Abstract:
A battery control module for use with a battery includes a voltage measuring module that measures battery voltage and a current measuring module that measures battery current. A power limit module communicates with the current and voltage measuring modules and once every time period estimates a battery current limit that corresponds with a future time period. The battery current limit is based on a predetermined voltage limit of the battery and a battery current and a battery voltage that correspond with a time period that precedes the future time period.

Description:
FIELD OF THE INVENTION 
     The present invention relates to battery systems, and more particularly to determining power delivery limits for battery systems. 
     BACKGROUND OF THE INVENTION 
     Battery systems may be used to provide power in a wide variety of applications. Exemplary transportation applications include hybrid electric vehicles (HEV), electric vehicles (EV), heavy duty vehicles (HDV) and vehicles with 42-volt electrical systems. Exemplary stationary applications include backup power for telecommunications systems, uninterruptible power supplies (UPS), and distributed power generation applications. 
     Examples of the types of batteries that are used include nickel metal hydride (NiMH) batteries, lead-acid batteries, and other types of batteries. A battery system may include a plurality of battery subpacks that are connected in series and/or in parallel. The battery subpacks may include a plurality of batteries that are connected in parallel and/or in series. 
     Inherent properties of a battery can dictate a minimum operating voltage specification (V min ) and/or a maximum operating voltage specification (V max ). In some applications V min  and V max  can be specified by engineers and based on other criteria. When taken together V min  and V max  indicate a voltage range that the battery voltage should be kept within to maximize the service life of the battery. V min  and V max  also infer limits on the battery&#39;s abilities to provide power during discharge and accept power during recharge. In some applications, such as HEVs, these limits on battery power can periodically limit the performance of the vehicle. For example, an HEV control system can limit the acceleration of the vehicle to keep the battery voltage above the minimum battery voltage V min . The HEV control system can also limit a regenerative braking function of the vehicle to keep the battery voltage below the maximum battery voltage V max  and thereby maintain some regenerative braking capacity. 
     One method of preventing the acceleration performance limit from occurring is to use an internal combustion engine to supplement the power delivered from the battery. Such a method introduces other issues, however. For example, the engine may be turned off when it is needed. In such a situation the vehicle performance will still be limited for a brief period while the HEV control system starts the engine. 
     A second method of preventing the performance limits is to leave the engine idling so that it is always available to supplement the battery power. However an idling engine wastes fuel and therefore does not provide an ideal solution. As such, there remains a need for predicting battery power limits and synchronizing supplemental power sources with load demands. 
     SUMMARY OF THE INVENTION 
     A battery control module for use with a battery includes a voltage measuring module that measures battery voltage and a current measuring module that measures battery current. A power limit module communicates with the current and voltage measuring modules and once every time period estimates a battery current limit that corresponds with a future time period. The battery current limit is based on a predetermined voltage limit of the battery and a battery current and a battery voltage that correspond with a time period that precedes the future time period. 
     A hybrid power supply system for a load includes a first power source that provides power to the load based on a power demand signal, a rechargeable battery that provides power to the load, and a control module in communication with the rechargeable battery. The control module includes a voltage measuring module that measures battery voltage, a current measuring module that measures battery current, and a power limit module that communicates with the current and voltage measuring modules. The power limit module estimates a battery current limit based on the battery current, the battery voltage, and a predetermined voltage limit of the battery. The power limit module generates the power demand signal based on the battery current limit. 
     A method for controlling current draw from a battery includes measuring a battery voltage, measuring a battery current, and periodically estimating a battery current limit that corresponds with a future time period and represents a maximum allowable battery current through the battery. The battery current limit is based on the measured battery current, the measured battery voltage, and a predetermined voltage limit of the battery. 
     A method for controlling a hybrid power supply system includes operating a first power source to provide power to a load based on a power demand signal, supplying power to the load from a rechargeable battery, measuring a battery voltage of the rechargeable battery, measuring a battery current of the rechargeable battery, periodically estimating a battery current limit that corresponds with a future time period and represents a maximum allowable battery current through the battery, and generating the power demand signal based on the estimated battery current limit. The battery current limit is based on the measured battery current, the measured battery voltage, and a predetermined voltage limit of the battery 
     A powertrain system for a hybrid electric vehicle includes a rechargeable battery that provides power to an electric motor for propelling the vehicle, an internal combustion engine that generates power for propelling the vehicle, an engine controller that starts the internal combustion engine in response to a power demand signal, and a control module in communication with the rechargeable battery. The control module includes a voltage measuring module that measures battery voltage, a current measuring module that measures battery current, and a power limit module that communicates with the current and voltage measuring modules. The power limit module periodically determines a battery current limit corresponding to a future period and based on the battery current and the battery voltage. The power limit module generates the power demand signal based on the battery current limit such that the engine controller starts the internal combustion engine prior to the future period. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of a battery system including battery subpacks, battery control modules and a master control module; 
         FIG. 2  is a functional block diagram of a battery control module; 
         FIG. 3  is an equivalent circuit of a battery; 
         FIG. 4  is a flowchart of a method for predicting a power limit of a battery; 
         FIG. 5  is a graph of actual and predicted battery parameters; 
         FIG. 6A-6C  are functional block diagrams of hybrid electric vehicles; and 
         FIG. 7  is a functional block diagram of a battery-powered supplemental power supply. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify the same elements. As used herein, the term module or device refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     An exemplary system that can be used to predict the maximum power output of a battery will be shown, although skilled artisans will appreciate that other systems may be used. Referring now to  FIG. 1 , an exemplary embodiment of a battery system  10  is shown to include M battery subpacks  12 - 1 ,  12 - 2 , . . . , and  12 -M (collectively battery subpacks  12 ). The battery subpacks  12 - 1 ,  12 - 2 , . . . , and  12 -M include N series connected batteries  20 - 11 ,  20 - 12 , . . . , and  20 -NM (collectively batteries  20 ). Battery control modules  30 - 1 ,  30 - 2 , . . . and  30 -M (collectively battery control modules  30 ) are associated with each of the battery subpacks  12 - 1 ,  12 - 2 , . . . and  12 -M, respectively. In some embodiments, M is equal to 2 or 3, although additional or fewer subpacks may be used. In some embodiments, N is equal to 12-24, although additional and/or fewer batteries may be used. 
     The battery control modules  30  sense voltage across and current provided by the battery subpacks  12 . Alternatively, the battery control modules  30  may monitor one or more individual batteries  20  in the battery subpacks  12  and appropriate scaling and/or adjustment is performed. The battery control modules  30  communicate with a master control module  40  using wireless and/or wired connections. The master control module  40  receives the power limits from the battery control modules  30  and generates a collective power limit. The SOC can be calculated for each module, in groups and/or collectively. The battery control module  30  may be integrated with the master control module  40  in some embodiments. 
     Referring now to  FIG. 2 , some elements of the battery control modules  30  are shown. The battery control modules  30  include a voltage and/or current measuring module  60  that measures battery voltage and/or current of the battery subpack  12  and/or one or more individual batteries  20  in the battery subpack  12 . A battery temperature sensing module  62  measures temperature at at least one location within battery subpack  12 . A battery state of charge (SOC) module  64  periodically determines the SOC of the batteries  20  in the battery subpacks  12 . SOC module  64  may employ a lookup table  66 , formulas and/or other methods to determine the SOC. 
     A power limit module  68  predicts a maximum current limit I LIM , battery voltage limit V LIM , and/or power limit P LIM  for the battery subpack  12  and/or one or more batteries  20  in the battery subpack  12 , as will be described further below. A contactor control module  70  controls one or more contactors (not shown) that are associated with the control and/or connection of the batteries  20  in the battery subpacks  12 . A clock circuit  72  generates one or more clock signals for one or more of the modules within the battery control module  30 . 
     Referring now to  FIG. 3 , an equivalent circuit for the battery  20  is shown. A resistor R 0  represents ohmic resistance of the battery, a voltage V P  represents a polarization voltage, a voltage V 0  represents an open circuit or relaxation voltage, a current I represents the battery current, and a voltage V represents the battery voltage. V and I are measured values. R p  varies with temperature, duration of applied current and SOC. V 0  and R 0  vary primarily with SOC. When current I is steady state, V p  is equal to measured current I times R p . Using the equivalent circuit and Kirchoff&#39;s voltage rules for the battery  20 , V=V 0 +V p +IR 0 . 
     Referring now to  FIG. 4 , a flowchart is shown of a method  400  for predicting battery performance limits. Method  400  can be implemented as a computer program that is stored in a computer memory associated with a computer. The computer and computer memory can be included in the power limit module  68 . Method  400  can be initiated each time battery system  10  is turned on. 
     Control begins in start block  402  and proceeds to block  404 . In block  404  control initializes V p  to zero. Control then proceeds to block  406  and initializes a loop counter i to −1. Control then proceeds to decision block  408  and determines whether a loop period T has lapsed. The loop period T determines an execution period of blocks  410 - 420 . If the loop period T has not lapsed then control re-enters decision block  408  to wait for the beginning of the next loop period. Lapsing of the loop period can be determined from clock  72 . An example value for loop period T is 100 mS, however it should be appreciated by those skilled in the art that other time periods can be used. Control branches to block  410  and increments the loop counter i upon determining that a new loop period T is starting. 
     Control proceeds from block  410  to block  412  and updates a running average battery voltage V avgi  based on the equation:
 
