Abstract:
A system for assuring safe use of a battery pack includes a display unit and a state of health (SoH) monitor connected to the battery pack, the SoH monitor. The SoH monitor a) instructs the SoH monitor to measure terminal voltage of the battery pack at a beginning of a current flow cycle; b) instructs the SoH monitor to measure terminal voltage of the battery pack after completion of the current flow cycle; c) instructs the SoH monitor to successively re-measure terminal voltage of the battery pack after completion of the current flow cycle until one of the successively re-measured terminal voltages is equal to a previous one of the re-measured terminal voltage; d) instructs the SoH monitor to determine and record ampere-hours (Ah) passing through the battery pack during the time period between steps a) and c); e) instructs the SoH monitor to determine SoH of the battery pack as a function of a quotient of a difference between terminal voltages determined in steps a) and c) divided by the Ah determined in step d), and f) instructs the SoH monitor to send the SoH determined in step e) to the display unit for display as an updated SoH of the battery pack. A switch is operable to disconnect the battery pack from the DC power system upon the display unit indicating SoH of the battery pack being below a predetermined value

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to assuring safe and efficient operation of a battery. More particularly, the invention relates to a system for continually assuring that a battery has a satisfactory state of health (SoH) while the battery is installed in its operating environment 
         [0002]    A battery pack&#39;s SoH degrades with use and over time because individual cells in the battery pack lose their ability to store and deliver electrical energy with use and over time. But, the battery pack operating voltage range remains the same even with degradation in a battery&#39;s capacity. Thus, while a battery pack may appear ostensibly to be in satisfactory operational condition, the battery pack may, in fact, be on the verge of failure. In some instances such failure may produce problems beyond mere lack of performance of the battery pack. For example, because individual cells in the battery pack are not identical, the individual cells may degrade at a different rate with time and use. This may lead to a condition known as cell unbalance. A battery pack with degraded cells may still deliver a desired output. However, the degraded cell(s) may discharge or charge at a faster rate when compared with the other cells in the pack. Temperature of degraded cell(s) during operation may have more variance when compared with healthy cells and may lead to unsafe conditions such as thermal runaway. 
         [0003]    Some battery packs are employed to deliver electrical power in applications where safety is of paramount importance. For example, numerous battery packs are used in modern “more electric aircraft” (MEA). To assure safe operation of such an aircraft, it would be desirable to provide a flight crew with a continuous report of the SOH and/or cell unbalance of battery packs on the aircraft so that the flight crew might take corrective action in the event of a report of a potential failure of a battery pack. 
         [0004]    As can be seen, there is a need for system that enables safe use of battery packs in environments such as aircraft. 
       SUMMARY OF THE INVENTION 
       [0005]    In one aspect of the present invention, a system for assuring safe use of a battery pack comprises: a switch connecting the battery pack to a DC power system; a display unit; a state of health (SoH) monitor connected to the battery pack, the SoH monitor including; a processor; and a memory block comprising a non-transitory computer-readable medium with instructions stored thereon, that when executed by the processor, performs the steps; a) instructs the SoH monitor to measure terminal voltage of the battery pack at a beginning of a current flow cycle; b) instructs the SoH monitor to measure terminal voltage of the battery pack after completion of the current flow cycle c) instructs the SoH monitor to successively re-measure terminal voltage of the battery pack after completion of the current flow cycle until one of the successively re-measured terminal voltages is equal to a previous one of the re-measured terminal voltage; d) instructs the SoH monitor to determine and record ampere-hours (Ah) passing through the battery pack during the time period between steps a) and c); e) instructs the SoH monitor to determine SoH of the battery pack as a function of a quotient of a difference between terminal voltages determined in steps a) and c) divided by the Ah determined in step d), and f) instructs the SoH monitor to send the SoH determined in step e) to the display unit for display as an updated SoH of the battery pack; wherein the switch is operable to disconnect the battery pack from the DC power system upon the display unit indicating SoH of the battery pack being below a predetermined value. 
