FIELD OF INVENTION

[0001] The present invention relates to but not limited to battery of an automobile and more particularly to a method of controlling and operating capacity balancing of battery cells.

BACKGROUND OF INVENTION

[0002] The main reasons for the loss of capacity in lithium-based batteries are loss of recyclable Li+, loss of active material and structural changes in the active material. Loss of active material occurs due to its parasitic reactions with the electrolyte, exposure of the cell to high temperature operation or wear and tear of the electrode surface because of repeated cycling. Prolonged cycling also results in structural deformation of the active materials which effects the cell capacity either by trapping some of the recyclable Li+ inside its interstitials that can no longer be extracted or structural changes which can no longer intercalate Li+ into them. The capacity loss occurring because of all the above-mentioned causes cannot be compensated for during the cell operation. However, such impacts can be minimized by suitable material selection/synthesis and cell operation under optimum conditions

[0003] US2011/0081563A1 Titled "Lithium reservoir system and method for rechargeable lithium-ion batteries" by Christensen et.al, outlines the arrangement of a secondary lithium-ion cell consisting of a lithium reservoir electrode (LRE) which can replenish the lost capacity of the cell as a result of loss of Li+ due to the formation of SEI layer or during other side reactions as well as store the excess capacity generated as a result of loss of active material. The BMS of the cell calculates the SOC of the working electrodes, closes a switch to complete the circuit between the working electrodes and the auxiliary electrode in case cell rebalancing or renewal is required, decides the direction and rate of transfer of Li+ as well as closes the switch in the event of completion of the Li+ distribution process in order to avoid excess Li+ insertion or extraction. Two ways of measuring the SOC of the electrodes has been presented, one in which the LRE is configured as a reference electrode and the other in which it is not configured as the reference electrode. In the prior art, the open circuit voltage (OCV) of the working electrodes are individually measured w.r.t. the LRE that is then used to calculate the SOC of the electrode. In the later arrangement, the SOC is calculated by measuring the full-cell voltage or current and then feeding these values to an appropriate cell model to determine the SOC of each working electrodes. Once the SOC of the electrodes are known, appropriate amount of Li+ can be transferred between the LRE and working electrodes. The rate of transfer of the Li+ is controlled by the variable load resistors, which are in turn controlled by the BMS of the cell.

[0004] US 8,241,793 B2 entitled "Secondary lithium-ion battery contains a pre lithiated anode" by Zhamu et.al, discloses a lithium-metal/lithium-ion battery with anode containing pre-lithiated and pre-pulverised active material. The anode contains very fine particles (average particle size < l\im) of a first non-carbonaceous active material which has been pre-pulverised and pre-lithiated and dispersed in the matrix of a second carbonaceous active material reinforced with a nano-scaled filler such as carbon nano-tube (CNT) or nano-graphene platelet (NGP). As disclosed in the invention, such anodes have several advantages including better rate capacity, rapid charging, higher cycle life, capacity enhancement by 10-20% compared to conventional batteries and better stability in normal operating conditions than pre-lithiated carbonaceous anodes.

[0005] US 2012/0107680 Al entitled "Lithium-ion battery with supplemental lithium" by Amiruddin et.al, discloses various ways to replenish the lost capacity of secondary lithium-ion battery electrodes and methods to generate lithium-replenished electrodes. One way to do so is by using a sacrificial electrode (either lithium foil and/or supplemental lithium source supported by a polymer binder) and loading supplemental lithium from this electrode to the anode by closing an external circuit. Another way is to incorporate supplemental lithium source into the electrodes (anode/cathode) in the form of a lithium-source bonded in a polymer matrix inside the electrode body or to provide a thin li-foil as supplemental lithium source either on the electrode surface or between the electrode and the respective current collector. Third method is to use a pre-lithiated electrode for the cell operation. The invention also discloses ways of preparation of electrodes with sacrificial lithium as well as ways of performing lithiation on electrode surface. A number of experimental evidences have also been provided to demonstrate the superior performance of pre-lithiated anodes.

