Abstract:
A charging system ( 108 ) supplies a source voltage (Vco, FIG.  5 ) and a source current (Ico, FIG.  5 ) to a plurality of battery cells ( 110 ). The charging system operates according to a method ( 200 ) including the steps of determining ( 202 ) a capacity for each of the plurality of battery cells ( 120  and  130 ), determining ( 204 ) a desired cutoff current (Ico 1 , FIG.  5 ) for a select one of the plurality of battery cells ( 120 ) having the smallest capacity, determining ( 206 ) an optimal source cutoff current according to the capacity of the select one of the plurality of battery cells, adjusting ( 208 ) the source current according to the optimal source cutoff current, and periodically applying ( 209 ) the source current after reaching the optimal source cutoff current.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to battery charging systems, and more particularly to a method and apparatus for improving cycle-life and capacity of a battery pack.  
       BACKGROUND OF THE INVENTION  
       [0002]      FIG. 1  is an illustration of a prior art system for charging conventional battery cells (depicted as CELL  1  and CELL  2 ). In this prior art system, two cells (CELL  1  and CELL  2 ) are charged by way of a source current (Ico) supplied by a conventional charging system (not shown). Prior art systems generally select the source current Ico according to the cutoff current of one of the cells. The reader&#39;s attention is directed to  FIG. 2 , which provides a diagram depicting the relationship of cycle-life (i.e., the number of functional charge and discharge cycles of a conventional battery cell) and the charging capacity of said cell as a function of source voltage and cutoff current. From this illustration, the cutoff current of a cell is preferably 40 mA and 4.2 volts.  
         [0003]     Prior art systems such as shown in  FIG. 1  set the source current Ico to cutoff current shown in  FIG. 2 . From the illustration of  FIG. 1 , CELL  1  and CELL 2  have asymmetric capacities of 500 mAh (milli-Ampere hours) and 1000 mAH, respectively. The cutoff current at each cell can be determined from a product of the source current Ico and the ratio of the capacity of the cell in question to the total capacity of the cells. Accordingly, the cutoff current of CELL  1  (Ic 1 ) is 13.3 mA, while the cutoff current of CELL  2  (Ic 2 ) is 26.7 mA.  
         [0004]     Referring back to  FIG. 2 , four curves are shown ( 10 ,  12 A-B, and  14 ) at a variety of source voltages and cutoff currents. Starting with curve  10 , a source voltage of 4.3V at a cutoff current of 40 mA provides a higher capacity charge (950 mAh), but a shorter cycle-life (500 cycles) than curves  12  and  14 . Curve  12 A provides a charge capacity of 875 mAh and a cycle-life of 750 cycles at a lower source voltage (4.2V), but the same cutoff current (40 mA). Thus, the lower source voltage (4.2V) provides a longer cycle-life, but a lower charge capacity. Curve  14  provides a charge capacity of 790 mAh and a cycle-life of greater than 1000 cycles at a source voltage of 4.1V and cutoff current of 40 mA.  
         [0005]     From these curves  10 - 14  it should be apparent that varying the source voltage results in an inverse relationship between charge capacity and cycle-life. It is also important to note that when the cutoff current is significantly reduced, the cycle-life of the battery cell is significantly impacted. Curve  12 B shows that when the cutoff current is reduced by half (20 mA) the cell&#39;s cycle-life is impacted by 20% (i.e., a cycle-life of 600 cycles-a reduction of 150 cycles from curve  12 A). This latter effect has an undesirable impact on the cycle-life of parallel cells of the prior art system of  FIG. 1 .  
       SUMMARY OF THE INVENTION  
       [0006]     Embodiments in accordance with the invention provide a method and apparatus for improving cycle-life and capacity of a battery pack having at least a smaller capacity cell and a larger capacity cell. Although further adjusting the cutoff current of the smaller or smallest cell in the battery pack can improve the cycle life and capacity of the smaller cell and even the cycle life of the larger cell, such techniques alone will not improve the capacity of the larger cell (and of the battery pack overall). Embodiments herein enable both cells to be fully charged while maintaining each cells&#39; optimum current cutoff point, thereby preserving cycle life performance.  
         [0007]     In a first embodiment of the present invention, a charging system supplies a source voltage and a source current to a plurality of battery cells. The charging system can operate according to a method including the steps of determining a capacity for each of the plurality of battery cells, determining a desired cutoff current for a select one of the plurality of battery cells having the smallest capacity, determining an optimal source cutoff current according to the capacity of the select one of the plurality of battery cells, adjusting the source current according to the optimal source cutoff current, and upon the source current reaching the optimal source cutoff current, periodically applying the source current to the plurality of cells. This periodic application of the source current can be done until the plurality of cells reach an optimal capacity and cycle life. Note, the plurality of battery cells can correspond to a plurality of parallel battery cells.  
