TECHNICAL FIELD 
     The present invention relates to a battery pack operating in a hybrid-electric powertrain for a vehicle. More specifically, the present invention relates to a method and apparatus to control at least one voltage output of a DC—DC converter to regulate the charge of individual battery modules of the battery pack. 
     BACKGROUND OF THE INVENTION 
     In today&#39;s automotive market there exist a variety of propulsion or drive technologies used to power vehicles. The technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy source, and hybrid systems utilizing a combination of internal combustion engines and electric drive systems. The propulsion systems each have specific technological, financial, and performance advantages and disadvantages, depending on the state of energy prices, energy infrastructure developments, environmental laws, and government incentives. 
     The increasing demand to improve fuel economy and reduce emissions in present vehicles has led to the development of advanced hybrid vehicles. Hybrid vehicles are classified as vehicles having at least two separate power sources, typically an internal combustion engine and an electric traction motor. Hybrid vehicles, as compared to standard vehicles driven by an ICE, have improved fuel economy and reduced emissions. During varying driving conditions hybrid vehicles will alternate between separate power sources, depending on the most efficient manner of operation of each power source. For example, a hybrid vehicle equipped with an ICE and an electric motor will shut down the ICE during a stopped or idle condition, allowing the electric motor to propel the vehicle and eventually restart the ICE, improving fuel economy for the hybrid vehicle. 
     Hybrid vehicles are broadly classified into series or parallel drivetrains, depending upon the configuration of the drivetrains. In a series drivetrain utilizing an ICE and an electric traction motor, only the electric motor drives the wheels of a vehicle. The ICE converts a fuel source to mechanical energy to turn a generator which converts the mechanical energy to electrical energy to drive the electric motor. In a parallel hybrid drivetrain system, two power sources such as an ICE and an electric traction motor operate in parallel to propel a vehicle. Generally, a hybrid vehicle having a parallel drivetrain combines the power and range advantages of a conventional ICE with the efficiency and electrical regeneration capability of an electric motor to increase fuel economy and lower emissions, as compared with a traditional ICE vehicle. 
     SUMMARY OF THE INVENTION 
     The present invention includes a vehicle having a parallel hybrid drive system incorporating an ICE and an electric motor generator (MoGen). The MoGen provides for propulsion of the vehicle during certain vehicle operating conditions, replaces an alternator to charge a battery pack in the vehicle, and replaces a conventional starter motor to start the ICE. The hybrid drive system of the present invention will utilize the ICE and MoGen to propel or motor the vehicle during the vehicle conditions which are most efficient for the ICE or MoGen operation. For example, during deceleration or a stopped condition, fuel flow to the ICE will be cut off, as these conditions are some of the least efficient conditions to run the ICE. The MoGen system becomes the active propulsion or motoring system during this fuel cut-off feature and powers the vehicle without noticeably disturbing the operation of the vehicle or sacrificing driveability. The MoGen will propel the vehicle and smoothly transition the vehicle from the idle or stopped state and start the ICE for ICE driving conditions. The transfer of power between the MoGen and ICE or vice versa is transparent to the operator or driver, as the vehicle will perform as if there is only one drive system propelling the vehicle. 
     During normal operation of the vehicle when the ICE is running, the MoGen will act as an electrical generator to supply electrical power to the vehicle&#39;s electrical infrastructure (fans, radios, instrumentation, control, etc.) as well as recharging the battery pack. The battery pack and a power supply, such as a DC—DC converter, will supply power to the vehicle electrical infrastructure the battery pack will power the MoGen when it is operating as the motoring device for the vehicle. 
     The present invention includes a method and apparatus for controlling the voltage output of a DC—DC converter (Vout) and its relationship to the battery pack and an underhood junction box (UHJB). The UHJB networks the vehicle&#39;s electric loads in the vehicle electrical infrastructure. The battery pack will store chemical energy in the form of reactive components that are designed to release the chemical energy as electric power to power the MoGen and the vehicle electrical infrastructure. Typically, a battery pack will comprise a series of individual batteries or battery modules connected in series, parallel, or a combination of both series and parallel, depending on the current and voltage needs of the vehicle. 
     The battery pack of the present invention is a secondary/rechargeable battery, incorporating the discharge and charge limitations associated with a rechargeable battery such as limited cycle life. The number of times a battery can be recharged and discharged before its fails to meet a selected performance criteria is referred to as the cycle life. A battery is likely to experience differing states of charge (SOC) (the percentage of the full capacity of a battery that is still available for further discharge), in accordance with the power demands of an electric vehicle. These SOCs and the number and depth of the discharges and the recharges will effect the life of the battery. For example, if a lead acid battery is operated in a state where it is only partially charged for an extended period of time, sulfation on the plates of the battery will occur. Sulfation in a lead acid battery involves lead sulfate in the battery developing into large crystals which cannot be readily converted back to an active material, decreasing the charge capacity of the battery and reducing the cycle life of the battery. 