[0006] The capacity loss occurring because of loss of Li+ alone can be recovered by supplying extra Li+ to the electrodes. The loss of Li-t- occurs primarily due to the formation of SEI layer on the anode surface and other side reactions. Although formation of SEI layer is also a side reaction, where Li+ combines with the electrolyte to form a passive layer on anode surface; its formation is necessary for stable cell operation and prevents the anode from further side reactions. The Li+ bonded in the SEI layer cannot be extracted for further cycling.

SUMMARY OF THE INVENTION

[0007] The present invention discloses a device and a method for controlling the operation and balancing of the capacity-balancing cell. The capacity-balancing cell disclosed herein consists of a capacity-balancing electrode (CBE) apart from the usual anode and cathode. The CBE consists of a lithium intercalation compound, which may or may not have similar chemical composition as that of the primary cathode and is of less than half the thickness of the primary cathode.

[0008] The CBE helps the cell to regain its lost capacity or remove its excess capacity by transferring Li+ between the CBE and the cathode, once the external circuit between them is closed. Capacity balancing improves the cycle life as well as safety of the lithium-based batteries as well as other batteries employing intercalation mechanism. A cell controller and a control strategy are proposed for controlling the usual cell operation as well as cell balancing.

[0009] The use of the capacity balancing battery cell and the related control strategy is mostly aimed at but not limited to automobiles using high capacity batteries including Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs).The capacity balancing cells mostly includes lithium-based chemistries (e.g. Lithium, lithium-ion, lithium-polymer etc.) or any other battery chemistry that works on intercalation mechanism.

[00010] The capacity balancing battery cell consists of an additional capacity-balancing electrode (CBE) apart from the usual cathode and the anode. The CBE provides Li+ to the cathode in case of capacity loss as a result of loss of recyclable Li+, hereafter referred to as negative capacity fade, C_ and extracts Li+ from the cathode in case of excess cell capacity due to the loss of electrode active material, hereafter referred to as positive capacity fade, C+; once the circuit between the cathode and the capacity balancing electrode (CBE) is closed.

[00011] Capacity balancing cells are most particularly suitable for Electric vehicles (EVs) and Hybrid Electric Vehicles (HEVs) which uses higher capacity batteries and have the requirement of long electric ranges and higher cycle lives.US 6,335,115 Bl entitled "Secondary lithium-ion cell with an auxiliary electrode" by Meissner, discloses a lithium-ion secondary cell arrangement consisting of a lithium rich auxiliary electrode that replenishes the lost Li+ as a result of SEI formation on anode surface or parasitic reactions during cycling. The said auxiliary electrode placed between the positive and the negative electrodes is kept isolated during the usual cell operation, as it is not dipped in electrolyte. It is brought in electronic contact (via a special potential connection with the concerned electrode) and electrolytic contact (by either tilting the cell or raising the electrolyte level such that the auxiliary electrode is dipped in electrolyte) with the concerned electrode in which introduction of additional lithium is desired during the charge/discharge cycle.

[00012] The present invention relates to a capacity-balancing cell and a control strategy to control the cell operation of the same. One of the important aspects of the present invention is that the capacity-balancing cell consists of an additional capacity-balancing electrode (CBE) apart from the usual anode and cathode, which can restore the lost Li+ content as well as withdraw the excess Li+ in the cell.

[00013] Another embodiment of the present invention discloses a controlling device, which in the event of fulfilling certain conditions calculates the capacity fade of the cell based on the SOC of the individual electrodes. In one of the important aspects of the present invention, the controlling device based on whether capacity balancing is required and the nature of the capacity fade (negative capacity fade, C_ or positive capacity fade, C+) decides the amount of Li+ that needs to be intercalated into the cathode or extracted from it in order to balance the capacity of the cell.

[00014] Another embodiment of the present invention discloses a control strategy for controlling the closing and opening of various switches during the usual cell operation as well as during cell balancing. The above mentioned and other features, aspects, and advantages of the present invention will be better understood with reference to the following description and appended claims. This summary is only to introduce a selection of concepts in a simplified form and is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. In another embodiment of the invention, this third mounting bracket is integrated with the rider footrest bracket thus reducing the number of parts as well as reducing the noise and vibrations.

