charger of the foregoing structure includes the
charge current supply circuit, the communicator and the charge
controller, and charges a secondary battery such as a lithium
ion battery in the battery pack by carrying out the constant
current (CC) charging with a constant charge current to charge
the secondary battery to the predetermined end voltage, and
carrying out the constant-voltage (CV) charging for maintaining
the end voltage by reducing the charge current when the
secondary battery reaches the end voltage. The foregoing
charger is arranged on the side of the battery pack such that
the end voltage is set to the OCV, and a request is made for
such charge voltage that the voltage across the charge terminals
of the battery pack becomes an overvoltage above the end
voltage. The foregoing charger is further arranged such that
upon receiving by the communicator, a request made for such
charge voltage for reducing the voltage across the charge
terminals to the end voltage when a transition is made to the
constant voltage (CV) charging, or the charge current is reduced
to or below the predetermined current level, and a request made
for such charge current for maintaining the voltage as reduced,
the charge control section controls the charge current supply
circuit to output the charge voltage and the charge current as

requested.
[0082] With the foregoing structure, although a voltage
above the end voltage is applied across the charge terminals,
such voltage is not applied to the respective cells in the
constant-current (CC) charging period. Moreover, a difference
in voltage between the voltage across the terminals and the cell
voltage can be consumed by a voltage drop caused by switches and
current detection resistances provided for safety control and
the charge/discharge control. With this arrangement, since the
charge current in the constant current (CC) charging period can
be reduced in a short period of time, a transition can be made
immediately to the constant-voltage (CV) charging period even
for almost fully charged battery packs. The foregoing charging
method of the present embodiment is therefore applicable to
battery packs in any state without a need of detecting a
residual capacity before the charging process is to be
performed, and the charge voltage to be applied in the constant-
voltage (CV) charging period can be increased, which in turn
increases an amount of charges to be injected while surely
preventing an application of an overvoltage to the respective
cells, and thereby preventing an overcharge of the respective
cells. Additionally, by setting a charge voltage and a
reduction in current to be detected to the same level as those
of the conventional method, as a final full charge condition,
the time required for an overall charging process can be

reduced, while maintaining the full charge capacity at the same
level.
[0083] In the foregoing charger, the charge controller sets
the charge current value for the constant-current charging in a
range of from 0.8 C to 4 C provided that a current value with
which a nominal capacity of the secondary battery is discharged
in an hour by carrying out constant current discharging, is 1 C.
[0084] As described, according to the foregoing structure, a
voltage above the end voltage is not applied to the secondary
battery in the constant-current (CC) charging, and overcharge is
reliably prevented regardless of the state of the secondary
battery. It is therefore possible to set the charge current
value in a range of 0.8 C to 4 C, which is higher than the
charge current value (0.7 C) adopted in the conventional
structure.
[0085] According to the foregoing structure, in addition to
the feature that the voltage across the charge terminals is set
higher than the end voltage at the time of the constant-current
(CC) charging, the charge current is increased. It is therefore
possible to inject a still larger amount of charges, thereby
reducing an overall charging time.
[0086] The charger of the foregoing structure may be further
arranged such that in response to an instruction for switching
the trickle charge current as received by the communicator in
the trickle charging process, the charge controller controls the

charge current supply circuit to directly output the charge
current to the battery pack, and to supply a charge current set
larger than the trickle charge current and is smaller than the
constant current for the constant-current charging.
[0087] The foregoing structure of the charger includes the
charge current supply circuit, the communicator and the charge
controller, and charges the secondary battery such as a lithium
ion battery in the battery pack by carrying out the constant
current/constant-voltage charging subsequent to the trickle
charging. The foregoing charger is arranged on the side of the
battery pack such that a switch voltage is set to a voltage
below the end voltage for the trickle charging, and when the
cell voltage reaches the switch voltage, a request for switching
the charge current is made to the charger, and in response to
the request, the charge control circuit outputs the charge
current from the charge current supply circuit directly to the
battery pack, and supplies to the charge current supply circuit,
a charge current of a larger value than the conventional trickle
charge current and of a smaller value than the constant current
value for the constant-current/constant-voltage charging.
[0088] Accordingly, when the residual capacity of the
secondary battery is not reduced significantly, a transition is
made immediately to the second half, and if the cell voltages of
the secondary battery are lower than the switch voltage and the
residual capacity is almost null, the charging is performed at