     The present invention regulates the voltages and the states of charge of battery modules in a battery pack to balance the charging and discharging of the individual battery modules in the battery pack. By regulating the charging and discharging of the individual battery modules in the battery pack, the cycle life of the battery pack will be extended. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawing FIGURE is a diagrammatic drawing of the balancing or charging system of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawing FIGURE is a diagrammatic drawing of a hybrid vehicle  10 , including a battery module balancing system  12  of the present invention. The battery balancing system  12  includes a battery pack  13  having individual battery modules  14 ,  16 , and  18 . In the preferred embodiment, the battery pack  13  comprises three 12 volt valve-regulated absorbent glass matt (AGM) lead-acid batteries connected in series to produce a 36 volt nominal bus. In alternate embodiments of the present invention, the battery pack  13  may comprise any known battery technology, including, but not limited to, lithium polymer batteries and nickel metal hydride batteries. The first battery module  14  is chassis grounded and the second and third battery modules  16  and  18  are connected in series with the first battery module  14 , as shown in the drawing figure. The respective voltages across the first, second and third battery modules  14 ,  16 , and  18  will be referred to as V 1 , V 2 , and V 3 . While the battery pack  13  has been described as having three battery modules, any number of battery modules of varying voltages are considered within the scope of this invention. Furthermore, a single battery such as a 36 volt battery with a plurality of access points to varying levels of potential (Ground, 12V, and 36V) within it may also be used in the present invention. 
     In one embodiment of the present invention, the first battery module  14  is tapped to supply power to an underhood junction box (UHJB)  19  which networks and supplies power to the electrical infrastructure of the vehicle  10 . The first battery module  14  provides the electrical energy necessary to drive the parasitic loads in the vehicle  10  such as the engine computer while the vehicle  10  is in a nonoperating or parked condition. A concern with tapping off one of the battery modules in a series string, such as found in the battery pack  13  of the present invention, is premature battery pack degradation due to an imbalance in the state of charge (SOC) among the battery modules  14 ,  16 , and  18 . The present invention balances the SOC by charging and discharging the battery modules  14 ,  16 ,  18  in response to their measured voltages V 1 , V 2 , and V 3 , as will be described later in the specification. 
     A motor generator (MoGen)  20  is dynamically coupled to an internal combustion engine (ICE)  22  and functions as either a motor to propel the vehicle  10  or a generator to charge the battery pack  13 , depending on the operating state of the vehicle  10  (i.e., braking, stopped, or operating at a constant speed on a highway). The MoGen  20  is preferably an AC induction machine but may comprise any known electrical motor/generator technology, including, but not limited to, DC machines, synchronous machines, and switched reluctance machines. 
     The MoGen  20  in generator mode generates electrical energy that is transferred to the battery pack  13  and the DC—DC converter  24  by a MoGen controller  23 . The MoGen controller  23  determines the direction of current flow for the MoGen  20 , according to the vehicle operating state. In a regeneration state (such as during braking) or charging condition, current will flow from the MoGen  20 , via the MoGen controller  23 , to charge the battery pack  13  and provide current to the DC—DC converter  24 . In a state where the MoGen  20  is needed to provide propulsion, current will flow from the battery pack  13  to the MoGen  20 , via the MoGen controller  23 , to power the MoGen  20 . In the present embodiment, the MoGen  20  operates at a nominal 36 volts. The operation of the MoGen  20  and its relationship to the battery pack  13  and DC—DC converter  24  is further detailed in the following paragraphs. 
     This hybrid powertrain of the present invention uses current i ER  or “Excess Regen,” supplied by the MoGen  20  determined through a single current-measuring device such as a current shunt  26 , as the main variable to manage the battery pack  13  state-of-usage (SOU)and SOC. The electrical power flow of the module balancing system  12  can dynamically change among four different modes of battery pack  13  SOU or mode of operation of the MoGen  20 , as determined by the MoGen controller  23 . 
     Excess Regen, 
     Zero Excess Regen, 
     MoGen  20  Neutral, 
     MoGen  20  Motoring Discharge. 