         [0008]     In a second embodiment of the present invention, a device can include a plurality of battery cells, and a charging system for supplying a voltage and a source current to the plurality of battery cells. The charging system can be programmed to determine a capacity for each of the plurality of battery cells, determine a desired cutoff current for a select one of the plurality of battery cells having the smallest capacity, determine an optimal source cutoff current according to the capacity of the select one of the plurality of battery cells, adjust the source current according to the optimal source cutoff current, and upon the source current reaching the optimal source cutoff current, periodically applying the source current to the plurality of cells.  
         [0009]     In a third embodiment of the present invention, a SCR (Selective Call Radio) can include a battery pack having a plurality of battery cells for supplying power to the SCR, a charging system for supplying a source voltage and a source current to the plurality of battery cells, a wireless transceiver for exchanging messages with a radio communication system, a memory for storing and processing data, and a processor for controlling the components of the SCR. The SCR can optionally include a display for conveying images to a user of the SCR and an audio system for conveying and receiving audible signals from the user of the SCR. The charging system under control of the processor can be programmed to determine a capacity for each of the plurality of battery cells, determine a desired cutoff current for a select one of the plurality of battery cells having the smallest capacity, determine an optimal source cutoff current according to the capacity of the select one of the plurality of battery cells, adjust the source current according to the optimal source cutoff current, and upon the source current reaching the optimal source cutoff current, periodically applying the source current to the plurality of cells. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is an illustration of a prior art system for charging battery cells;  
         [0011]      FIG. 2  is a diagram depicting the relationship of cycle-life and charging state of battery cells according to source voltage and cutoff current;  
         [0012]      FIG. 3  is a block diagram of a device in accordance with an embodiment of the present invention;  
         [0013]      FIG. 4  is a flow chart depicting a method operating in the device in accordance with an embodiment of the present invention; and  
         [0014]      FIG. 5  is circuit diagram of a battery pack in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     While the specification concludes with claims defining the features of embodiments of the invention that are regarded as novel, it is believed that the embodiments of the invention will be better understood from a consideration of the following description in conjunction with the figures, in which like reference numerals are carried forward.  
         [0016]      FIG. 3  is a block diagram of a device  101  in accordance with an embodiment of the present invention which can reside within a selective call receiver (SCR)  100  as will be further detailed below. The device  101  comprises a plurality of conventional battery cells  110  and a charging system  108 . The charging system  108  includes, for example, a conventional regulation circuit (not shown) with conventional charge pumps if needed. The charging system  108  is coupled to the cells  110  for supplying an adjustable source voltage and source current for charging said cells  110 . The battery cells  110  can be interconnected as shown in  FIG. 5  and can be carried in a conventional battery pack. The battery cells  110  can be coupled in parallel to form the battery pack and each of the cells in the battery pack can have different capacities and be charged in accordance with a method  200  as shown in  FIG. 4 .  
         [0017]      FIG. 4  is a flow chart depicting the method  200  operating, for example, in the device  101  in accordance with an embodiment of the present invention. The method  200  begins with step  202  where the charging system  108  is programmed to determine a capacity for each of the cells  120  and  130  among the plurality of cells  110  (See  FIG. 5 ). In step  204 , a desired cutoff current is determined for a select one of the battery cells  120  having the smallest capacity. In step  206 , an optimal source cutoff current is determined according to the capacity of the select one of the cells  120  (among the plurality of cells  110 ). In step  208 , the source current is adjusted according to the optimal source cutoff current determined in step  206 . Upon reaching the optimal source cutoff current, the method  200  can then apply the source current periodically thereafter at step  209 . The source current can be applied periodically until the plurality of cells  120  and  130  reach an optimal capacity while preserving the optimal cycle life. Note, the periodically applied source current can be a pulsing current that can continue indefinitely or terminate in any number of ways as contemplated herein as long as the plurality of cells reach the optimal capacity while preserving the optimal cycle life. For example, the current pulsing can be terminated based on a fixed time duration or a fixed voltage differential or based on any number of other criteria.  
         [0018]     In other words, when the optimal source cutoff current is reached, instead of terminating the charge, the algorithm can go into a series of short wait and recharge states. This allows the cells  120  and  130  which are at different potentials based on their different charge currents to equalize. When the pause occurs, the smaller cell charges the bigger cell to reach voltage equilibrium. So by pulsing the cell pack periodically, the larger cell ( 130 ) is allowed to be fully charged via the smaller cell ( 120 ). In this fashion, both cells ( 120  and  130 ) are fully charged without exceeding their respective current cutoff thresholds to assure optimal capacity and cycle life performance. Another way of viewing several of the embodiments herein is that the methods and systems disclosed assure optimal cycle life while enabling the “topping off” of cells at the end of their recharge cycles to provide optimum capacity for all cells in a plurality of parallel cells.  