         [0006]    In another aspect of the present invention, apparatus for controlling electrical power distribution on an aircraft comprises. a starter motor for an engine of the aircraft; a DC power system connected to the starter motor; a battery pack; a first switch connecting the battery pack to the DC power system; a second switch for connecting the starter motor to an external power source; a state of heath (SoH) monitor connected to the battery pack; a display unit connected to the SoH monitor for displaying SoH of the battery pack; and a disconnect control unit configured to open the first switch and close the second switch in the event that displayed SoH of the battery pack is below a predetermined value. 
         [0007]    In still another aspect of the present invention, a method for assuring safe use of a battery pack in a DC power system comprising the steps: determining terminal voltage change values for n cells of the battery pack during a current flow cycle; determining terminal voltage change value for the battery pack during the current flow cycle; dividing the battery pack voltage changes value by each of n cell voltage change values to develop terminal-voltage-change indices for the n cells; determining if the terminal-voltage-change indices for the n cells are equal; generating a cell-unbalance display in the event of inequality of any of the n terminal-voltage-change indices; disconnecting the battery pack from the DC power system in the event of said inequality. 
         [0008]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is block diagram of a system for assuring safe use of a battery in accordance with an exemplary embodiment of the invention; 
           [0010]      FIG. 1A  is a block diagram of a system for assuring safe use of a battery in accordance with a second exemplary embodiment of the invention; 
           [0011]      FIG. 2  is a block diagram of a state of health (SoH) monitor of the system of  FIG. 1  in accordance with an exemplary embodiment of the invention; 
           [0012]      FIG. 3  is a plot of time versus battery voltage of a battery pack of the system of  FIG. 1  in accordance with an exemplary embodiment of the invention; 
           [0013]      FIG. 4  is an electrical equivalent 2-RC model of the battery pack of the system of  FIG. 1  in accordance with an exemplary embodiment of the invention; and 
           [0014]      FIG. 5  is a flow chart of a method for assuring safe use of a battery pack in accordance with an exemplary embodiment of the invention; and 
           [0015]      FIG. 6  is a flow chart of a method for assuring safe use of a battery pack in accordance with another exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0017]    Various inventive features are described below that can each be used independently of one another or in combination with other features. 
         [0018]    The present invention generally provides a system for assuring safe use of a battery pack. More particularly, the invention provides for monitoring and displaying of SoH while the battery pack is installed and operational to determine if continued use of the battery should be terminated. Still further, the invention provides for monitoring SoH of individual cells and cell unbalance of the battery pack to determine if the battery pack should be disconnected from a DC power system. 
         [0019]    Referring now to  FIG. 1 , an exemplary embodiment of a safety-control system  100  for a battery pack  102  is illustrated. The control system  100  may include a state-of-health monitor  104  (hereinafter SoH monitor  104 ) coupled to the battery pack  102  and to individual cells  108  of the battery pack  102 . A display unit  112  may be coupled to SoH monitor  104 . A disconnect control  114  may be connected to operate a disconnect contactor or switch  116  to disconnect the battery pack  102  from a DC power system  118 . If an unsafe condition within the battery pack  102  or the SoH of the battery pack  102  is below a predetermined value, the battery pack  102  may be disconnected from the DC power system  118 . 
