Abstract:
A method and system for controlling temperature in an electric vehicle battery pack which preserves battery pack performance and longevity while maximizing vehicle driving range. A controller prescribes a minimum allowable operating temperature in the battery pack, where the minimum operating temperature increases as battery pack state of charge and remaining useful life decrease. During vehicle driving operations, the minimum allowable temperature is computed, and a thermal management system is used to warm the battery pack only if necessary to raise its temperature above the calculated minimum level. By minimizing use of the thermal management system to warm the battery pack, energy consumption is reduced and vehicle driving range is increased, while not adversely affecting battery pack performance or durability. The same strategy is employed during charging, which reduces the amount of energy consumed from the grid for warming the battery pack.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to electric vehicle battery pack thermal management and, more particularly, to a method and system for managing the temperature in an electric vehicle battery pack which allows a battery pack at a high state of charge and/or high remaining useful life to be operated at a lower temperature than a battery pack at a lower state of charge and/or lower remaining useful life, thus reducing energy expenditure on battery pack heating and maximizing vehicle driving range, while still optimizing battery pack performance and durability. 
     2. Discussion of the Related Art 
     Electric vehicles are rapidly gaining popularity in today&#39;s automotive marketplace. Electric vehicles offer several desirable features, such as eliminating local emissions and usage of petroleum-based fuels at the consumer level, and potentially lower operating costs. A key component of electric vehicles is the battery pack, which can represent a substantial proportion of the vehicle&#39;s cost. Battery packs in these vehicles typically consist of numerous interconnected cells, which are able to deliver a lot of power on demand. Maximizing battery pack performance and life are key considerations in the design and operation of electric vehicles. 
     In order to maximize battery pack performance and durability, the temperature in the battery pack must be maintained at a certain minimum level when the battery pack is discharging during vehicle operation. Control systems in existing electric vehicles typically prescribe a constant minimum temperature in the battery pack, irrespective of state of charge or remaining useful life of the battery pack, and use an onboard thermal management system to ensure battery pack temperature is at or above the minimum set point level during vehicle operation. However, using the thermal management system to warm the battery pack before or during driving operation consumes energy, which detracts from the driving range of the vehicle. It has now been observed that the performance of lithium-ion battery packs can still be sufficient to meet vehicle requirements if the minimum set point temperature in the battery pack is lowered for battery packs with a high state of charge and/or remaining useful life. In addition, lowering the temperature set point is beneficial for battery durability. 
     There is an opportunity to employ a battery pack thermal management methodology which determines minimum set point temperature as a function of state of charge and remaining useful life, thus ensuring optimum battery pack performance and life, but also maximizing vehicle range by not expending energy to warm the battery pack unnecessarily. Such a method could increase customer satisfaction through improved vehicle driving range and decreased power consumption from the grid. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method and system are disclosed for controlling temperature in an electric vehicle battery pack such that battery pack performance and longevity are preserved, while vehicle driving range is maximized. A controller prescribes a minimum allowable temperature in the battery pack as a function of state of charge and remaining useful life of the battery pack. This control strategy reflects evidence that lithium-ion battery packs can be operated at lower temperatures when battery pack state of charge and/or remaining useful life are high, without having a detrimental effect on battery pack performance or durability. During vehicle driving operations, the minimum allowable temperature is computed, and a thermal management system is used to warm the battery pack only if necessary to raise its temperature above the calculated minimum level. By minimizing use of the thermal management system to warm the battery pack, energy consumption is reduced and vehicle driving range is increased. The same thermal management strategy is employed during charging, which can reduce the amount of energy consumed from the grid by not unnecessarily warming the battery pack. 
     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 schematic diagram of an electric vehicle battery pack thermal management system; 
         FIG. 2  is a graph of minimum allowable temperature in the battery pack as a function of state of charge and remaining useful life, according to the present invention; and 
         FIG. 3  is a process flow diagram of a method for managing the temperature in the battery pack as a function of state of charge and remaining useful life. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a battery thermal system control strategy is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the present invention has particular application to electric vehicle battery packs, but may be equally applicable to battery packs in gasoline/electric or fuel cell/electric hybrid vehicles, battery packs in utility vehicles, such as forklifts, and non-vehicle-based battery packs. 