BRIEF DESCRIPTION OF DRAWINGS

Figurel illustrates a control system for capacity balancing cell according to the present invention.

Figure2 illustrates a controlling device 200 for controlling the operation and balancing of the capacity-balancing cell.

Figure 3 illustrates a method of operation and interaction between the various modules of the cell controller 200, according to an implementation of the present invention.

Figure 4 illustrates a method 400 for controlling the operation and balancing of capacity balancing cell.

DETAILED DESCRIPTION OF THE INVENTION

[00015] Descriptions of various embodiments of the present invention are made in reference to the accompanying drawings, and are shown by way of illustration in which the invention may be practiced. The following description is only for the sake of understanding and is not to limit the present invention. By adding or utilizing other embodiments, structural or dimensional changes are possible without eliminating the scope of the present invention.

[00016] Figure 1 is the schematic representation of control system for capacity balancing cell illustrating the cross-sectional view of the capacity-balancing cell and its connections to the loads and power sources. The anode 102 consisting of anode active material is coated onto the anode current collector film 101. The cathode 105 comprising of lithium intercalating active material is pressed onto a porous foam based cathode substrate 104. The capacity-balancing cell consists of the additional capacity-balancing electrode (CBE) 107 with its active material coated onto the CBE current collector 108. The CBE 107 is an alternate lithium source in the form of a secondary cathode consisting of a lithium intercalation compound, which may or may not have the same chemical composition as the primary cathode 105. The thickness of the CBE 107 is less than half of the thickness of the primary cathode 105.Two separator films 103 and 106 are placed, one between the anode 102 and the porous cathode substrate 104 and another between the porous cathode substrate 104 and the CBE 107 respectively.

[00017] The anode active material consists of a non-pre-lithiated compound which is either carbonaceous (graphene, carbon black, Meso carbon micro beads-MCMB, etc.) or non-carbonaceous (Si, Sn, Sn02etc.) along with conductive additive and binder. The cathode active material consists of either LiFeP04, LiMn204, Li (NiCoMn)l/302, LiCo02 or Li(Ni0.5Co0.2Mn0.3)O2 along with binders. The porous cathode substrate may be nickel foam, nickel-plated steel foam, copper foam or the like. The separator films 103 and 106 are polymer membranes mostly poly-tetrafluoroethylene or polypropylene membrane. Aluminium or nickel foil is used as CBE current collector 108 whereas copper foil is used as the anode current collector 101.

[00018] In Figure 1, the circuit 109 between the anode 102 and the cathode 105 represents the main circuit whereas the circuit 110 between the cathode 105 and the CBE 107 represents the alternate circuit. Further, the capacity balancing cell system in Figure 1 can be considered to be constructed of two different portions, the main cell portion CI which is connected by the main circuit 109 and the capacity balancing portion C2 which is connected by the alternate circuit 110. In both cell portions, the cathode 105 is the common entity. The main cell portion CI caters to the vehicle load, Lvlllwhereas the capacity-balancing portion C2 serves for balancing the main cell portion CI in order to improve the overall cell life.

[00019] During the usual cell operation; the main switch SI remains closed and the alternate switch S2 remains open such that the main circuit 109 is close and the alternate circuit 110 is open. Whereas during cell balancing; the main switch Sland main circuit 109 remains open and the alternate switch S2 and the alternate circuit 110 remains closed. The switch logic control module 205, which is one of the modules of the cell controller 200, controls the opening and closing of the switches (Figure 2).

[00020] During usual operation, the main cell portion CI of the capacity balancing cell system is discharged via the main circuit 109 in order to cater to the vehicle load, Lvlll. In the event of low SOC, the cell can be charged on-board with the help of an auxiliary DC power source 115 via a DC/DC buck and boost converter 113.For large battery capacities as in the case of Electric Vehicles (EVs) and Plug-in Hybrid Electric Vehicles (PHEVs); on-board charging is rendered impractical. In such cases, an external charger using power from the AC source/mains 116 via an AC/DC converter 114 primarily carries out charging.