 V   avgi   =[V   avg(i-1)   +K ( V   i   −V   avg(i-1) )],
 
     where 0≦K≦1, V i  is the the measured battery voltage V corresponding to the loop count i, and V avg(i-1)  is the value of the running average voltage corresponding to the previous loop period. The running average battery voltage V avg  can be used as an approximation for the relaxation voltage V 0 . 
     Control proceeds from block  412  to block  414  and predicts a battery current limit I LIM  based on the equation:
 
 I   LIM =( V   LIM   −V   avgi   −V   pi )/ R   o  
 
where V LIM  refers to a selected operating voltage limit of the battery and V pi  is the value of the polarization voltage V p  corresponding to the loop count i. The value for V LIM  may be one of the minimum operating voltage specification (V min ) and maximum operating voltage specification (V max ) of battery  20 . Control then proceeds to block  416  and updates the polarization voltage V p  based on the equation:
 
 V   pi   =V   p(i-1) +( I   LIM   R   p   −V   p(i-1) )| I|T/τ 
 
where R p  can be estimated from lookup tables  66  based on battery temperature and/or battery SOC, V p(i-1)  is the polarization voltage V p  corresponding to the previous loop period, and T is a time constant that is experimentally determined based on the battery voltage V and the battery voltage limit V LIM .
 
