Abstract:
A system and method for discharging a vehicle battery following a vehicle damaging event. The method includes determining that the vehicle has been involved in the vehicle damaging event and then discharging cells in the battery to a predetermined cell voltage based on the severity of the vehicle damaging event. The battery cells can be discharged by resistors that are already existing in the vehicle battery for cell balancing purposes, or by resistors that have been added for the cell discharge purposes. The voltage of the cells are monitored when they are being dissipated, and once the particular voltage of a cell has reached the desired voltage, then a switch is opened to disconnect the resistor from the cell.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a system and method for discharging a high voltage vehicle battery following a crash or other vehicle damaging event and, more particularly, to a system and method for discharging a lithium-ion vehicle battery following a crash or other vehicle damaging event that includes dissipating cell voltages in the battery using cell balancing resistors. 
     2. Discussion of the Related Art 
     Electric vehicles are becoming more and more prevalent. These vehicles include hybrid vehicles, such as the extended range electric vehicles (EREV), that combine a battery and a main power source, such as an internal combustion engine, fuel cell systems, etc., and electric only vehicles, such as the battery electric vehicles (BEV). All of these types of electric vehicles employ a high voltage battery that includes a number of battery cells. These batteries can be different battery types, such as lithium-ion, nickel metal hydride, lead-acid, etc. A typical high voltage battery system for an electric vehicle may include several battery cells or modules to provide the vehicle power and energy requirements, where each battery module may include a certain number of battery cells, such as twelve cells. Different vehicle designs include different battery designs that employ various trade-offs and advantages for a particular application. 
     During a vehicle crash or other vehicle damaging event, the rearrangement and/or damage to various vehicle parts and the like may cause unwanted electrical connections and the discharge of various fluids from the vehicle to occur. Because of the electrical power that may be stored in the vehicle battery, these things could cause potentially hazardous situations, such as electrifying of vehicle parts, chemical hazards, fire hazards, etc. 
     Fault isolation detection systems are sometimes employed in electrical circuits to provide fault detection. Electric vehicles are one electrical system that typically employ fault isolation detection systems to prevent a person from being harmed by the system. 
     In order to provide electrical fault isolation, some vehicles are equipped with a battery disconnect unit (BDU) that automatically disconnects or removes battery power from the vehicle after a crash event by opening battery contactors. Also, it is known in the art to employ a manual service disconnect (MSD), which is a device that separates a battery into two parts, where a trained responder responding to a vehicle crash could remove the MSD to isolate the battery. However, these known isolation techniques do not remove the charge from the battery, only contain it. 
     As mentioned above, a high voltage battery for a vehicle typically includes several battery cells electrically coupled in series. As a result of many factors, such as internal cell resistance, electrical connections, battery aging, etc., the state-of-charge (SOC) of the cells in the battery may drift apart during operation of the battery over time. A battery management system may be provided to monitor the state-of-charge of each battery cell and the temperature of the battery, and control how much the battery can be charged and discharged based on the state-of-charge of the maximum charged cell and the minimum charged cell. The battery cannot be used to provide power if the cell with the lowest state-of-charge drops below some minimum state-of-charge because that cell may become damaged, and the battery cannot be over-charged beyond some maximum state-of-charge for the cell with the highest state-of-charge because that cell may become overheated and damaged. Thus, a cell with a low state-of-charge may prevent the battery from being used even though the other cells may have a suitable or significant state-of-charge. 