     I. Excess Regen: Of the total regenerative current “Total Regen” i TR  provided by the MoGen  20 , a portion powers the DC—DC converter  24  i DC—DC , and the remaining current (or the Excess Regen i ER ) recharges the battery pack  13 . This is the state that the system  12  will default to for a large majority of its operation time (e.g., cruising on highway). If the battery pack  13  SOC is low, the Excess Regen can be commanded up to a set value; if the battery pack  13  SOC is high, the Excess Regen is tapered down to a minimal value. The upper limit for Excess Regen is determined by the driveability of the vehicle  10 ; i.e., if the Excess Regen is too high, the powertrain will feel sluggish. This SOU is active anytime the battery  13  SOC is not full, and the MoGen  20  is being backdriven by the ICE  22  or an automotive transmission. 
     II. Zero Excess Regen: The MoGen  20  provides just enough Total Regen to power the DC—DC converter (i TR =i DC—DC ). The Excess Regen to charge the battery pack  13  is zero (i ER =0). Zero Excess Regen is used when the battery modules  14 ,  16 , and  18  are fully charged. Determination of when the battery modules  14 ,  16 , and  18  are fully charged can be estimated from charge voltage, charge amperage, open-circuit voltage, and charge integration coupled with the Peukert relationship. In actuality, since the DC—DC converter  24  loads can be constantly fluctuating, Excess Regen can not be held to exactly zero. Thus, above a certain voltage threshold (temperature dependent), the current control system changes to a voltage control system. This SOU is active when the battery modules  14 ,  16 , and  18 , are at 100% SOC and after crank starting the ICE  22  when the coolant temperature or the SOC of the battery modules  14 ,  16 , and  18  is medium or high. The MoGen  20  is controlled to Zero Excess Regen after the MoGen  20  is done motoring the ICE  22 , but before the combustion is deemed fully stabilized. 
     III. MoGen Neutral: The MoGen  20  is free spinning, thus i M =i TR =0. Since the accessory loads are still supported by the DC—DC converter  24 , i DC—DC  is still positive. The power for i DC—DC  is provided by i DC—DC+m , thus the battery pack  13  is being discharged. This SOU is active when: during some transmission shift events where neutral is commanded to eliminate aliasing, due to possible ICE  22  torque variabilities, of automotive transmission adaptives; neutral is commanded at the end of MoGen  20 -powered downshift synchronizations, and during a no-MoGen  20  downshifts; after crank starting the ICE  22  when the coolant temperature or the SOC of the battery modules  14 ,  16 , and  18  are low; the MoGen  20  is controlled to neutral after the MoGen  20  is done motoring the ICE  22 , but before the combustion is deemed fully stabilized to minimize engine load; and the vehicle  10  is keyed-on with the ICE  22  off. 
     IV. Motoring Discharge: The MoGen  20  delivers mechanical work to the ICE  22 . The electrical charge flowing out of the battery pack  13  i DC—DC+m  (in the drawing figure) is the sum of this MoGen  20  motoring load i m  and the DC—DC converter  24  input load i DC—DC . This can occur under the following conditions: during key-up crank start, during a hybrid launch from a stop, during a fuel-off downshift, and during an inertia eliminator routine. 
     The MoGen  20  and battery pack  13  provide power to the DC—DC converter  24  and the DC—DC converter  24  regulates the SOCs of the battery modules  14 ,  16 , and  18  of the battery pack  13 . The DC—DC converter  24  will provide a voltage output Vout wired in parallel to the first battery module  14  to charge the first battery module  14  and provide supplemental current capacity to the UHJB  19  when load demands are high. In normal operation, Vout will be substantially equal to 12 volts nominal to charge the first battery module  14 . The DC—DC converter  24  may comprise any known DC—DC converter known in the art and is sized according to the current ratings of a vehicle electrical system to which it is coupled. In the preferred embodiment, the first battery module  14  will supply 12 volts nominal to the UHJB and its corresponding parasitic loads. As previously discussed, a concern with tapping off one of the battery modules in a series string is premature battery pack performance degradation. The DC—DC converter  24  will manage the SOC of the battery modules  14 ,  16 , and  18  of the battery pack  13 , notably the first battery module  14 , to balance the SOC of the battery module  14  to those of battery modules  16 , and  18 . The DC—DC converter  24  will transfer charge between the battery modules  14 ,  16 , and  18  by monitoring the voltage levels of each battery module  14 ,  16 , and  18  and transferring charge to the first battery module  14  by controlling Vout. Any type of voltage monitor or sensor known in the art is considered within the scope of this invention. 
     Many hybrid electric powertrain systems control a battery pack&#39;s state-of-charge (SOC) near 50 to 80% so that the charge acceptance and efficiency during regenerative braking can be realized. Though this type of strategy can result in energy efficiency gains, long term battery life can be compromised, particularly in the case of lead-acid batteries. 