         [0020]    Referring now to  FIG. 1A , an exemplary embodiment of battery-pack safety control system  200  is illustrated as it might be employed in an aircraft (not shown). The system  200  may differ from the system  100  in that a disconnect control  214  may be connected to operate both the switch  116  and a second switch  117 . When the switch  116  is open and the switch  117  is closed, the battery pack  102  may be disconnected from a starter motor  119  of an engine (not shown) of an aircraft. The starter motor  119  may then be powered from an external power source  121  in the event that SoH of the battery pack  102  is lower than a predetermined value. 
         [0021]    The system  200  may be useful in an aircraft that may be employed in short-haul operations. The flight crew may observe, from the display unit  112 , that SoH of the battery pack  102  may be low, but still above a predetermined replacement limit. In that case, the flight crew may elect to use external power source  121  for driving a starter motor  119  of an auxiliary power unit (APU) of the aircraft instead of using the on-board battery pack  102  for such APU starting. In this manner, the flight crew may extend useful life of the battery pack  102  and avoid a need for a non-scheduled battery pack replacement. In other words, replacement of the low-SoH battery pack may be delayed until the aircraft is taken out of service for regularly scheduled maintenance. 
         [0022]    Referring now to  FIG. 2 , a block diagram illustrates an exemplary embodiment of the SoH monitor  104 . The SoH monitor  104  may include a measurement block  120  having a voltage sensor  122 , a current sensor  124  and a temperature sensor  126 . The measurement block  120  may be adapted to take on-line measurements from the battery pack  102 . The SoH monitor  104  may also include a computer  128  with a non-volatile memory  132  and a processor  136 . 
         [0023]    Referring now to  FIGS. 1 and 2 , it may be seen that individual cells  108  of the battery pack  102  may be connected to the SoH monitor  104 . More particularly, individual cells  108  may be connected to a cell measurement block  106  of the SoH monitor  104 . The cell measurement block  106  may include a plurality, n, of voltage sensors  140  coupled individually to each of n cells  108 . 
         [0024]    In operation, the SoH monitor  104  may perform an SoH determination each time the battery pack  102  experiences completion of a cycle of current flow resulting from either charging or discharging. Such a cycle may include an initiation of current flow followed by a termination of the current flow. In some applications, the battery pack  102  may experience multiple cycles of current flow with relatively short time periods between cycles. For example, the battery pack  102  may provide power to a load for a period of five minutes and then after a lapse on only a few seconds the battery pack  102  may again provide power to a load for a five minute period. Such rapidly changing and repeating load powering may be considered to be a dynamic mode of operation. 
         [0025]    Alternatively, the battery pack  102  may be installed in a vehicle such as an aircraft which may be parked at a terminal for a period of an hour or more, during which time the battery pack  102  may not be subjected to any cycles of current flow. In such a context, the battery pack  102  may be subjected to current flow cycles at intervals that may spaced apart in time by periods of hours. In other words, the current flow cycles may be separated in time by rest periods. Such a mode of operation may be referred to herein as a resting mode of operation. 
         [0026]    The SoH monitor  104  may utilize a first SoH estimation technique for determining SoH for a cycle of current flow during a resting mode of operation of the battery pack  102 . The SoH monitor  104  may utilize a second SoH estimation technique when a cycle of current flow occurs in the context of a dynamic mode of operation of the battery pack  102 . 
         [0027]    Referring now to  FIG. 3 , a graph  300  illustrates a time versus battery voltage relationship for the battery pack  102  as it undergoes a cycle of current flow in a rested mode of operation. As the battery pack  102  discharges, changes in terminal voltage may be observed due to the following phenomena:
       1. V d(res) —Instantaneous drop in voltage due to series ohmic resistance at the start of discharge;   2. V d(pol+dis1) —Drop in voltage due to combined effect of polarization and discharge phenomena;   3. V d(dis2) —Drop in voltage due to discharge phenomenon only;   4. V r(res) —Instantaneous rise in voltage due to series ohmic resistance at the end of discharge; and   5. V r(depol) —Rise in voltage due to depolarization phenomenon during rest period.       
 
