Claims:1) The Charger is handy, easily portable, light weight and cost effective.

2) Highly stabilized regulator circuit with particular attention to limit the maximum current. This will ensure overcharging of the battery and thus increases its longevity.

3) The product is very much useful in the locality where electricity is not available. In remote and underdeveloped village areas users of mobile phones need not depend upon the electric power to charge the mobile battery. Solar energy is direct alternative to them. , Description:3. Design methodology:

The main design work is based on design of an electronic circuit for the purpose of charging cell phone batteries using suitable technique with control of both fast charging and trickle charging current. The proposed system uses a 12V, 5W solar panel as the main power source. This system will initially charge a Li-ion 3.7V, 1400mAH cell phone battery for about 98% of its rated voltage within 3 hours by using fast charging method with restricted current magnitude and then switches to trickle charging circuit for slow charging to avoid overcharging so that battery life is safe.

4. Block Diagram:

The block diagram for the proposed design is given in figure 1. It consists of a solar panel module, a voltage regulator stage, a control circuit to control the charging currentflowing through the cell phone battery.

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Figure 1
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4.1 The solar panel:

A solar panel comprises of a set of solar photovoltaic cells electrically connected and mounted on a supporting structure. The solar cells are connected in series and parallel combination to give a required capacity of solar panel. Each panel is rated by its DC power output with maximum voltage and current. A solar panel produces current in proportion to the amount of sunlight falling on it, while the panel’s open circuit voltage remains relatively constant. For the purpose of selecting the solar panel to charge mobile battery, the specification of the solar panel to be used is given below:
Peak power output = 5W
Maximum voltage, Vmax: 12V
Maximum current, Imax: 417 mA
Open Circuit voltage: 15V (25% of Vmax)
Short circuit current: 110% of Imax i.e., 450 mA (We have considered 466 mA)
Size: (With 600w/m2): 10cm x 8.5cm.

4.2 Series voltage regulator:

For a 3.7V mobile battery, it is assumed that the required charging voltage i.e., set point voltage (normally recommended by the manufacturer) must be 15% more than this voltage, i.e., charging voltage is 4.2V. The series voltage regulator circuit is so designed that it will supply the required voltage to the cell.

4.3 Control circuit:

The Circuit diagram of the charger is shown in figure 2.

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For a 3.7V battery, the required charging voltage i.e., 4.2V will be available at the emitter of transistor Q3(point C). The series regulator supplies a constant voltage (4.7V) at the emitter of transistor Q1 (point B) so that the regulated voltage at point C will be 4.2V.
The base of Q4 is provided a bias of 4.3V through resistors R5 and R6.
When initially the mobile battery voltage is low, voltage at point C is low which makes base-emitter junction of transistor Q3 forward biased and thus Q3 is ON. The charging current flows through Q3, mobile battery and maximum current detector R4 till the voltage across R4 i.e., base of Q4 is less than or equal to 0.7V, in addition to the trickle charging current ITC. In this duration of fast charging the maximum current which can flow through the battery is given by Imax= 0.7V/1.5 Ohm = 466 mA and during this period of fast charging current is restricted to 466mA while transistor Q4 will remain OFF. When the charging current tends to cross Imax i.e., 466mA, Q4 will be in the active region which in turn decreases current through Q2 and also through Q1 and thus R4 limits the charging current Imax.
When battery voltage almost reaches 98% of its maximum value i.e., 3.6V, base-emitter junction of transistor Q3 becomes reverse biased i.e., Q3 will be OFF and only slow charging current i.e., trickle charging current, ITC (= 140 mA approx.) will be flowing through the battery thus prohibiting chance of overcharging of the cell battery.

4.4 Mobile Battery:

It is assumed that the mobile battery is having specification of 3.7V, 1400 mAH.