low speed with the conventional trickle charge current to
increase the cell voltages. When the cell voltages are
increased to the predetermined level, the charging is performed
with a current larger than the conventional trickle charge
current. As a result, the time required for the trickle
charging can be reduced, thereby reducing a time required for an
overall charging process.
INDUSTRIAL APPLICABILITY
[0089] The present invention is applicable to battery packs
in any state, and permits an increase in amount of charges to be
injected while surely preventing an application of an
overvoltage to cells of a secondary battery, or overcharging the
cells, and realizes a reduction in overall time required for
charging. Therefore, the present invention can be suitably
applied to a battery pack and a charger of the same capable of
performing constant current/constant-voltage charging subsequent
to a trickle charging.

WHAT IS CLAIMED IS:
1. A charging method, comprising:
a constant-current charging step wherein a constant charge
current is supplied to a secondary battery to be charged to a
predetermined end voltage; and
a constant-voltage charging step wherein the predetermined
end voltage is maintained by reducing the charge current after
said secondary battery is charged to the end voltage, wherein:
said constant-current charging step includes a charging
step to be carried out with the end voltage set to an open
circuit voltage (OCV) which is a voltage when no current is
flowing, and with a voltage across charge terminals of said
battery pack set to an overvoltage above the OCV, and
said constant-voltage charging step includes a step of
reducing the voltage across the charge terminals to the OCV
after the voltage across the charge terminals is increased to
the overvoltage or after the charge current across the charge
terminals is reduced to or below a predetermined current level.
2. A charging method according to claim 1, wherein the
charge current for said constant-current charging step is set in
a range of from 0.8 C to 4 C provided that a current value with
which a nominal capacity of said secondary battery is discharged
in an hour by carrying out constant current discharging, is 1 C.

3. A charging method according to claim 1 or 2, further
comprising:
a trickle charging step to be carried out in an initial
stage of a charging process of said secondary battery,
wherein said trickle charging step includes the steps of:
setting a switch voltage to a voltage below the end voltage
for said trickle charging step and carrying out a trickle
charging with a trickle charge current from a beginning of the
charging process,
charging with a current which is larger than the trickle
charge current after the voltage across the charge terminals is
increased to said switch voltage, and
terminating the trickle charging step when the voltage
across the charge terminals is increased to the end voltage for
said trickle charging step.
4. A battery pack, comprising:
a secondary battery;
a current detector which detects a charge current of said
secondary battery;
a communicator which communicates with a charger; and
a charge controller which carries out a constant-current
charging wherein a constant charge current is supplied to said
secondary battery to be charged to a predetermined end voltage
by sending a request for a charge voltage and a charge current

to the charger via said communicator and which carries out a
constant-voltage charging wherein the end voltage is maintained
by reducing the charge current after said secondary battery is
charged to the end voltage,
wherein said charge controller sets the end voltage to an
OCV, which is a voltage when no current is flowing, and requests
said charger via said communicator, i) for a charge voltage with
which the voltage across the charge terminals can be increased
to an overvoltage above the OCV when carrying out said constant-
current charging, and ii) for a charge voltage with which the
voltage across the charge terminals can be maintained at the OCV
after the voltage across the charge terminals reaches the
overvoltage and the current detector detects that the charge
current is reduced to or below a predetermined current level.
5. A battery pack according to claim 4, wherein the charge
current for said constant-current charging step is set in a
range of from 0.8 C to 4 C provided that a current value with
which a nominal capacity of said secondary battery is discharged
in an hour by carrying out constant current discharging, is 1 C.
6. A battery pack according to claim 4 or 5, further
comprising:
a voltage detector which detects a cell voltage of said
secondary battery; and