         [0033]    The term “V d(pol+dis1) ” may be resolved into two components “V d(pol) ” and “V d(dis1) ”. The components “V d(pol) ” may represent voltage drop due to polarization effect and “V d(dis1) ” may represent voltage drop due to charge delivering phenomenon during polarization period. Among the five phenomena listed above, only the “V d(dis1) ” and “V d(dis2) ” components represent terminal voltage lost due to charge delivering phenomenon. The other components are not associated with charge delivering phenomenon but still, they may affect a total change in terminal voltage. 
         [0034]    Terminal voltage drop or gain per Ampere-Hour (Ah) delivered or received changes with battery health. Therefore, SoH function may be determined in accordance with the expression, 
         [0000]    
       
         
           
             
               
                 
                   SoH 
                   = 
                   
                     f 
                      
                     
                       ( 
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             V 
                             bat 
                           
                         
                         Ah 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0035]    In this context, SoH is most accurately determined if the term “ΔV bat ” represents the change in terminal voltage due only to charge delivering phenomenon during discharge or charging operation. But, the total drop in terminal voltage during discharge operation is given by: 
         [0000]      Δ V   bat(total)   =V   d(res)   +V   d(pol) +( V   d(dis1)   +V   d(dis2) )=Total Voltage Drop   (2)
       where V d(res) =Instantaneous drop in voltage due to series ohmic resistance;   V d(pol) =Drop in voltage due to polarization effect; and   V d(dis1) +V d(dis2) =Drop in voltage due to charge delivering phenomenon       
 
         [0039]    As a practical matter, it may not be possible to separate and extract the voltage drop only due to charge delivering phenomenon (i.e. “V d(dis1) +V d(dis2) ”) in the above equation. Instead, the first two terms may be compensated for or cancelled out using rising voltage components that occur after discharge or charging operation. 
         [0040]    There may be an instantaneous voltage rise  “V   r(res) ” in the battery terminal voltage at the end of discharge. This may be equal in magnitude and opposite in polarity to the instantaneous drop observed at the start of discharge operation. So, “V r(res) ” component can compensate  “V   d(res) ”. 
         [0000]        V   r(res)   =−V   d(res)    (3)
 
         [0041]    Also, depolarization that occurs after the current flow cycle may compensate the polarization effect that occurs at a beginning of the current flow cycle. The battery pack  102  may get depolarized during a rest period to the same extent it is polarized during the current flow cycle, provided that the battery pack  102  is sufficiently rested after the current flow cycle. The voltage drop “V d(pol) ” due to polarization is equal to the voltage rise “V r(depol) ” during depolarization. 
         [0000]        V   r(depol)   =−V   d(pol)    (4)
 
         [0042]    Net change in terminal voltage during discharge operation is given by, 
         [0000]    
       
         
           
             
               
                 
                   
                       
                   
                    
                   
                     
                       Δ 
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                         V 
                         bat 
                       
                     
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                         V 
                         
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                             ( 
                             res 
                             ) 
                           
                         
                       
                       + 
                       
                         V 
                         
                           d 
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                             ( 
                             pol 
                             ) 
                           
                         
                       
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                         ( 
                         
                           
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                               d 
                                
                               
                                 ( 
                                 
                                   dis 
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                                 ( 
                                 
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                       + 
                       
                         V 
                         
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                             ( 
                             res 
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                       + 
                       
                         V 
                         
                           r 
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                             ( 
                             depol 
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   Thus 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   SoH 
                   = 
                   
                     
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                         ( 
                         
                           
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                              
                             
                                 
                             
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                               V 
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                         ( 
                         
                           
                             
                               V 
                               
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                                   ( 
                                   res 
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                             + 
                             
                               V 
                               
                                 d 
                                  
                                 
                                   ( 
                                   pol 
                                   ) 
                                 
                               
                             
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                                   ( 
                                   
                                     dis 
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                                   ( 
                                   
                                     dis 
                                      
                                     
                                         
                                     
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                                   ) 
                                 
                               
                             
                             + 
                             
                               V 
                               
                                 r 
                                  
                                 
                                   ( 
                                   res 
                                   ) 
                                 
                               
                             
                             + 
                             
                               V 
                               
                                 r 
                                  
                                 
                                   ( 
                                   depol 
                                   ) 
                                 