     Battery pack performance and durability, and vehicle driving range, are key considerations in the design of an electric vehicle. It is well known in the industry that a lithium ion battery pack&#39;s performance can suffer if the battery pack is operated at too low of a temperature. Therefore, electric vehicles typically include an onboard thermal management system to maintain battery pack temperatures above a certain level, which is known to be conducive to optimum performance and battery pack life. Thermal management systems in existing electric vehicles typically prescribe a single minimum allowable temperature in the battery pack, and the thermal management system is engaged as necessary to maintain battery pack temperature at or above this fixed limit. However, operating the battery pack thermal management system consumes energy, which detracts from the driving range of the vehicle. A new thermal management strategy is proposed, in which lower battery pack temperatures are allowed under many conditions, thus reducing the amount of heating energy consumed by the thermal management system. 
     In addition to the minimum battery pack temperature considerations described above, electric vehicle battery packs must also be controlled to prevent the temperature from becoming too high. While a battery pack cooling strategy is not the subject of the present invention, the battery pack thermal management strategy disclosed herein has a favorable impact on the cooling strategy, as will be discussed below. 
     Recent testing has revealed that the performance of lithium-ion battery packs is a function of not only temperature but also the state of charge and the age of the battery pack. That is, a battery pack in a high state of charge can be operated at a lower temperature than a battery pack in a lower state of charge, and a new battery pack can be operated at a lower temperature than an old battery pack. The present invention exploits this phenomenon with a thermal management system designed to preserve battery pack performance and longevity, but also maximize electric vehicle driving range. This is done by judiciously using the thermal management system and allowing the battery pack to be operated at a somewhat lower temperature than would have been allowed previously, if the battery pack is in a high state of charge and/or has most of its useful life remaining. 
       FIG. 1  is a schematic diagram of an electric vehicle battery pack thermal management system  10 . A vehicle  12  uses electric motors (not shown) for propulsion, with electrical energy for the motors provided by a battery pack  14 . The battery pack  14  is fitted with a plurality of sensors  16 , including devices which monitor temperature and state of charge in the battery pack  14 . The sensors  16  can also monitor various battery pack properties that are used to compute the age of the battery pack  14 , or alternatively, its remaining useful life. Other parameters may also be monitored by the sensors  16 , and temperature and state of charge may be measured at multiple locations throughout the battery pack  14  by the sensors  16 . A heating and cooling system  18  is used to warm or cool the battery pack  14  as necessary to maintain the temperature in the battery pack  14  within a prescribed range, as will be discussed below. 
     The heating and cooling system  18  is connected to the battery pack  14  by fluid lines  20 , which deliver a chilled or warmed fluid to the battery pack  14  and return the fluid to the heating and cooling system  18 . The fluid in the fluid lines  20  may be a liquid, such as a refrigerant; air or another gas; or a two-phase mixture of a liquid and a gas. The heating and cooling system  18  may be of any type suitable for the purpose described herein, including a traditional refrigerant-based cooling system with a supplemental heater, a two-stage system which delivers a secondary fluid through the fluid lines  20 , a thermal storage system which stores heat from the battery pack  14  in a fluid and either dissipates the heat to the environment through a radiator or uses the heat to subsequently warm the battery pack  14  or the interior of the vehicle  12 , and other types. 
     A controller  22  is used to control the heating and cooling system  18 , based on conditions in the battery pack  14  as monitored by the sensors  16 . The controller  22  is in communication with the sensors  16  via connections  24 , and with the heating and cooling system  18  via a connection  26 . The connections  24  and  26  may be wired or wireless. The controller  22  is configured to engage the heating and cooling system  18  as necessary to warm or cool the battery pack  14  in order to maintain temperature in the battery pack  14  within a prescribed range, where the prescribed temperature range is dependent on the state of charge and the remaining useful life of the battery pack  14 . In the remainder of the discussion below, only the portion of the thermal management strategy which deals with warming the battery pack to the prescribed minimum set point temperature, which is the subject of the present invention, will be addressed. 