[00021] The cell controller 200 initiates capacities balancing by closing the alternate switch S2 and opening the main switch SI. As the alternate switch, S2 is closed; the active load, LA112 connected in series with the alternate circuit 110 becomes operational. The active load, LA112 is controlled by the capacity-balancing module 206 which is another module of the cell controller 200 (Figure 2). The working of the various modules of the cell controller 200 is explained in details in the subsequent embodiments.

[00022] Figure 2 illustrates the system architecture of the cell controller 200 for the capacity balancing cell control system 100. The cell controller 200 can be implemented as a microcontroller, a microcomputer, and/or any device that manipulates signals based on operational instructions. According to an embodiment, the cell controller 200 includes a processor 201 and a device memory 202. The processor 201 can be a single processing unit or a number of units, all of which could include multiple computing units. The processor 201 may be implemented as atleast one microprocessor, digital signal processor, central processing unit, state machine or by using logic circuits. Among other capabilities, the processor 201 is configured to fetch and execute computer-readable instructions and data stored in the device memory 202.

[00023] In an embodiment, the device memory 202 includes a usual cell operation module 203, SOC measurement module 204, switch logic control module 205, capacity balancing module 206 and other modules 207. The cell controller 200 is operatively connected to various components to obtain input signals and to exercise control over the capacity balancing cell control system 100. For example, the cell controller 200 is connected to temperature, voltage, current, SOC sensors (not shown in Figure 1) and switches SI, S2.Further, the cell controller 200can also include input-output (I/O) interfaces (not shown in figure). The I/O interfaces may include a variety of software and hardware interfaces, which may enable the cell controller 200 to communicate with other components of the capacity balancing cell control system 100.

[00024] The usual cell operation module 203 remains operational during the usual vehicle operation, i.e. when main switch SI is close. The usual cell operation module 203 monitors and controls the discharging and charging of the capacity balancing cell during its usual operation. It monitors and controls the various parameters of the main cell portion CI of the capacity balancing cell control system 100 such as cell temperature (TN), voltage (V), charge or discharge current (Ic/Id), cycle number (N), internal resistance (Ri), etc. which are inputs to the other modules of the cell controller 200.

[00025] The SOC measurement module 204 measures the state-of-charge (SOC) of the electrodes using methods such as impedance spectroscopy, voltage measurement, current integration, etc. The SOC of the full cell, SOCcell is equal to the SOC of the anode, SOC_; as SOCcell refers to the amount of charge remaining in the anode for discharge. Also, SOC+ and SOC_ must be equal to 100, i.e.
SOCcell = SOC_
SOC+ + SOC_ = 100
[00026] As previously mentioned, the switch logic control module 205 controls
the closing and opening of the switches SI and S2 in order to trigger either usual cell operation or cell balancing based on the switching logic or certain set of conditions, mentioned elsewhere in the invention.

[00027] The capacity-balancing module 206 is activated only in the event of fulfilling any of those set of conditions. The capacity-balancing module 206 controls the active load, LA112 that is connected in series to the alternate circuit 110. Depending upon the nature of the capacity fade, i.e. positive, C+ or negative, C_; the capacity balancing module 206 manipulates the active load, LA112to control the direction and the amount of Li+ movement in order to replenish or balance the cell.
The other modules 207 may include programs or coded instructions that supplement applications and functions of the cell controller 200.

[00028] Figure 3 illustrates the method of operation and interaction between the various modules of the cell controller 200 according to an implementation of the present invention. The bold lines in the figure refer to data transfer and the dashed lines refer to transfer of control signals between the modules. Unidirectional arrows indicate one way transfer of data/control signal whereas bidirectional arrows indicate transfer of data/control signals to and from both the modules.

[00029] As previously mentioned the usual cell operation module 203 monitors and controls the discharging and charging of the main cell portion CI of the capacity balancing cell control system 100 during its usual operation. The usual cell operation module 203 also gathers and monitors various parameters of the main cell portion CI of the capacity balancing cell control system 100 such as cell temperature (TN), voltage (V), charge or discharge current (Ic/Id), cycle number (N), internal resistance (Ri), etc. obtained from the processor 201 and number of sensors installed in the main cell portion CI of the capacity balancing cell control system 100 (not shown in figure). The usual cell operation module 203 also has the capability to shut down the cell operation in the event of any abnormality such as temperature rise or voltage drop beyond permissible limits.

