Abstract:
A power system for a vehicle may include at least one controller and a battery having a plurality of cells. The at least one controller may, for each of the cells, determine a voltage of the cell, determine a discharge time to reduce the voltage to a value approximately equal to a minimum of the determined voltages, and cause the cell to discharge for the discharge time to balance the battery.

Description:
BACKGROUND 
     Vehicle battery rebalancing is performed to correct cell imbalance conditions. The voltage of each of the cells is measured and the cell having the minimum voltage identified. All other cells are bled down via resistive circuitry associated with each cell until the other cells have a measured voltage approximately equal to the minimum. Continuous/periodic cell voltage measurements are taken during the bleed down process to monitor change in the cell voltages. Once all of the cell voltage readings are approximately equal, the battery is charged. 
     SUMMARY 
     A vehicle may include an electric machine that generates motive power for the vehicle, a battery having a plurality of cells that store energy for the electric machine, and at least one controller. The at least one controller may, for each of the cells, determine a value of an electrical parameter of the cell, determine a cell discharge time to reduce the determined value of the electrical parameter to a target value, and cause the cell to discharge for the determined duration of time to balance the battery. 
     A method of balancing a vehicle battery having a plurality of cells may include, for each of the cells, determining a value of an electrical parameter of the cell, determining a discharge time to reduce the determined value of the electrical parameter to a target value, and discharging the cell for the discharge time to balance the vehicle battery. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an alternatively powered vehicle. 
         FIG. 2  is a flow chart illustrating an algorithm for determining rebalancing times associated with the battery of  FIG. 1 . 
         FIG. 3  is a schematic diagram of a battery cell and its resistive circuitry. 
         FIG. 4  is a flow chart illustrating an algorithm for bleeding down cell voltages of the battery of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Charge may build up at the surface of cell contacts as energy is removed from cells of a vehicle traction battery, such as during the cell bleed down process described above. This polarization charge may affect attempts to accurately measure cell voltage as energy is removed from the cell: voltage readings may be greater than actual, for example, due to polarization charge. Moreover, other factors such as temperature may introduce error into cell voltage measurements. Inaccurate cell voltage readings may thus confound attempts to rebalance a battery. 
     Certain embodiments disclosed herein may determine, for each of the cells of a traction battery, the discharge time necessary to bleed the cell down from its current voltage to a target voltage. Hence, continuous/periodic cell voltage measurements during the bleed down process need not be taken. Rather, resistive circuitry associated with each of the cells is activated for the times determined. Once the discharge time associated with each of the cells has expired, the resistive circuitry is deactivated. 
     Referring to  FIG. 1 , an embodiment of a plug-in hybrid electric vehicle (PHEV)  10  may include an engine  12 , a plurality of cells  13  forming a traction battery  14 , battery charger  15  and electric machine  16 . The PHEV  10  may also include a transmission  18 , wheels  20 , controller(s)  22 , and electrical port  24 . The engine  12 , electric machine  16  and wheels  20  are mechanically connected with the transmission  18  (as indicated by thick lines) in any suitable/known fashion such that the engine  12  and/or electric machine  16  may drive the wheels  20 , the engine  12  and/or wheels  20  may drive the electric machine  16 , and the electric machine  16  may drive the engine  12 . Other configurations, such as a battery electric vehicle (BEV) configuration, etc., are also possible. 
     The battery  14  may provide energy to or receive energy from the electric machine  16  (as indicated by dashed line). The battery  14  may also receive energy from a utility grid or other electrical source (not shown) via the electrical port  24  and battery charger  15  (as indicated by dashed line). 
     The controller(s)  22  are in communication with and/or control the engine  12 , battery  14 , battery charger  15 , electric machine  16 , and transmission  18  (as indicated by thin lines). 
     Referring to  FIGS. 1 and 2 , the controller(s)  22  determine, in any suitable/known fashion, whether the vehicle  10  is off (e.g., key off) at operation  24 . If no, the controller(s)  22  return to operation  24 . If yes, the controller(s)  22  determine whether the vehicle  10  is on plug (plugged in), in any suitable/known fashion, at operation  25 . If no, the controller(s)  22  return to operation  25 . If yes, the controller(s)  22  determine whether a specified wait time, T 1 , has passed at operation  26 . The wait time, T 1 , may be selected to ensure that any polarization charge accumulated on contacts of the cells  13  has dissipated to the point where its impact on voltage measurement is negligible. In the embodiment of  FIG. 2 , a wait time, T 1 , of 1 min. is used. This wait time, however, may vary depending on the type of cells  13 , etc. and may be determined, for example, via testing. If no, the controller(s)  22  return to operation  26 . If yes, the controller(s)  22  read the voltages of each of the cells  13  at operation  28 . 
     At operation  30 , the controller(s)  22  determine whether a specified number of (per cell) voltage measurements, n, have been collected. As explained below, these voltage measurements will be averaged on a per cell basis to generate an average voltage value for each of the cells  13 . This time averaging is intended to further minimize the error associated with measuring voltage. n may therefore be selected so as to provide a sufficient number of data points to average out at least some of the effects of measurement error. n is equal to twelve in the embodiment of  FIG. 2 . n, however, may vary depending on design, etc. and may be determined, for example, via testing, simulation, etc. 
     If no, the controllers(s)  22  determine whether a wait time, T 2 , has passed at operation  32 . As explained above, time averaging of the voltage measurements seeks to reduce the effects of measurement error. The wait time, T 2 , may therefore be selected so as to temporally space out the voltage measurements to maximize the benefits of time averaging in reducing the effects of measurement error. A wait time, T 2 , of 30 sec. is used in the embodiment of  FIG. 2 . This wait time, however, may vary depending on the type of cells  13 , design configuration, etc. and may be determined, for example, via testing, etc. If no, the controller(s)  22  return to operation  32 . If yes, the controller(s)  22  return to operation  28 . 
     Returning to operation  30 , if yes, the controller(s)  22  determine an average voltage for each of the cells  13  at operation  34 . The controllers(s)  22 , for example, may sum the voltage measurements for each of the cells  13  and divide the sums by n. At operation  36 , the controller(s)  22  determine a cell balancing discharge time for each of the cells  13  based on the voltage measurements determined at operation  30  as discussed below. 
     Referring to  FIGS. 1 ,  2  and  3 , each of the cells  13  (and its associated resistive bleed down circuitry) may be modeled as a standard RC circuit. Hence, the relationship between a current voltage, v current , of one of the cells  13  and a target voltage, v target , may be given by
 