       FIG. 2  is a graph  40  showing minimum allowable temperature, also known as minimum set point temperature, in the battery pack  14  as a function of state of charge and remaining useful life of the battery pack  14 . The graph  40  plots state of charge percentage in the battery pack  14  on horizontal axis  42 , and minimum allowable temperature in the battery pack  14  on vertical axis  44 . Curve  46  represents the minimum temperature at which the battery pack  14  may be operated, at any particular state of charge, if the battery pack  14  is at its beginning of life (BOL), or 100% remaining useful life. Curve  48  represents the minimum temperature at which the battery pack  14  may be operated, at any particular state of charge, if the battery pack  14  is at its end of life (EOL), or near 0% remaining useful life. The minimum allowable temperature shown by the curves  46  and  48  is determined by the required performance of discharge and charge power. Line  50  represents a constant or fixed minimum operating temperature which is typical of existing thermal management systems used in electric vehicles today. 
     Point  52  represents a set point temperature for an operating condition at which the battery pack  14  is at a state of charge of 75% and a remaining useful life of 65%. That is, the point  52  is 65% of the distance from the curve  48  to the curve  46 . Distance  54  shows the difference in set point temperature between the strategy of the present invention, which takes into account the state of charge and remaining useful life of the battery pack  14 , and a traditional thermal management strategy, which prescribes a fixed minimum operating temperature in the battery pack  14 . The distance  54  is proportional to the amount of heating energy saved by the variable set point strategy, which translates directly into increased driving range for the electric vehicle  12 . While units have been omitted from the graph  40 , it is noted that when the battery pack  14  has a high state of charge and is near beginning of life, the difference between the disclosed variable set point temperature and the traditional fixed set point temperature can be 20 Celsius degrees or more. 
     Point  56  represents a set point temperature for an operating condition at which the battery pack  14  is at a state of charge of 25% and a remaining useful life of 50%. Even in this state of relative discharge and age, the battery pack  14  can be operated at a temperature lower than that prescribed by a traditional fixed set point strategy. Only when the battery pack  14  reaches a very low state of charge and is near the end of its useful life does the minimum allowable operating temperature approach the traditional fixed set point temperature, as indicated at the far left end of the curve  48 . 
     In addition to the direct savings in heating energy described above, there is a secondary benefit to the strategy of operating the battery pack  14  at lower temperatures if battery pack conditions permit. The secondary benefit is that less energy may need to be expended on cooling the battery pack  14  later in an operational cycle. As is well known in the art, lithium ion battery packs generate heat when they are being discharged. Therefore, in a typical cool-weather scenario involving a traditional thermal management system with a fixed minimum temperature set point, the battery pack  14  would have to be heated up to the temperature represented by the line  50  on the graph  40  before vehicle operation could begin. Then, as the vehicle  12  is driven and the battery pack  14  discharges, the temperature in the battery pack  14  would rise, possibly to the point where the battery pack heating and cooling system  18  would have to be operated in cooling mode to prevent the battery pack  14  from getting too hot, to avoid impact on durability. The thermal management strategy of the present invention, by allowing driving operations to begin with the battery pack  14  at a much lower temperature when near full charge, greatly reduces the chance that the battery pack  14  will have to be cooled later in the same driving cycle. Also, using the thermal management strategy of the present invention, the overall temperature distribution is lower, which is beneficial for battery pack durability. 
     The shape of the curve  46  shown on the graph  40  was derived from extensive testing of lithium-ion battery packs. The testing measured battery pack performance as a function of temperature, state of charge, and life time degradation. While it is known in the art that the performance of lithium ion battery packs drops off significantly with decreasing temperature, the testing showed that the threshold temperature, below which the battery pack  14  no longer provides the required performance, is lower for a fully charged battery pack than for a nearly discharged battery pack, and that this threshold temperature is lower for a beginning of life battery pack than for a near end of life battery pack. In other words, a temperature which was low enough to cause significant performance degradation in a tested lithium-ion battery pack which was nearly discharged caused no significant performance degradation in the same test battery pack which was near full charge. Plotting the test data as the minimum allowable operating temperature at any given state of charge which maintains good battery pack performance, for a beginning of life battery pack, yields the curve  46 . Similarly, plotting the test data as the minimum allowable operating temperature at any given state of charge which maintains good battery pack performance, for an end of life battery pack, yields the curve  48 . 