[00030] The SOC measurement module 204 obtains various parameters from the usual cell operation module 203 and calculates the state-of-charge (SOC) of the individual electrodes using either advanced methods such as impedance spectroscopic techniques, extended Kalman filter based methods, etc. or primitive methods like voltage measurement, current integration techniques, etc. Apart from calculating the SOC of the individual electrodes and the cell, SOCcell; the SOC measurement module 204 can calculate the SOC of the capacity balancing electrode, CBE 107 i.e. SOCCBE by obtaining a set of parameters from the capacity balancing module 206. The SOC calculated is further conveyed to other modules of the cell controller 200 such as the usual cell operation module 203 and the capacity balancing module 206 and is imperative for the usual cell operation as well as capacity balancing.

[00031]The switch logic control module 205 obtains various parameters from the usual cell operation module 203 and controls the closing and opening of the switches SI and S2 based on the conditions mentioned below:
A. Charge cycle, NC =2
B. Cycle no., N = n.x
C. Operating temperature, TN >z °C
D. Shelf life, ts>yhrs.

[00032] In the event of fulfillment of any of the above conditions, the switch logic control module 205 sends control signals to the capacity balancing module 206 and activates it or else usual cell operation continues. It is to be noted that capacity balancing takes place only prior to charging. In accordance to (A), the first cell balancing is required after the first charge cycle and prior to the second charge cycle, i.e. when nc = 2 because almost 15-20% of the recyclable lithium is lost during the first charge cycle as a result of formation of the Secondary Electrode Interface (SEI) layer on the anode surface. In accordance to (B), further balancing is required after every fixed N number of cycles where N is calculated by the formula N=n.x. Here, x=100, 150 or any other integer value depending upon the cell type and application and n is the nth charging cycle. In accordance to (C), the cell controller will evaluate the requirement for cell balancing in case the operating temperature at any cycle, TN exceeds z °C. Typically, z > 65°C but its value can vary depending upon the battery chemistry and nature of application. Finally, in accordance to (D), cell balancing may require if the shelf life, ts of the cell exceeds y hours, where y can have values like 2000, 3000... or any other integer value depending upon the cell type, storage conditions and application demands. Many more conditions can be used to control the switches SI and S2 depending upon user preferences and specific application requirements.

[00033] The capacity balance module 206 is activated only in the event of fulfillment of any of the conditions mentioned above. Based on the input of SOCcell or SOC_ value from the SOC measurement module 204, the capacity balancing module 206 calculates the capacity fade; CO as:
Capacity fade, CO = (SOC+ + SOCJ - 100

[00034] Capacity fade less than 0, i.e. C0< 0; indicates negative capacity fade or C_ and capacity fade greater than 0, i.e. C0> 0; indicates positive capacity fade or C+. Thereafter, the capacity balancing module 206 compares the C+ and C_ values with the reference positive capacity fade, X+ and reference negative capacity fade, X_ values respectively. In case, C+ > X+ and C_ < X_; the capacity balancing module 206 sends control signal to the switch logic control module 205 to close the alternate switch S2 and open the main switch SI in order to initiate the capacity balancing process.

[00035] The comparison of the cell capacity fade values with the reference capacity fade values is essential because cell balancing may not be required if the cell capacity fade values are very small. Many times the capacity fade can be recovered during subsequent cycling. This is also the reason why cell balancing may require only after few hundreds of cycles in accordance to condition (B) and not too frequently. The reference capacity values can be in the range of +10 to +15 for X+ and -10 to -15 for X_ indicating 10 to 15% of positive and negative capacity fade respectively.

[00036] The capacity balance module 206 also obtains and monitors different control parameters of the cell balancing portion C2 of the capacity balancing cell control system 100 such as its temperature (TCB,N), voltage (VCB), charge or discharge current (ICB, c/ ICB,d), internal resistance (RCB,i), etc. obtained from a number of sensors (not shown) installed in the capacity balancing portion C2 of the capacity balancing cell control system 100. These control signals are then transferred to the SOC measurement module 204, which calculates the SOC of the CBE 107, i.e. SOCCBE.