                               
                             
                           
                           Ah 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0043]    Using eq. (3) and (4), rewriting eq. (6), 
         [0000]    
       
         
           
             
               
                 
                   SoH 
                   = 
                   
                     
                       f 
                        
                       
                         ( 
                         
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               V 
                               bat 
                             
                           
                           Ah 
                         
                         ) 
                       
                     
                     = 
                     
                       f 
                        
                       
                         ( 
                         
                           
                             
                               V 
                               
                                 d 
                                  
                                 
                                   ( 
                                   
                                     dis 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                   ) 
                                 
                               
                             
                             + 
                             
                               V 
                               
                                 d 
                                  
                                 
                                   ( 
                                   
                                     dis 
                                      
                                     
                                         
                                     
                                      
                                     2 
                                   
                                   ) 
                                 
                               
                             
                           
                           Ah 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0044]    It may be noted that accuracy of the rested-mode estimation technique described above may be dependent upon the occurrence of a sufficiently long rest period between successive current flow cycles. As explained hereinbelow and illustrated in  FIG. 5 , such sufficiency of length of a rest period may be determined to exist when successive measurements of V r(depol)  (See  FIG. 3 ) at an end of a current flow cycle are equal to one another. 
         [0045]    If the battery pack  102  is operated with only insufficient rest periods between successive current flow cycles, the second or dynamic-mode estimation technique may be performed. This dynamic-mode estimation may be performed in a manner similar to that described in US Patent Application Publication 2013/0138369, which publication is incorporated herein by reference in its entirety. In the dynamic-mode estimation technique, the computer  128  may employ a battery dynamic model  130  stored in the non-volatile memory  132 . The battery dynamic model  130  may include various values of parameters R s , C st , R st , C lt  and R lt  determined at various temperatures, terminal voltages and currents while the battery pack  102  is off line. The parameters may be elements of a battery model  133  illustrated in  FIG. 4 . 
         [0046]    The computer  128  may include a processor  134  adapted to receive the on-line measurements from the measurement block  120  and to calculate an open circuit voltage online, using an equation 
         [0000]        V   OC   =V   BAT   +ΔV   R   +ΔV   P    (8)
 
         [0000]    where 
         [0047]    V BAT =Battery terminal voltage; 
         [0048]    ΔV R =Voltage drop due to battery resistance; 
         [0049]    ΔV P —Voltage drop due to polarization phenomenon; and 
         [0050]    wherein ΔV P  is computed using an equation 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       V 
                       P 
                     
                   
                   = 
                   
                     I 
                     * 
                     
                       [ 
                       
                         
                           ( 
                           
                             
                               R 
                               st 
                             
                             
                               1 
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       R 
                                       st 
                                     
                                     · 
                                     
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                                  
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                           ( 
                           
                             
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                                   ( 
                                   
                                     
                                       R 
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         [0000]    in which values of R s , C st , R st , C lt  and R lt  are determined from the battery dynamic model  130  by comparing the on-line measurements to the battery dynamic model  130 . 
         [0051]    Accuracy of a resultant calculation of Voc may be enhanced by recursive application of mathematical filtering within the processor  134 . In an exemplary embodiment of the invention the following adaptive filter equations may be employed: 
       State Estimate Equation (Time Update: 
       [0052]    
       
         
           
             
               
                 
                   
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                                   ) 
                                 
                               
                             
                             ] 
                           
                            
                           
                               
                           
                            
                           … 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     
                       V 
                       
                         OC 
                          
                         
                             
                         
                          
                         
                           ( 
                           k 
                           ) 
                         
                       
                       - 
                     
                     = 
                     
                       
                         V 
                         OC 
                       
                        
                       
                         ( 
                         
                           SoC 
                           
                             ( 
                             k 
                             ) 
                           
