Abstract:
A topology is described in which each pair of cells in a string shares a single inductor. Switches permit the single inductor to selectively charge one or the other of the cells. In a variant of the topology, the inductor together with additional switches permit selectively charging multiple cells simultaneously (even one or both cells simultaneously in a pair of cells), drawing upon either an external energy source or upon one or multiple other cells in the string. In this way the number of inductors is minimized while providing isolation among the charging circuits.

Description:
[0001]    This application claims the benefit of U.S. patent application Ser. No. 61/495,988 filed Jun. 11, 2011, which application is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND  
       [0002]    Inevitably as a series string of cells goes through its service life, a variety of events and conditions conspire to ensure that during discharge, one cell discharges fully sooner than its neighbors, and that during charge, one cell charges fully sooner than its neighbors. This prompts investigators to try to devise ways to balance the charge among the cells in the string. Experience shows, however, that it is not easy to balance the state of charge when several electrochemical cells are in series. For example if one wishes to selectively charge particular cells (for example to “top up” a particular cell that needs topping-up), the charging module for any particular cell needs to have isolation relative to any charging modules for other cells. The charging modules for the cells likewise need to have isolation relative to any external energy source being drawn upon for charging purposes. 
         [0003]    But it is not enough merely to find a way to provide isolation mechanisms for the various charging modules. It is also necessary to find a way for each module to be individually controlled as to the current being applied by that module to its respective cell. The control mechanisms might be “local” to the respective cell or might be centralized. If centralized, then the control mechanisms must also be electrically isolated as needed. 
         [0004]    A reader hoping to gain valuable background in the area of cell balancing and charge redistribution will find it helpful to review the following patent documents:
       U.S. Pat. No. 6,518,725 B2 Marten issued Feb. 11, 2003   U.S. Pat. No. 6,511,764 Marten issued Jan. 28, 2003   WO 2008-137764 A1 published Nov. 13, 2008   U.S. Pat. No. 7,936,150 B2 to Milios issued May 3, 2011   WO 2012-042401 published on Apr. 5, 2012   WO 2012-056417 published on May 3, 2012       
 
         [0011]    Investigators have proposed any of a wide variety of approaches for such balancing and charging. A patent of possible interest is US 2010-0295509 A1 to Moussaoui et al. published Nov. 25, 2010. A review of past proposed approaches reveals many drawbacks to various approaches. For example many approaches using inductive coupling require “snubbers”, circuits to fight and to absorb transients that develop when current to an inductor is cut off. Snubbers for high-voltage circuits are particularly tricky to design. In the absence of a snubber, or in the absence of a snubber that is good enough to do the job, such a transient can lead to failure of the controlling device such as a switch. 
         [0012]    Some approaches are costly in terms of the number or physical bulk of switches, inductors, or capacitors employed (per cell) to bring about the balancing or charging. Some approaches are disappointing in terms of the energy losses suffered during the balancing process. Some approaches only achieve charging based upon an external energy input but cannot redistribute charge between cells in a string. Some approaches only serve to discharge particular cells, throwing away energy merely to ensure that no cell performs better than the weakest cell in the strong. 
         [0013]    It would be helpful if a family of approaches could be devised that would permit selective charging and balancing of cells in a string, using a minimum of expensive or bulky components per cell, with maximum efficiency and minimal operational losses. 
       SUMMARY OF THE INVENTION 
       [0014]    A topology is described in which each pair of cells in a string shares a single inductor. Switches permit the single inductor to selectively charge one or the other of the cells. In a variant of the topology, the inductor together with additional switches permit selectively charging multiple cells simultaneously (even one or both cells simultaneously in a pair of cells), drawing upon either an external energy source or upon one or multiple other cells in the string. In this way the number of inductors is minimized while providing isolation among the charging circuits. 
     
    
     
       DESCRIPTION OF THE DRAWING 
         [0015]    The invention is described with respect to a drawing in several figures, of which: 
           [0016]      FIG. 1  shows a prior-art approach for balancing according to the above-mentioned U.S. Pat. No. 7,936,150; 
           [0017]      FIG. 2  shows a first approach for an isolated charger; 
           [0018]      FIG. 3  shows a second approach for an isolated charger; 
           [0019]      FIG. 4  shows a third approach for an isolated charger, drawing from the above-mentioned US published patent application number 2010-0295509; 
           [0020]      FIG. 5  shows a fourth approach for an isolated charger that is also able to discharge cells; 
           [0021]      FIG. 6  shows a first approach according to the invention; 
           [0022]      FIG. 7  shows a second approach according to the invention in actual use in a first regime; 
           [0023]      FIG. 8  shows current and voltage plots over time during charge/discharge operations; 
           [0024]      FIG. 9  shows the second approach according to the invention in actual use in a second regime; 
           [0025]      FIG. 10  shows a drive mechanism for driving individual switches in an isolated fashion; and 
           [0026]      FIG. 11  shows a cell assembly of the second approach in greater detail. 
       