     Control proceeds from block  416  to block  418  and determines a predicted battery power limit P LIM  based on the equation:
 
P LIM =I LIM V LIM  
 
Control then proceeds to block  420  and communicates P LIM  and/or I LIM  values to the master control module  40 . It should be appreciated that the values of P LIM  and I LIM  correspond to the selected battery voltage limit V LIM . Method  400  can therefore be used to maintain values of P LIM  and I LIM  for corresponding values of V min  and V max .
 
     The master control module  40  can use the P LIM  and/or I LIM  values to take appropriate action before the battery voltage V violates the selected battery voltage limit V LIM . For example, in a hybrid electric vehicle, master control module  40  can start the vehicle engine to assist the batteries before the battery voltage V falls below the battery voltage limit V LIM . when V LIM  is set equal to V min . 
     Referring now to  FIG. 5 , a sample plot shows a comparison of the predicted battery power limit P LIM  and the actual power delivered by batteries  20 . The sample plot is taken from a hybrid vehicle application where the load power is shared between batteries  20  and an internal combustion engine. A horizontal axis  500  represents time in seconds. A left vertical axis  502  represents battery power. An upper half of left vertical axis  502  represents batteries  20  absorbing or regenerating power from the load. A lower half of left vertical axis  502  represents batteries  20  providing or discharging power into the load. A trace  504  represents load power and is referenced to the left vertical axis  502 . A trace  506  represents the predicted battery power limit P LIM  and is referenced to the left vertical axis  502 . The predicted battery power limit P LIM  was determined in accordance with method  400 . 
     A first pair of points  508 - 1 , a second pair of points  508 - 2 , and third pair of points  508 - 3 , collectively referred to a pairs of points  508 , shows a relationship between the predicted battery power limit P LIM  and the actual battery power. A left point in each pair of points  508  shows the predicted battery power limit P LIM . A right point in each pair of points  508  indicates the actual power reaching the corresponding predicted battery power limit P LIM . 
     A right vertical axis  510  is scaled in volts. The batteries used to generate the sample plot of  FIG. 5  have a battery voltage limit V LIM =9V. A battery voltage trace  512  indicates the measured battery voltage V. The predicted battery power limit P LIM  provides the master controller  40  with ample time to control load sharing between batteries  20  and the internal combustion engine, thereby keeping the measured battery voltage V above 9V. 
     Referring now to  FIGS. 6A-6C , various embodiments of hybrid electric vehicles (HEV) are shown. The present invention can be implemented in battery systems  10  incorporated in the HEVs.  FIG. 6A  depicts a functional block diagram of a parallel-architecture HEV  600 . HEV  600  includes an internal combustion engine  602  and an electric motor  604 . Internal combustion engine  602  includes an output shaft  606  that provides rotational power to a transmission  608 . A generator  610  is driven by internal combustion engine  602  and provides a charging current to battery system  10 . MCM  40  controls and/or sends a control signal to an electronic control module (ECM)  612 . ECM  612  controls internal combustion engine  602  based on the control signal from MCM  40 . 
     Electric motor  604  converts power from battery system  200  to mechanical power. The mechanical power is applied to an input shaft of transmission  608 . Transmission  608  combines power from internal combustion engine  602  and electric motor  604  to provide power to a drive axle  614 . 
     Referring now to  FIG. 6B , a functional block diagram of a serial-architecture HEV  620  is shown. HEV  620  includes internal combustion engine  602  that drives generator  610 . Generator  610  provides charging current to battery system  10 . MCM  40  generates a control signal that is communicated to ECM  612 . Electric motor  604  receives power from battery system  200 . 
     Referring now to  FIG. 6C , a functional block diagram of an indirect serial-architecture HEV  650  is shown. Internal combustion engine  602  provides power to a second drive axle  632  and generator  610 . Generator  610  provides a charging current to battery system  10 . Electric motor  604  provides power to drive axle  614 . MCM  40  coordinates operation of internal combustion engine  602  and electric motor  604  to propel the vehicle. 
     Referring now to  FIG. 7 , a functional block diagram is shown of a supplemental power supply  640 . A load  642  receives power from a utility line  644 . Load  642  can also receive battery power from battery system  10 . An inverter  644  converts the battery power to alternating current. A phase matching network  646  matches a phase of the power from inverter  644  and a phase of the power from utility line  644 . MCM  40  can include an input  650  that monitors load  642  for power shortages. MCM  40  controls battery system  10  based on the power shortages. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.