                           - 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
       Output Equation 
       [0053]        V   Bat(k)   =V   OC ( SoC   (k) )− V   ST(k)   −V   LT(K)   −└I   Bat(k)   *R   ser ┘  (12)
 
       State Correction Equation (Measurement Update): 
       [0054]    
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             SoC 
                             
                               ( 
                               k 
                               ) 
                             
                             + 
                           
                         
                       
                       
                         
                           
                             V 
                             
                               ST 
                                
                               
                                   
                               
                                
                               
                                 ( 
                                 k 
                                 ) 
                               
                             
                             + 
                           
                         
                       
                       
                         
                           
                             V 
                             
                               LT 
                                
                               
                                   
                               
                                
                               
                                 ( 
                                 k 
                                 ) 
                               
                             
                             + 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               SoC 
                               
                                 ( 
                                 k 
                                 ) 
                               
                               - 
                             
                           
                         
                         
                           
                             
                               V 
                               
                                 ST 
                                  
                                 
                                     
                                 
                                  
                                 
                                   ( 
                                   k 
                                   ) 
                                 
                               
                               - 
                             
                           
                         
                         
                           
                             
                               V 
                               
                                 LT 
                                  
                                 
                                     
                                 
                                  
                                 
                                   ( 
                                   k 
                                   ) 
                                 
                               
                               - 
                             
                           
                         
                       
                       ] 
                     
                     + 
                     
                       
                         L 
                         k 
                       
                        
                       
                         [ 
                         
                           
                             V 
                             
                               Bat 
                                
                               
                                   
                               
                                
                               
                                 ( 
                                 mes 
                                 ) 
                               
                             
                           
                           - 
                           
                             V 
                             
                               Bat 
                                
                               
                                   
                               
                                
                               
                                 ( 
                                 k 
                                 ) 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
             
               
                 
                   
                     V 
                     
                       OC 
                        
                       
                           
                       
                        
                       
                         ( 
                         k 
                         ) 
                       
                     
                     + 
                   
                   = 
                   
                     
                       V 
                       OC 
                     
                      
                     
                       ( 
                       
                         SoC 
                         
                           ( 
                           k 
                           ) 
                         
                         + 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
       Where: 
       [0000]    
       
         R ser  is ohmic resistance of the battery pack; 
         Q is the full capacity of the battery; 
         L k  is the filter gain matrix; and 
         V Bat(mes)  is the measured battery voltage. 
       