    
    
       [0027]    To the extent possible, like reference numerals are employed for like elements among the figures. 
       DETAILED DESCRIPTION 
       [0028]    The beneficial aspects of the invention will be best appreciated with a brief review of earlier approaches for cell balancing. 
         [0029]      FIG. 1  shows a prior-art approach  59  for balancing according to the above-mentioned U.S. Pat. No. 7,936,150. Each cell B 1 , B 2 , B 3  . . . Bn has its own respective charger (for example charger  41 ). Each charger is DC/DC isolated from the other chargers and from a power source (shown here as the external positive and negative current terminals  42  for the cell string). If the number of cells is n, then the number of circuit elements in the chargers is n times the circuit elements in a particular single charger. For example if there is an inductor in each charger, then the system has n such inductors. 
         [0030]      FIG. 2  shows a first approach for an isolated charger  43  using a “flyback” approach. A low voltage current output  51  is provided to a cell that is to be charged. Transformer  45  provides galvanic isolation. A high voltage current input  47  is provided to the transformer  45  as controlled by switch  50 . A high voltage snubber  46  has to be provided and these are tricky to design and implement. A gate drive and control circuit  48  is controlled by a control line  49  from a battery management system, omitted for clarity in  FIG. 2 . The diode  44  provides operational simplicity but at a cost, namely that efficiency is degraded due to loss of any ability to draw power from any voltage swing that happens to go in the opposite direction in the inductor, and due to losses in the diode itself. 
         [0031]      FIG. 3  shows a second approach for an isolated charger  58  using a “flyback” approach. Here again a low voltage current output  51  is provided to a cell that is to be charged. Transformer  46  provides galvanic isolation. A high voltage current input  47  is provided to the transformer  45  as controlled by switch  50 . A high voltage snubber  46  has to be provided and, again, these are tricky to design and implement. A gate drive and control circuit  48  is controlled by a control line  49  from a battery management system, omitted for clarity in  FIG. 3 . Instead of a simple diode, a switch  53  is provided which is switched on and off in synchronous fashion, providing a synchronous rectifier function. For this to work, the switch  53  has to be driven by a synchronous gate drive and control circuit  54 . Yet another snubber  52  must be provided to protect switch  53  from transients. 
         [0032]      FIG. 4  shows a third approach for an isolated charger  57 , drawing from the above-mentioned US published patent application number 2010-0295509. Each cell  56  has a respective inductor (winding  59 ) and switch  53 . Snubber  52  is also required as mentioned in connection with  FIG. 3 . The various inductors  59  are inductively coupled with inductor  60 . Each switch  53  is controlled by a synchronous rectifier gate driver and control circuit  54 . 
         [0033]      FIG. 5  shows a fourth approach for an isolated charger  61  that is also able to discharge cells. Each cell  56  has a respective inductor (winding  59 ) and switch  53 . Snubber  52  is also required as mentioned in connection with  FIG. 3 . The various inductors  59  are inductively coupled with inductor  60 . Each switch  53  is controlled by control from a centralized control and driver circuit  62 . Individually isolated drives D 1 , D 2 , DN are provided to the switches  53 . The high-voltage drive requires a high-voltage snubber  46  as discussed above. The circuit  62  is rather complex and probably needs to be a high-gate-count field-programmable gate array. 
         [0034]    Advantageously, this approach  61  permits pulling energy from any one of the cells such as  56  and permits pumping that energy (or most of that energy) into any other one of the cells such as  56 . But this approach still has drawbacks like some other approaches just mentioned above, for example that if the number of cells is n, then the number of chargers is n and the component count (such as the number of inductors  59 ) is also n.  FIG. 6  shows a first approach  81  according to the invention. (It should be noted that to avoid having to squeeze too many reference numerals too closely together, typical reference numerals are spread out among the various cell assemblies but the alert reader will have no difficulty understanding that these are typical and indicative of important elements of each of the various cell assemblies.) 
         [0035]    We see a plurality of cell assemblies (of which  84  is typical) each having a positive current terminal (of which  98  is typical) and a negative current terminal (of which  99  is typical), the assemblies  84  connected in series by their current terminals to form a string with string positive terminal  96  and string negative terminal  97 . 
         [0036]    Each cell assembly such as  84  comprising a first storage cell (of which  82  is typical) and a second storage cell (of which  83  is typical) each having a respective positive and negative terminal (for example terminals  101  and  102 ), the positive terminal of the first cell (typical  101 ) defining the positive current terminal (typical  98 ), the negative terminal of the second cell (typical  104 ) defining the negative current terminal (typical  99 ), the negative terminal of the first cell (typical  102 ) connected with the positive terminal of the second cell (typical  103 ), thereby defining a node (typical  100 ). 