     In the preferred embodiment of the present invention, the SOC is dynamically tracked by voltage control and charge integration using the Peukert relationship. The SOC estimation is periodically reset with open-circuit voltage (Voc) readings after a timed shutdown, or via loaded voltage (Vload) during a known discharge action such as an engine start or a transmission downshift synchronization. 
     To increase the life of the batteries, the battery control system keeps the SOC near full charge, The advantages of implementing such a strategy include: 
     1. The failure mode of lead-acid batteries through minor gassing is more favorable than that through plate sulfation (realized through consistently using the batteries at partial SOC). 
     2. The charge imbalance between the battery modules  14 ,  16 , and  18  is not as detrimental if lowest battery is not very low. For example, if two batteries are at 95% SOC, and the other one is 85%, the imbalance is not as detrimental to the health of the entire pack as if two of the batteries were at 40% SOC and the other at 30% SOC. 
     3. Similar to item 2 above, the parasitic loads through the UHJB  19  may drain battery module  14  during a long park period. If the battery pack  13  were maintained at a high SOC with the battery modules  14 ,  16 , and  18  balanced, the first battery module  14  upon return (after an extended park) will have a greater chance of being high enough to perform its share in the starting tasks. 
     4. If the SOC is high, the amount of regenerative braking is reduced. Though this can result in a vehicle energy efficiency reduction, the driveability control strategy is greatly simplified since modulation and blending (with the hydraulic brakes) of the regenerative braking torque is not necessary. 
     To prevent premature battery sulfation, the battery pack  13  may need to be periodically fully charged (e.g. every x keyup cycles). If the SOC is consistently high, the driveability (regenerative braking torque) is consistent from day to day regardless of whether the vehicle  10  is operative normally or if it is in midst of a battery pack  13  equalization routine. 
     The present invention balances SOC by imposing uniform module voltages across the battery pack  13 . There are a plurality of modes of operation for control of Vout for the DC—DC converter  24 . 
     The first mode occurs when Vout is set to match the lower of the voltages of the second and third battery modules  16  and  18 , Vout=min(V 2 , V 3 ). During discharging and/or a low SOC charge of the battery pack  13 , the DC—DC converter  24  will set Vout to match the lower of the voltages of the second or third battery modules  16  and  18 . Generally, during the discharge of a series string of battery modules such as in the battery pack  13  of the present invention, the weakest or lowest voltage battery module limits the performance of the entire battery pack  13 . The DC—DC converter  24  prevents a single module&#39;s voltage from dropping off, relative to the remaining battery modules, by transferring charge from the remaining battery modules to the weak module by manipulating Vout. For example, if the first battery module  14  is weakened by the UHJB  19  parasitic loads, the DC—DC converter  24  will transfer charge from the battery modules  16  and  18  to the first battery module  14 . The active Vout control is especially valuable when the ICE  22  is off and there is no charging by the MoGen  20  and the first battery module  14  SOC is relatively low. 
     During low SOC charging (e.g., V 1 &lt;13 volts), and the SOC is medium to high the 36 volt generating power from the MoGen  20  can more effectively recharge the entire battery pack  13  without undercharging one of the battery modules  14 ,  16 , and  18 , especially the second and third battery modules  16  and  18 , as it is not possible to individually charge them. If the first battery module  14  voltage V 1  is greater than V 2  or V 3 , then the first battery module  14  will provide current to the UHJB  19  electrical load until V 1  more closely matches V 2  and V 3 , thus preventing the first module  14  from being overcharged by the MoGen  20 . 
     In a second mode of operation when charging at medium to high SOC with the DC—DC converter  24 , Vout is set to match the higher of V 2  and V 3  to limit the MoGen  20  from overcharging the highest module, Vout=max(V 2 , V 3 ). For example, if V 2 =14 volts and V 3 =15 volts, setting Vout to 14 volts leads to a total battery pack  12  voltage of 43 volts. If the MoGen  20  can provide up to 45 volts, the third battery module  18  can be overcharged (the voltage rise of a battery near full charge increases rapidly) while depriving the first and second battery modules  14  and  16  of a full charge. When Vout is set to the higher of the two voltages V 2  and V 3  (i.e., 15 volts in this example), V 3  would be dragged down by current drain through the DC—DC converter  24  to the first battery module  14 . The MoGen  20  regeneration voltage lid would not be hit with a single battery module&#39;s voltage sharply rising by itself since a portion of the charge of the third battery module  18  would be transferred to the first battery module  14 . This scenario is used when the ICE  22  is running. 