     It is known in the art to provide a cell balancing system that attempts to control the charging and discharging of the cells within a high voltage vehicle battery so that they have about the same level charge at any given point in time. In one known cell balancing circuit, cell balancing resistors are provided in parallel with the battery cells that allow the cells to be electrically dissipated to a desired level so that the charge matches other battery cells. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a system and method are disclosed for discharging a vehicle battery following a vehicle damaging event. The method includes determining that the vehicle has been involved in the vehicle damaging event and then discharging cells in the battery to a predetermined cell voltage based on the severity of the event. The battery cells can be discharged by resistors that are already existing in the vehicle battery for cell balancing purposes, or by resistors that have been added for the cell discharge purposes. The voltage of the cells are monitored when they are being dissipated, and once the particular voltage of a cell has reached the desired voltage, then a switch is opened to disconnect the resistor from the cell. Alternately, the cell voltages are not monitored, but are passively discharged to zero volts. In one embodiment, the predetermined cell voltage is three separate voltages depending on event severity, including a minimum operating voltage of the cells for a low severity event, a 0% SOC of the cells for a medium severity event and a zero voltage of the cells for a severe event. 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simple illustration of a vehicle including a high voltage battery; 
         FIG. 2  is a schematic diagram of a high voltage battery including balancing resistors; and 
         FIG. 3  is a flow chart diagram showing an operation for dissipating the electrical charge of battery cells in a vehicle battery. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a system and method for discharging battery cells in a high voltage vehicle battery following a crash or other vehicle damaging event is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the technique of the present invention has particular application for dissipating charge from a high-voltage vehicle battery, and in particular a lithium-ion vehicle battery. However, as will be appreciated by those skilled in the art, the technique of the invention may have application for other systems other than vehicle systems. 
       FIG. 1  is a simple illustration of a vehicle  10  intended to represent any electric vehicle or electric hybrid vehicle. The vehicle  10  includes a high voltage battery  12  mounted to a suitable support within the vehicle  10 , where the battery  12  includes a plurality of battery cells  14 . The battery  12  can be any battery suitable for an electric vehicle, such as a lead-acid battery, lithium-ion battery, nickel metal hydride battery, etc. The vehicle  10  may also include a separate power source  16 , such as an internal combustion engine, fuel cell system, etc., for an electric hybrid vehicle. The vehicle  10  includes an electronic control unit (ECU)  18  that controls various aspects of the operation of the vehicle  10 . In the embodiment discussed below, the ECU  18  receives signals from one or more sensors  20  indicating that a crash or other vehicle damaging event, such as a fire, has occurred, and possibly the severity of the event, where the sensor  20  can be part of any suitable crash detection system, such as an airbag deployment system, accelerometer, temperature detection system, etc. 
       FIG. 2  is schematic diagram of a high voltage energy storage system, such as a battery  30 , for example, a lithium-ion battery, that can be used as the high voltage battery  12  on the vehicle  10 . The battery  30  includes a negative terminal  32  and a positive terminal  34 . The battery  30  also includes a plurality of battery cells  36  electrically coupled in series between the terminals  32  and  34  that store electrical charge in a manner that is well understood by those skilled in the art. In one non-limiting example, each cell  36  may operate with a cell voltage in the range of 3 to 4.15 volts suitable for operation of the vehicle  10 , where 2-3 volts is considered 0% SOC of the cell  36  and a cell voltage less than 2 volts is an over-discharge of the cell  36 . 
     As mentioned above, it is desirable to provide cell voltage balancing in a vehicle battery so that each of the cells  36  discharge and charge to about the same level. In order to accomplish this, each cell  36  includes a cell balancing circuit  38  electrically coupled in parallel with the particular cell  36 . In this example, the cell balancing circuit  38  includes a switch  42 , typically a solid-state switch, such as a MOSFET, electrically coupled in series with a suitably sized resistor  40 . A cell-balancing control algorithm is provided to control the switch  42  during charging and discharging of the battery  30  so that the charge on the battery cell  36  is either dissipated or not dissipated by the resistor  40  to cause the charge on each cell  36  to be substantially the same. This will prevent some of the cells  36  from over-charging or over-discharging compared to the other cells  36 . 
     The present invention proposes using the already existing resistors  40  provided for cell voltage balancing as discussed above to dissipate the charge on the battery cells  36  to a desired level following a crash or vehicle damaging event. As mentioned, the balancing circuit  38  including the resistors  40  can already be an existing part of the battery  30 . Alternately, the resistors  40  can be added to the battery  30  for the purposes discussed herein. 
