Traction battery cooling system for an electrified vehicle

Abstract

Cooling system (10) for an electrified motor vehicle, comprising the following: a coolant circuit (12) that transfers coolant between a traction battery (14) and circulates in a battery cooler (15) or a cooling device (16); a refrigerant circuit (18) circulating refrigerant between a compressor (20), a condenser (22) and a first evaporator (241) or the cooling device (16); several flow control valves in the coolant circuit (12) and the refrigerant circuit (18); and a control system (50) comprising a controller (52) designed to (a) control the operation of the multiple flow control valves, and (b) prioritize interior cooling over traction battery cooling, wherein the multiple flow control valves comprise an electronic expansion valve (46) in the refrigerant circuit (18) between the condenser (22) and the cooling device (16), wherein the controller (52) is designed to include a first data input (54) for an ambient air temperature, a second data input (58) for an HVAC fan speed, a third data input (62) for an evaporator temperature, and a fourth data input (66) for a refrigerant temperature, which is monitored in the refrigerant circuit (18) between the cooling device (16) and the compressor (20), wherein the control system (50) further comprises an ambient temperature sensor (56) and an evaporator temperature sensor (64).

Description

TECHNICAL FIELD

[0001] This document relates generally to the technical field of automotive equipment and, more particularly, to a traction battery cooling system for an electrified motor vehicle. The cooling system may include an electronic expansion valve and a refrigerant-to-coolant heat exchanger cooling device. BACKGROUND

[0002] Hybrid electric vehicles and electric vehicles use an electric motor to propel the vehicle. The electric motor is powered by a traction battery. The traction battery is designed to store an electrical charge and can also be used to power other vehicle components. Efficient use of the traction battery can significantly increase the vehicle's range. Because battery energy availability is influenced by battery temperature, hybrid and electric vehicles generally incorporate a liquid cooling system for the traction battery. Many such systems utilize a refrigerant-to-coolant cooling device coupled with the vehicle's interior climate control system (heating, ventilation, and air conditioning system, HVAC system).Thus, using the cooling device to cool the traction battery can lead to temperature fluctuations in the conditioned air released into the vehicle interior. Such temperature fluctuations can be noticeable to the vehicle occupants and therefore undesirable.

[0003] The documents US 2013 / 0 298 583 A1 and DE 10 2017 101 218 A1 describe systems for battery cooling and interior air conditioning of a vehicle.

[0004] This document relates to a new and improved traction battery cooling system and controls for an electrified motor vehicle that limits or substantially eliminates these temperature variations under normal operating conditions, thereby increasing the comfort and satisfaction of vehicle occupants. SUMMARY

[0005] In accordance with the purposes and advantages described herein, a traction battery cooling system for an electrified motor vehicle is provided. The cooling system includes a coolant circuit, a refrigerant circuit, a plurality of flow control valves, and a control system.

[0006] The coolant circuit circulates coolant between the traction battery and a battery cooler and / or a cooling device. The refrigerant circuit circulates refrigerant between a compressor, a condenser, and a first cabin evaporator and / or the cooling device. The plurality of flow control valves are provided in the coolant circuit and in the refrigerant circuit. The control system includes a controller configured to (a) control operation of the plurality of flow control valves and (b) manage air conditioning capacity under normal vehicle operating conditions by prioritizing cabin cooling or battery cooling.Advantageously, the cooling system delays use of the cooling device for battery cooling until the HVAC load for the vehicle interior has decreased to the point where excess refrigerant evaporation capacity may exist, thereby reducing or substantially eliminating undesirable fluctuations in the conditioned air delivered by the HVAC system to the vehicle interior.

[0007] The plurality of flow control valves include an electronic expansion valve provided in the refrigerant circuit between the condenser and the cooling device.

[0008] The controller is configured to include a first data input for ambient air temperature. The controller is further configured to include a second data input for an HVAC fan speed. Additionally, the controller is configured to include a third data input for an evaporator temperature. Additionally, the control system further includes an ambient temperature sensor and an evaporator temperature sensor connected to the first and third data inputs, respectively.