     
         [0059]    SoH of the battery pack  102  may then be determined in the manner described in US Patent Application Publication 2013/0138369 by employing the value of Voc determined in accordance with the dynamic-mode estimation technique described above. 
         [0060]    Referring now to  FIG. 5 , a flow chart illustrates an exemplary embodiment of a method  500  for assuring safe use of a battery pack. The method  500  may employ either or both of the rested-mode SoH estimation technique and the dynamic-mode SoH estimation technique. In a step  502 , terminal voltage of the battery pack may be measured at a beginning of a current flow cycle (e.g. the terminal voltage may be measured by the voltage sensor  122  of the SoH monitor  104 ). In a step  504 , an end-of-cycle terminal voltage of the battery pack may be measured at an end of the current flow cycle (e.g. V r(depol)  of  FIG. 3  may be measured by the voltage sensor  122 ). In a step  508 , the end-of-cycle terminal voltage may be re-measured (e.g. V r(depol)  of  FIG. 3  may be re-measured by the voltage sensor  122 ). In a step  510 . the end-of cycle terminal voltage of step  506  may be compared to the end-of-cycle terminal voltage of step  506 . If the measured voltage of step  508  is unequal to the measured voltage of step  506 , the steps  506  and  508  may be successively repeated until the voltages of steps  504  and  506  are found to be equal. Equality of voltage measurements of steps  506  and  508  may be indicative of passage of sufficient time to assure that the rested-mode of estimation may be properly employed. 
         [0061]    In a step  510 , a determination may be made as to whether a new current flow cycle has begun prior to achieving re-measured terminal voltage equality in step  508 . In that event SoH of the battery pack may be estimated in step  512  in accordance with the dynamic-mode of SoH estimation. If, in step  508 , end-of-cycle terminal voltage of steps  504  and  506  are found to be equal, then SoH of the battery pack may be estimated in step  514  in accordance with the rested-mode of SoH estimation. 
         [0062]    If step  514  is performed, then in a step  516 , rested-mode SoH estimations may be displayed at the nominal value determined in step  514 . If step  516  is performed, then in a step  518 , a dynamic mode estimation of SoH may be displayed with an adjusted value of the actual SoH determined in step  512 . Rested-mode estimations may have a higher likelihood of being accurate as compared to dynamic-mode SoH estimations. Thus, the dynamic-mode SoH estimation may be adjusted to account for a potential margin of error. For example, a adjusted dynamic-mode SoH value may be about 0.01% to about 0.015% lower than a nominal SoH value determined by the rested-mode estimation technique. In a step  520 , the battery pack may be disconnected from a DC power system if SoH of the battery pack is below a predetermined value. 
         [0063]    Referring now to  FIG. 6 , a flow chart illustrates another exemplary method  600  for assuring safe use of a battery. The method  600  may differ from the method  500  in that cell unbalance and SoH values of individual cells of the battery are monitored and displayed. In a step  602 , terminal voltage change for each of n cells may be determined during a current flow cycle (e.g., terminal voltage change may be measured with the n voltage sensors  140  of the cell measurement block  106 ). A step  604  may be performed simultaneously with step  602 . In the step  604 , terminal voltage change for the battery pack may be determined during the current flow cycle (e.g., terminal voltage change may be measured with the voltage sensor  122  of the measurement block  120  during the same current flow cycle for which terminal voltage change is measured in step  602 ). 
         [0064]    In a step  606 , the voltage changes measured in steps  602  and  604  may be combined to develop a terminal-voltage-change index for each of the n cells in accordance with the expression: 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       V 
                       
                         index 
                          
                         
                           ( 
                           cn 
                           ) 
                         
                       
                     
                   
                   = 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         V 
                         pack 
                       
                     
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         V 
                         cn 
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
         [0065]    In a step  608 , a determination may be made as to whether the terminal-voltage-change indices for all the n cells are equal. In a step  610 , a cell-unbalance display may be generated in the event of inequality of any of the n terminal-voltage-change indices. In the step  610 , the identity of one or more degraded cells may also be displayed. SoH for each of the n cells may be determined in accordance with the expression; 
         [0000]    
       
         
           
             
               
                 
                   
                     SoH 
                     cn 
                   
                   = 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       V 
                       
                         index 
                          
                         
                           ( 
                           cn 
                           ) 
                         
                       
                     
                     × 
                     
                       
                         SoH 
                         pack 
                       
                       n 
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
         [0066]    In a step  614 , SoH for each of the n cells may be displayed (e.g., the display unit  112  of  FIG. 1  may be employed to display the SoH information from step  614 ). In step  616 , a disconnect alarm may be generated if a high unbalance is present (e.g., the display unit may provide a high unbalance display if cell unbalance exceeds about 0.1 volts to about 0.15 volts). In that event, a step  618  may be performed in which the battery pack may be disconnected from a DC power system. 
         [0067]    Referring back to  FIG. 1 , it may be seen that with such a collection of battery-related information on the display unit  112 , an observer of the display unit  112  may have an opportunity to make a real-time decision as to whether or not to allow the battery to continue performing its role. Consider, for example, the battery pack  102  being installed in an aircraft. The flight crew might elect to operate the disconnect control  114  if the display unit  112  showed that a cell unbalance condition exceeding about 0.1 to about 0.15 volts had developed. The battery pack  102  would then be disconnected from the DC power system  118  and a risk of overheating of the battery pack  102  might be precluded. 
         [0068]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.