[00037] Upon the initiation of capacity balancing, based on the values of SOCcell and SOCCBE and the nature of capacity fade, the capacity balancing module 206 decides the direction and amount of transfer of the Li+ ion between the cathode 105 and the CBE 107by varying the value of the active load, LA112. In case of positive capacity fade, C+ excess recyclable Li+ resides within the electrodes and Li+ needs to be transferred from the cathode 105 to the CBE 107 in order to remove the excess lithium. Alternately, in case of negative capacity fade, C_ Li+ is lost as a result of formation of SEI layer or other by-products and hence Li+ needs to be transferred from the CBE 107 to the cathode 105 in order to compensate for the lost Li+.

[00038] Capacity balancing is continued until the below mentioned condition is fulfilled, after which the capacity balancing module 206 triggers a signal to the switch control logic module 205 to open the alternate switch S2 and terminate the balancing process.
SOCcbe = 100 + C+ + C_

[00039] Figure 4 illustrates a method 400 for controlling the operation and balancing of capacity balancing cell control system 100 according to an implementation of the present invention. In one example, the method is executed by the cell controller200 to regulate operation of the capacity balancing cell control system 100.

[00040] At block 401; the switch logic control module 205 obtains the information of the temperature (TN), cycle number (N), charge cycle number (NC) and shelf life (ts) from the usual cell operation module 203. Prior to charging, at block 402; the switch logic control module 205 evaluates the four conditions (A), (B), (C) and (D) based on which switching operation is carried out. In case any of the four conditions is true (YES path from block 402), the switch logic control module 205 sends control signals to activate the capacity balancing module 206 at block 403. In case all the conditions are false (NO path from block 402), usual charging operation is continued at block 410.

[00041] At block 404, the capacity-balancing module 206evaluates the capacity fade and the nature of capacity fade based on the SOCcell value obtained from the SOC measurement module 204. The calculated capacity fade values are then compared with the reference capacity fade values at block 405. If C+ > X+ or C_ < X_ (YES path from block 405), at block 406; the capacity balancing module 206 sends control signals to the switch logic control module 205 to close the alternate switch S2 in order to initiate capacity balancing. If the condition at block 405 is not true (NO path from block 405), usual cell charging is continued at block 410.

[00042] The capacity balancing at block 406 is controlled by the capacity balancing module 206 and is continued until the condition at block 407 is satisfied. If the condition at block 407 is not satisfied (NO path from block 407), capacity balancing is continued until the condition at block 407 is satisfied. Once the condition at block 407 is satisfied (YES path from block 407), at block 408; the capacity balancing module206 sends control signals to the switch logic control module 205 to open the alternate switch S2 and terminate cell balancing. At block 409; after terminating cell balancing, the switch logic control module 205 closes the main switch SI in order to resume usual cell operation.

[00043] The order in which the methods are described is not intended to be construed as a limitation and any number of the described method blocks can be combined in any order to implement the method, or an alternative method. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein. Furthermore, the method can be implemented in any suitable hardware, software, firmware, or combination thereof.

We claim:

1. A control system (100) for capacity balancing of a cell comprising:
a cell controller (200);
a cell operation module (203);
a SOC measurement module (204);
switch logic control module (205);
a capacity balancing module (206);
a main cell (CI), wherein the said main cell (CI) comprises of atleast one anode terminal (102) and atleast one cathode terminal (105) which are connected to a vehicle load (Lv) via a first switch (SI);
a capacity balancing cell (C2) wherein the said capacity balancing cell (C2) and the said main cell (CI) have a common cathode (105) and the said capacity balancing cell (C2) has a separate capacity balancing electrode (107) in such a way that the terminals of the capacity balancing cell (C2) are the said common cathode (105) and the said capacity balancing electrode (107) which are connected to an active load (La) via a second switch (S2);
wherein the said controller (200) measures the SOC (State of Charge) of the main cell (CI) and depending upon the SOC of the said main cell (CI), the said controller (200) operates the said first switch (SI) and the said second