 v   target   =v   current ( e   −t/RC )  (1)
 
where v current  is the determined cell voltage from operation  34 , t is the time constant for the circuit, R is the resistance of the resistive circuitry (e.g., 4 kΩ), and C is the equivalent cell capacitance. C may be written as
 
                   C   =     I   ·       ⅆ   t       ⅆ   v                 (   2   )               
where I·dt is the capacity (A·hrs) in the cell and
 
 dv=V   max   −V   min   (3)
 
where V max  is the cell voltage at full state of charge (e.g., 4.0 V) and V min  is the cell voltage at 0% state of charge (e.g., 3.1 V). Hence, (2) may be rewritten as
 
                   C   =       Ihr     ma   ⁢           ⁢   x           V     ma   ⁢           ⁢   x       -     V     m   ⁢           ⁢   i   ⁢           ⁢   n                   (   4   )               
where Ihr max  is the cell&#39;s maximum capacity and may be found according to the relationship
 
                     Ihr     ma   ⁢           ⁢   x       =       Δ   ⁢           ⁢   Ihr       Δ   ⁢           ⁢   SOC               (   5   )               
where ΔIhr is the change in capacity in the cell and ΔSOC is the change in state of charge of the cell. As an example, the SOC of a given cell may be determined before and after 1 A·hr of capacity is provided to it. Assuming a measured ΔSOC of 10% for this example, the cell&#39;s maximum capacity, Ihr max , would be 10 A·hrs according to (5).
 
     (1) may be rewritten as 
                   t   =       -   RC     ⁢           ⁢   ln   ⁢           ⁢       v   target       v   current                 (   6   )               
Substituting (4) into (6) and assuming that v target  is equal to the minimum of the average cell voltages determined at operation  34 , the time, t, necessary to discharge one of the cells  13  from its current voltage to the minimum of the voltages determined at operation  34  may thus be determined. (6) may be evaluated for each of the cells  13  of the traction batter  14 .
 
     Referring to  FIGS. 1 and 4 , the controller(s)  22  activate, for each of the cells  13 , the resistive circuitry to bleed down the cell voltages to a minimum at operation  38 . The minimum, in this example, is equal to the minimum of the cell voltages determined at operation  34  ( FIG. 2 ). At operation  40 , the controller(s)  22  determine whether, for each of the cells  13 , the cell&#39;s discharge time has expired. If no, the algorithm returns to operation  40 . That is, for any of the cells  13  whose discharge time has yet to expire, the algorithm returns to operation  40 . If yes, the controller(s)  22  deactivate the cell resistive circuitry at operation  42 . That is, for any of the cells  13  whose discharge time has expired, the controller(s)  22  deactivate their resistive circuitry. 
     Once the resistive circuitry for all of the cells  13  has been deactivated, the controller(s)  22  may then operate to charge the battery to some desired level. 
     The algorithms disclosed herein may be deliverable to/implemented by a processing device, such as the battery charger  15  or controller(s)  22 , which may include any existing electronic control unit or dedicated electronic control unit, in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The algorithms may also be implemented in a software executable object. Alternatively, the algorithms may be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, or other hardware components or devices, or a combination of hardware, software and firmware components. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.