     The variable set point temperature strategy can also save energy when recharging the battery pack  14 . When the battery pack  14  needs to be recharged, it will often be in a condition of low state of charge. In low ambient temperature conditions, the battery pack  14  may also be at a relatively low temperature. This low state of charge, low temperature condition is represented by point  58  on the graph  40 . Using a traditional thermal management strategy, the battery pack  14  might be recharged and warmed up to the temperature of the constant set point line  50 . But under the strategy of the present invention, the battery pack  14  can be recharged to a full charge condition, without additional heating, by adaptation of the recharge power to the battery performance according to state of charge, degradation, and temperature. Battery pack recharging naturally generates some heat, so that the battery pack temperature, when fully recharged, would be within the allowable operating range. By not adding heat from the heating and cooling system  18  during recharging, the variable set point temperature strategy reduces the amount of energy consumed from the electrical grid. 
     A control strategy based on the graph  40  can be programmed into the controller  22  to increase the driving range of the vehicle  12 , while ensuring good performance of the battery pack  14 , per the process described below. 
       FIG. 3  is a flow chart diagram  80  of a process which can be used by the controller  22  to manage the operation of the heating and cooling system  18 . At box  82 , the state of charge of the battery pack  14  is measured by the sensors  16 . At box  84 , the remaining useful life of the battery pack  14  is determined, either from the sensors  16 , or by way of a timetable built into the controller  22 . At box  86 , the controller  22  computes the minimum set point temperature at which the performance of the battery pack  14  will not drop off significantly, as a function of the state of charge from the box  82  and the remaining useful life from the box  84 . 
     While the curves  46  and  48  are shown on the graph  40  as generic shapes with no values given for temperature or state of charge, in actual practice the curves  46  and  48  would be specifically defined so that a minimum set point temperature (in degrees Celsius, for example) could be looked up for any given state of charge from 0% through 100%. The performance-based set point temperature would then be determined at the box  86  by interpolating between the curves  46  and  48 , based on the percentage of remaining useful life of the battery pack  14 . 
     At box  88 , the actual temperature in the battery pack  14  is measured by the sensors  16 . Battery pack temperature is provided to box  90  where an overall battery pack temperature control is defined. The box  90  determines whether to command heat from the heating and cooling system  18  at box  92 , or command cooling from the heating and cooling system  18  at box  94 , or neither. 
     As discussed above, the performance of lithium ion battery packs varies as a function of state of charge, remaining useful life, and temperature. In addition to this performance consideration, the durability of lithium ion battery packs may also be affected by operating temperature. To this end, a minimum allowable operating temperature based on durability considerations may also be defined. This durability-based minimum set point temperature is calculated at box  96  of the flow chart diagram  80 . The durability-based lower set point temperature is provided to the overall battery pack temperature control box  90 , which determines whether to command heating at the box  92  or cooling at the box  94 , or neither. The overall battery pack temperature control box  90  determines whether heating or cooling is needed based on both performance-based minimum temperature considerations and durability-based minimum temperature considerations, as well as maximum temperature limits which may need to be considered. Various rules can be defined for determining whether the performance-based set point temperature or the durability-based set point temperature takes precedence. 
     By using the control strategy described above, significant improvements in electric vehicle driving range can be realized, as energy is not used unnecessarily to add heat to the battery pack  14  before driving. Also, by using the variable set point temperature control strategy described above, the amount of energy consumed from the electrical grid is reduced, as the battery pack  14  is not unnecessarily heated during the charging process. 
     Along with electric vehicles, the thermal management strategy described above is also applicable to battery packs in hybrid gasoline/electric or fuel cell/electric vehicles, where the energy savings due to reduced battery pack heating result in improved fuel economy, which ultimately translates into increased driving range. 
     The foregoing discussion discloses 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.