     The ECU  18  can employ logic and suitable algorithms to provide varying levels of discharge of the cells  36  through the resistors  40  depending on the severity of the event. For example, in a high severity collision, as possibly determined by the rate-of-change of vehicle velocity or other crash-related parameters, the battery  30  can be discharged to about zero volts since it is likely that the battery  30  will be damaged by the collision and will not be able to be used thereafter. 
     For a less severe collision or crash event, the battery  30  may be discharged to a 0% state-of-charge below the normal operating state-of-charge of the battery  30 , since it is likely that the battery  30  will not be damaged by the event and may still be able to be used. For example, the battery cells  36  may be discharged to a level slightly less than 3 volts in the example given above, where the battery  30  will have to be charged at a service location to be able to be used again. 
     In a third option where the collision is minor and the battery discharge may just be a precaution, the ECU  18  may cause the battery cells  36  in the battery  30  to be discharged to the lower end of the cell operating range, which could be about 3.3 volts in a non-limiting example for a specific system. 
     Other parameters can be used to determine the level of cell voltage discharge. For example, a loss of 12 volt power to the battery  30  when combined with collision severity information may be used to determine the appropriate voltage discharge level to discharge the battery cells  36 . In addition, if in the future, a battery state-of-charge health metric is available, this parameter can also be used to determine the appropriate amount of discharge of the battery  30 . Also, cell temperature can be a parameter that can be used to determine the level of cell discharge, where the vehicle  10  may be in a fire. 
     An additional potential application is to implement a function that initiates discharge of the battery cells  36  upon removal of the 12 volt power from the battery  30 . In this case, to prevent battery damage and customer dissatisfaction, the level of discharge would need to be no more than the lower end of the normal voltage operating range of the battery  30 . This would allow the battery  30  to be reinstalled or reconnected to the vehicle  10  without adversely affecting performance. However, it would be necessary to make sure that a battery in long term storage, for example, has a lower state-of-charge. 
       FIG. 3  is a flow chart diagram  50  showing an operation for dissipating the battery cells  36  to a certain state-of-charge level in response to a vehicle crash or other vehicle damaging event, depending on severity. At box  52 , the algorithm detects the crash or collision event through any suitable crash sensor or combination of crash sensors to determine that the crash or collision event has occurred. The sensors, or other devices, also determine the level of severity of the crash event and select a cell voltage dissipation level based on that severity. As discussed above, these dissipation levels can be a minimum cell operation voltage where the battery  30  is still operational, a 0% state-of-charge level where the cell voltage is just below the operating range of the cells  36 , but where the cells can be recharged, and a 0 volt level where the cells will become damaged and not be able to be reused. Other dissipation levels may be desirable for other applications. The crash sensors can include airbag deployment sensors, accelerometers, etc. on the vehicle  10 . 
     Upon detection of the event, the algorithm performs the normal battery isolation process at box  54 , such as opening a BDU. Further, the algorithm also closes all of the switches  42  to discharge the battery cells  36  through the resistors  40  at box  56 . The algorithm monitors the voltage level of each of the individual cells  36  in the battery  30  at box  58  so that if the voltage of a particular cell  36  is greater than the predetermined voltage dissipation level based on the event, the algorithm will maintain the switch  42  in the closed position at the box  56 . Once a particular cell  36  reaches the predetermined voltage dissipation level at box  60 , the algorithm opens the switch  42  for that particular cell  36  at box  62 . Thus, even though the cells  36  should be somewhat balanced at any point in time, the present invention still allows unbalanced cells to be discharged separately by using the separate resistor  40  for each cell  36 . 
     As will be well understood by those skilled in the art, the several and various steps and processes discussed herein to describe the invention may be referring to operations performed by a computer, a processor, or other electronic calculating device that manipulates and/or transforms data using electrical phenomenon. Those computers and electronic devices may employ various volatile and/or non-volatile memories including non-transitory computer-readable medium with an executable program stored thereon including various code or executable instructions able to be performed by the computer or processor, where the memory and/or computer-readable medium may include all forms and types of memory and other computer-readable media. 
     The foregoing discussion disclosed and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.