[0009] The controller is also configured to include a fourth data input for a refrigerant temperature between the cooling device and the compressor and optionally a fifth data input for a refrigerant pressure between the cooling device and the compressor. Furthermore, the control system may include a refrigerant temperature sensor and a refrigerant pressure sensor in the refrigerant circuit between the cooling device and the compressor. In yet other embodiments, the refrigerant circuit may include a second evaporator provided in parallel with the first evaporator and the cooling device. For example, a minivan may include a front evaporator and a rear evaporator.

[0010] According to another aspect, a method is provided for controlling traction battery cooling while limiting temperature fluctuations of the conditioned air delivered to a cabin of an electrified motor vehicle. This method comprises the steps of (a) monitoring, by a first device, the ambient air temperature; (b) monitoring, by a second device, the HVAC fan speed; (c) monitoring, by a third device, the evaporator temperature; and (d) prioritizing, by a controller, cabin cooling over traction battery cooling based on HVAC load and evaporator fault.

[0011] The method may further include the step of determining, by the controller, an HVAC load based on an indicated HVAC fan speed and an indicated ambient air temperature. Additionally, the method may include the steps of (a) determining, by the controller, an evaporator fault by comparing an indicated evaporator temperature with a target evaporator temperature, and (b) determining, by the controller, an air conditioning capacity of the refrigeration device as a function of the evaporator fault and HVAC load.

[0012] The method may further comprise the step of monitoring, by a fourth device, a refrigerant temperature between the cooling device and the compressor. The method may further comprise the step of monitoring, by a fifth device, a refrigerant pressure between the cooling device and the compressor.

[0013] Furthermore, the method may also include other steps such as circulating coolant between the traction battery and the cooling device, and circulating refrigerant between a compressor, a condenser, and this cooling device. Furthermore, the method may include the step of controlling the flow of refrigerant through the cooling device by means of an electronic expansion valve provided in the refrigerant circuit between the condenser and the cooling device. Furthermore, the method may include the step of controlling, by the controller, the operation of the electric expansion valve based on (a) the available cooling device capacity, (b) the coolant temperature upstream of the traction battery, and (c) the refrigerant temperature between the cooling device and the compressor.

[0014] Furthermore, the method may include the step of determining, by the controller, a maximum opening position of the electronic expansion valve as a function of the air conditioning capacity of the refrigeration device and the compressor speed of the air conditioning system. Further, the method may include determining a position of the electronic expansion valve by summing a coolant temperature proportional-integral (PI) control output and a superheat PI control output. Then, a final electronic expansion valve position is determined as a function of the maximum opening position of the electronic expansion valve and the target opening position of the electronic expansion valve.

[0015] In the following description, several preferred embodiments of the cooling system and method of controlling traction battery cooling while limiting temperature fluctuations of the conditioned air delivered to the interior of an electrified motor vehicle are shown and described. As should be appreciated, other, differing embodiments of the cooling system and method are possible, and their several details are capable of modification in various obvious aspects without departing from the cooling system and method as set forth and described in the following claims. Accordingly, the drawings and descriptions are to be considered as illustrative and not restrictive. SHORT DESCRIPTION OF THE DRAWING FIGURES

[0016] The accompanying drawing figures, which form a part of the specification, illustrate various aspects of the cooling system and method and, together with the description, serve to explain certain principles of the invention. In the drawing figures: Fig. 1 a schematic block diagram of the traction battery cooling system. Fig. 2 a schematic block diagram of the control system for the Fig. Cooling system shown in Figure 1. Fig. 3 is a control logic flow diagram illustrating operation of a possible embodiment of the cooling system. Fig. 3a a detailed view of box 92 of the Fig. 3 shown control logic flow diagram. Fig. 4 a table indicating an HVAC load for a possible embodiment of the cooling system. Fig.5 is a table illustrating four different stages of chiller operation management for a possible embodiment of the refrigeration system based on load and evaporator fault.