a trickle charge circuit capable of varying a charge
current to be supplied to said secondary battery and trickle-
charging said secondary battery while limiting the charge
current from the charger in a period from a begging of the
charging process until said voltage detector detects that the
cell voltage of said secondary battery reaches a predetermined
end voltage for the trickle charging,
wherein the charge controller controls said trickle charge
circuit to increase the charge current when said voltage
detector detects that the cell voltage reaches a predetermined
switch voltage set below the end voltage for the trickle
charging, and to terminate the trickle charging when the cell
voltage reaches the end voltage for said trickle charging.
7. A battery pack according to claim 6, wherein:
said trickle charge circuit includes two current limiting
resistors and FETs paired with said two current limiting
resistors, and
said charge controller switches a resistance value of said
trickle charge circuit by controlling ON/OFF of the FETs,
thereby varying the charge current to be supplied to said
secondary battery.
8. A battery pack according to claim 6, wherein:
said charge controller i) requests said charger via said

communicator for a charge current which is larger than a charge
current for said trickle charging and is smaller than a charge
constant current for said constant-current charging, and
directly outputs the charge current to said trickle charge
circuit when said voltage detector detects that the cell voltage
reaches the predetermined switch voltage set below the end
voltage for the trickle charging, and ii) makes a transition
from the trickle charging to the constant-current charging and
requests said charger for a constant charge current when said
voltage detector detects that the cell voltage reaches the end
voltage for the trickle-charging.
9. A charger, comprising:
a charge current supply circuit which supplies a charge
current to a battery pack;
a communicator which communicates with said battery pack;
and
a charge controller which carries out a constant-current
charging wherein a constant charge current is supplied to a
secondary battery of the battery pack to be charged to a
predetermined end voltage by controlling a charge current from
said charge current supply circuit in response to a request from
said battery pack inputted via said communicator and which
carries out a constant-voltage charging by reducing the charge
current so as to maintain the end voltage after said second

battery is charged to the end voltage,
wherein when the constant-current charging is to be carried
out, said charge controller sets the end voltage to an OCV,
which is a voltage when no current is flowing, in response to a
request from said battery pack inputted via said communicator,
and controls said charge current supply circuit so as to output
such charge current that the voltage across the charge terminals
of said battery pack becomes higher than the OCV, and
when the voltage across the charge terminals reaches the
overvotlage, and a transition is made from the constant-current
charging to the constant-voltage charging, or when the charge
current across the charge terminals is reduced to or below a
predetermined current level, said charge controller controls
said charge current supply circuit so as to reduce the voltage
across the charge terminals to the OCV and to supply such a
charge current to maintain the voltage across the charge
terminals at the OCV.
10. A charger according to claim 9, wherein:
said charge controller sets the charge current value for
said constant-current charging in a range of from 0.8 C to 4 C
provided that a current value with which a nominal capacity of
said secondary battery is discharged in an hour by carrying out
constant current discharging, is 1 C.

11. A charger according to claim 9 or 10, wherein in
response to an instruction for switching the trickle charge
current as received by the communicator in a trickle charging
process, said charge controller controls the charge current
supply circuit to directly output the charge current to said
battery pack, and to supply a charge current set larger than the
trickle charge current and is smaller than the constant current
for said constant-current charging.

A charging method includes a constant-current charging step
wherein a constant charge current is supplied to a secondary
battery to be charged to a predetermined end voltage; and a
constant-voltage charging step wherein the predetermined end
voltage is maintained by reducing the charge current after said
secondary battery is charged to the end voltage, wherein: said
constant-current charging step includes the charging step to be
carried out with the end voltage set to OCV which is a voltage
when no current is flowing, and with a voltage of a charge
terminal of said battery pack set to an overvoltage above said
OCV, and said constant-voltage charging step includes the step
of reducing the voltage across the charge terminals to the after
the voltage across the charge terminals is increased to the
overvoltage or after the charge current of the charge terminal
is reduced to or below a predetermined current level.