         [0037]    Each cell assembly such as  84  further comprises an inductor (typical  88 ) and first and second switches (typical  85  and  86 ), the inductor  88  having first and second leads (typical  106  and  105  respectively), the second lead  105  of the inductor connected to the node  100 , the first lead  106  of the inductor connected by the first switch  85  to the positive terminal  101  of the first cell  82  and connected by the second switch  86  to the negative terminal  104  of the second cell  83 . 
         [0038]    As communicated by hatching  87 , the various inductors  88  are inductively coupled to each other as well as to inductor  89 , discussed in more detail below. 
         [0039]    A controller  92  is provided, which uses control lines  94  to selectively open and close the various first switches  85  and the various second switches  86  of the cell assemblies  84 . 
         [0040]    An energy sharer  107  is also shown in  FIG. 6 . The sharer  107  comprises an inductor  89  inductively coupled (as mentioned above) with the inductors  88  of the cell assemblies  84 , the inductor  89  of the sharer  107  connected by at least one switch to an energy sharing bus  91 . The controller  92  selectively opens and closes the at least one switch of the sharer  107 . In  FIG. 6  the connection of the inductor  89  of the sharer  107  to the energy sharing bus  91  is a full-wave bridge comprising four switches  90 , and the controller  92  selectively opens and closes the four switches  90  of the full-wave bridge of the sharer  107 . 
         [0041]    Turning ahead to  FIG. 11 , what is shown is a cell assembly  110  of a second approach according to the invention in some detail. The cell assembly  110  has two cells  82 ,  83  in series as discussed above in connection with  FIG. 6 , defining node  100 . Inductor  88  has a third lead  107  which has the same inductive sense relative to the second lead  105  as the inductive sense of the second lead  105  relative to the first lead  106 . Each cell assembly  110  further comprises third and fourth switches  108 ,  109 , the third lead  107  connected by the third switch  108  to the positive terminal  101  of the first cell  82  and connected by the fourth switch  109  to the negative terminal  104  of the second cell  83 . The controller ( 92  in  FIG. 6 , omitted for clarity in  FIG. 11 ) selectively opens and closes the third switches  108  and the fourth switches  109  of the cell assemblies  110 . 
         [0042]      FIG. 7  shows the second approach according to the invention in actual use in a first regime. In this figure there are eight cells defining four cell assemblies. There are two switches per cell and one inductor per cell assembly. In this regime, during a first time interval (shown as portion  129 ) a switch  121  is closed, drawing charge from cell  127 . During a second time interval (shown as portion  130 ) a switch  122  is closed, pumping charge into cell  128 .  FIG. 8  shows current and voltage plots over time during such charge/discharge operations. Current during the first time interval is shown by plot  123 , and current during the second time interval is shown by plot  124 . The voltage drawn from cell  127  is shown by plot  125  and the voltage provided to cell  128  is shown by plot  126 . 
         [0043]    Control of the switches is carried out so that the energy pumped “into” the transformer (which we may think of as the volt-seconds area under a voltage curve as a function of time) is countered by a later extraction of energy back out of the transformer. The two areas (area associated with pumping energy into the transformer and area associated with extracting energy back out of the transformer) need to be the same. 
         [0044]      FIG. 9  shows the second approach according to the invention in actual use in a second regime. In this figure, as in  FIG. 7 , there are eight cells defining four cell assemblies, and there are two switches per cell and one inductor per cell assembly. In this regime, during a first time interval (shown as portion  141 ) switches  144 ,  143 , and  145  are closed, drawing charge from cells  146 ,  147 , and  148 . During a second time interval (shown as portion  142 ) switches  149 ,  151 ,  153 , and  155  are closed, pumping charge into cells  150 ,  152 ,  154 ,  156 . 
         [0045]    The thoughtful reader will appreciate from the example regimes of  FIGS. 7 and 9  that the topology shown permits almost any combination of charge-discharge actions. Charge could be transferred from a first cell in a first cell assembly to a second cell in a second cell assembly. Charge could be transferred from two different cells (in the same assembly or in different assemblies) to a third cell. Charge could be transferred from one cell to two other cells (in the same assembly or two different assemblies). 
         [0046]      FIG. 10  shows a drive mechanism for driving individual switches in an isolated fashion. The sharer has a single winding  165 , and the cell assemblies each have a winding  166 . Isolation between the sharer and the cell assemblies, shown at  167 , is for example a  1  kilovolt isolation, selected to be well higher than the maximum voltage of the series cell stack and well higher than the maximum voltage provide to the sharer on the energy share bus  163 . Traditional high-current half-bridge/full-bridge drivers  164  are provided, “bootstrapped” on the high side. 
         [0047]    Each switch  171  is driven by a driver such as driver  162 , which is a transformer-coupled driver. A transformer  170  is driven by low-current bipolar drivers  168 , controlled by a control field-programmable gate array  161 . 
         [0048]    While the invention is described with respect to electrochemical storage cells, the teachings of the invention offer themselves equally to other energy storage devices. 
         [0049]    The thoughtful reader will have no difficulty, after having considered the teachings herein, devising myriad obvious variants and improvements upon the invention, all of which are intended to be encompassed by the claims which follow.