         [0019]      FIG. 5  is circuit diagram that illustrates the operation of the charging system  108  in accordance with method  200  of  FIG. 4 . The plurality of cells  110  are depicted as two parallel battery cells  120  and  130  (CELL  1  and CELL  2 ). Like the prior art system of  FIG. 1 , the capacity of these cells is 500 mAh and 1000 mAh, respectively, each having an ideal cutoff current in this example of 50 mA (or higher). In a supplemental embodiment of the invention, the capacity of each cell  120  and  130  and other relevant characteristics can be supplied to the charging system  108  by the cells  120  and/or  130  in step  202 . That is, one or both cells ( 120  and/or  130 ) can include intelligent circuitry  111  such as a small conventional memory that can be programmed to supply the characteristics of one or both cells ( 120  and/or  130 ). Such characteristics can include one or more cutoff currents with its corresponding expected cycle-life performance for each current, and one or more source voltages and corresponding charge capacity for each voltage. This in turn provides flexibility to select a source voltage (Vco) and a source current (Ico) that optimizes cycle-life and charge capacity for the cells  110 .  
         [0020]     From this step, a designer of the charging system  108  can choose to balance the need for charge capacity and cycle-life of battery cells  110  or possibly implement an algorithm that can provide optimum charge capacity and cycle-life characteristics for all the battery cells  120  and  130  under certain circumstances. In determining this balancing effect, the designer considers the expected use behavior of the device  101 , and determines therefrom a source voltage (Vco) and a cutoff current (Ic 1 ) of the smallest capacity cell  120  (CELL  1 ). In the present example, the designer is assumed to choose the source voltage (Vco) at 4.2V in order to achieve a first predetermined charge capacity. Similarly, the designer is assumed to choose a cutoff current (Ico 1 ) of the smallest cell  120  at 50 mA to achieve a predetermined cycle-life. It will be appreciated by an artisan with skill in the art that the source voltage (Vco) and cutoff current for the smallest cell (Ico 1 ) (or cell having the smallest capacity) can be chosen differently as may be dictated by the use behavior of the device  101  and a desired outcome sought by the designer.  
         [0021]     In step  206 , an optimal source current (Ico) can be determined from the product of the desired cutoff current (Ic 1 =50 mA) and a ratio of a total capacity of the cells  120  and  130  (1500 mAh) and a capacity of the smallest cell  120  (500 mAh). This calculation provides a source current (Ico) of 150 mA. For a simple parallel cell configuration as shown in  FIG. 5 , the cutoff current of the second cell  130  (Ic 2 ) can be determined from the difference of the source current (Ico) and the cutoff current of the smallest cell  120  (Ic 1 ). Thus, providing a cutoff current for the second cell  130  (Ic 2 ) of 100 mA. For a structure having more than two parallel cells, the cutoff current of the second cell  130  (Ic 2 ) can be determined from the product of the source current (Ico) and the ratio of the capacity of said cell  130  (1000 mAh) and the total capacity of the cells  120  and  130  (1500 mAh). A similar calculation can be applied to determine the cutoff currents for third, fourth, and up to n th  parallel cells. Although 100 mA may be twice a desired cutoff current of the second cell  130 , note that the second cell  130  is not fully charged yet. This is why the current pulsing is used to allow the first cell  120  (which is at a higher (unloaded) voltage based on its lower cutoff current) to charge the second cell  130  during the wait periods to reach voltage equilibrium. It should also be noted that where parallel cells do not have asymmetric charge capacities such as shown in  FIG. 5  (i.e., each cell has the same charge capacity), any cell could be selected in step  204  as the smallest cell of method  200 . In other words, symmetric charge capacities among cells enable the selection of any of the cell in a battery as the smallest cell (the cell having the smallest capacity) for the purposes herein.  
         [0022]     In a supplemental embodiment of the present invention, the device  101  can be embodied in a selective call radio (SCR)  100  having conventional technology comprising the device  101 , a wireless transceiver  102  for communicating with a conventional radio communication system, a display  104  for conveying images to a user of the SCR  100 , an audio system  106  for receiving and conveying audible signals to and from the user of the SCR, a memory  112  for storing and processing data, and a processor  114  coupled to the foregoing components  102 - 112  for control thereof. The charging system  108  of the device  101  operates under the control of the processor  114  and is programmed according to the aforementioned method  200  of  FIG. 4 .  
         [0023]     In light of the foregoing description, it should be recognized that embodiments could be realized in hardware, software, or a combination of hardware and software. These embodiments could also be realized in numerous configurations contemplated to be within the scope and spirit of the claims below. It should also be understood that the claims are intended to cover the structures described herein as performing the recited function and not only structural equivalents.