     In a third mode of operation, if all the battery module voltages V 1 , V 2 , and V 3  are being drawn low (for example, 8 volts on a 12 volt nominal module), then Vout is set to a minimum voltage such as 9 volts. This minimum voltage is high enough to ensure that the powertrain computer and the ignition/fuel systems remain active. This can be particularly important when starting the ICE  22  on a cold winter morning. 
     During a MoGen  20  regeneration voltage limit operating mode, the MoGen controller  23  limits the total regeneration voltage Vlid (for example, Vlid=44 volts) to prevent overcharge of the battery pack  13 . For this application, Vlid is set to approximately three times the higher voltage of V 2  and V 3  (i.e., Vlid=3×max (V 2 , V 3 )=3×Vout in discrete ratcheted increments of 0.5V (e.g., 41.5V, 42.0V, 42.5V, etc.), although any voltage increment is considered within the scope of this invention. This is done to allow all three battery modules  14 ,  16 , and  18  to converge to close voltage values as the total battery pack  13  is charged. For example, if V 3  is 13 volts, and Vout=V 2 =14 volts and Vlid is set to 45 volts, it is likely that V 1  and V 2  will climb and leave V 3  lower than optimal. However, if Vlid is set to 14*3=42 volts, V 1  and V 2  would not climb as high, and allow V 3  to catch up. Vlid is ratcheted up until the proper top off voltage value is reached (e.g., 14.8 volts per module for a lead acid battery module application). 
     In alternate embodiments of the present invention, a controller in the vehicle  10  is capable of learning the duty cycles of the battery pack  13  and compensating for varying SOCs. Depending on the sizing of the DC—DC converter  24  and the duty cycle of the loads demanded by the UHJB  19 , the first battery module  14  may end up being systematically undercharged. In this case, the engine computer can learn the accessory usage pattern of the driver and adapt the Vout of the DC—DC converter  24  such that the SOC of the first battery module  14  (via running voltages and Voc measurements) is more consistent with that of the second and third battery modules  16  and  18 . Instead of matching the lower of V 2  and V 3 , Vout can match successively higher increments between V 2  and V 3 , while monitoring the SOC between the second and third battery modules  16  and  18 . Another application of the adaptive Vout is to compensate for parasitic loads (e.g., engine computer) when the vehicle is parked and keyed off. 
     In another embodiment of the present invention, if the ignition is keyed off, lower power devices such as dome lights and other parasitic loads are powered by the first battery module  14 . However, if a high-powered device such as headlights is turned on, or if the voltage of the first battery module  14  drops a calibrated amount below that of V 2  and V 3 , the body computer signals the DC—DC converter  24  to turn on to help power the accessories and to rebalance the battery modules  14 ,  16 , and  18 . Once the ICE  22  is running, the DC—DC converter  24  powered by the MoGen  20  powers the UHJB  19  in parallel with the first battery module  14 , as previously described. 
     In a further embodiment of the present invention, if the battery modules  14 ,  16 , and  18  are located such that they will not be at similar temperatures (i.e., one battery module underhood and the remaining battery modules in the trunk), the DC—DC charging scheme can incorporate a temperature compensation algorithm to balance the SOCs for the battery modules  14 ,  16 , and  18 . 
     As a backup measure for monitoring the voltages of the battery modules  14 ,  16  and  18 , if a voltage sense wire(s) of a voltage input or sensor of the present invention becomes disconnected, Vout can be set to the average of V 2  and V 3  read through the power wires or connections. The V 1  voltage is read through the DC—DC converter  24  output cable, and the difference between it and the total battery pack  13  voltages gives twice the average voltage of V 2  and V 3 . Similarly, a single 36 volt battery can be used that has three posts: Ground 12V, 36V. In this case, the Vout would be set equal to the average of V 2  and V 3 . Dedicated voltage taps can be used with the same backup measure mentioned above. 
     While this invention has been described in terms of some specific embodiments, it will be appreciated that other forms can readily be adapted by one skilled in the art. Accordingly, the scope of this invention is to be considered limited only by the following claims.
An apparatus for balancing a battery module in a vehicle including a plurality of batteries in the battery module, the plurality of batteries connected in series, the plurality of batteries each having a battery voltage, and at least one battery providing power to an electrical system of the vehicle, an electric generator producing a generator voltage to charge the plurality of batteries, and a DC--DC converter coupled to the electrical generator and receiving power from the electrical generator, the DC--DC converter monitoring at least one of the battery voltages of at least one of the plurality of batteries and controlling the battery voltage of the battery providing power to the electrical system of the vehicle with reference to the monitored battery voltage.