[0017] Reference will now be made in detail to the present preferred embodiments of the traction battery cooling system, examples of which are illustrated in the accompanying drawing figures. DETAILED DESCRIPTION

[0018] Now reference is made to the Fig.1, which schematically illustrates the traction battery cooling system 10 designed for an electrified motor vehicle, such as a hybrid electric vehicle or an electric vehicle. The traction battery cooling system 10 includes a coolant circuit 12 for circulating a coolant between a traction battery 14 and a traction battery cooler 15 and / or a refrigerant-to-coolant cooling device 16 for heat exchange (refrigerant and coolant do not mix). The coolant may be a conventional coolant mixture, such as water and ethylene glycol.

[0019] The traction battery cooling system 10 further includes a refrigerant circuit, generally designated by reference numeral 18. The refrigerant circuit 18 circulates a refrigerant between an air conditioning compressor 20, a condenser 22, and one or both of the two evaporators 241 or 242 and / or the cooling device 16. The refrigerant may be a conventional refrigerant such as R134a or R1234yf.

[0020] As further stated in Fig.1, the traction battery cooling system 10 further includes a plurality of flow control valves in the coolant circuit 12 and the refrigerant circuit 18. In particular, the traction battery coolant valve (TBKV) 26 is provided in the coolant circuit 12 downstream of the traction battery 14, where it is configured to direct coolant flow through a first loop 30 between the traction battery 14 and the traction battery cooler 15 or a second loop 34 between the traction battery and the cooling device 16. A traction battery coolant pump 36 circulates the coolant through one or both loops, depending on the position of the traction battery coolant valve 26. A sensor 37 monitors the temperature of the coolant upstream of the traction battery 14.

[0021] The plurality of flow control valves further includes the front evaporator isolation valve 38 between the condenser 22 and the front evaporator 241 and the rear evaporator isolation valve 40 between the condenser and the rear evaporator 242. Additionally, a first expansion device 42 is provided in the refrigerant circuit 18 between the front evaporator isolation valve 38 and the front evaporator 241. Similarly, a second expansion device 44 is provided in the refrigerant circuit 18 between the rear evaporator isolation valve 40 and the rear evaporator 242. It should be understood that the evaporator isolation valve 38 and the first expansion device 42 could be combined into one device, if desired, to minimize potential refrigerant leakage paths. The isolation valve 40 and the second expansion device 44 could be similarly combined.Furthermore, an electronic expansion valve (EXV) 46 is provided in the refrigerant circuit 18 between the condenser 22 and the cooling device 16. The electronic expansion valve 46 has a full range of motion from zero flow to an open, full flow.

[0022] As in Fig. 2, the traction battery cooling system 10 further includes a control system, generally designated by the reference numeral 50. As shown, the control system 50 includes a controller 52. The controller 52 is a data processing device, such as a dedicated microprocessor or an electronic control unit (ECU) that operates according to instructions provided by control software. Such a controller 52 may include one or more processors, one or more memories, and one or more network interfaces, all of which communicate with each other via a communications bus.

[0023] The controller 52 is configured to (a) control operation of the plurality of flow control valves, including, among others, the traction battery coolant valve 26 and the electronic expansion valve 46, and (b) prioritize interior cooling over traction battery cooling during normal vehicle operation. To this end, the controller 52 is configured to include a first data input 54 connected to a sensor or other device 56, such as another controller, that provides data regarding ambient air temperature. The controller 52 is also configured to include a second data input 58 connected to a sensor or other device 60, such as another controller, for providing data input for an HVAC fan speed.

[0024] As further shown, the controller 52 is also configured to include a third data input 62 connected to a sensor or device 64, such as another controller, that provides data regarding a temperature of the evaporators 241, 242 (one sensor or device 64 per evaporator). The controller 52 is also configured to include a fourth data input 66 connected to a sensor or device 68, such as another controller, for providing data regarding the temperature of the refrigerant. Furthermore, the controller 52 is configured to include a fifth data input 70 connected to a sensor or device 72, such as another controller, for providing data regarding the pressure of the refrigerant in the refrigerant circuit 18.The devices 68 or devices 72 are located between the outlet of the cooling device 16 and the inlet of the air conditioning compressor 20.

[0025] Although this is not Fig. 2, it will be understood that the controller 52 may include additional data inputs connected to other sensors or devices, including other controllers, that provide data input regarding other system operating parameters, including, but not limited to, a battery temperature, a battery coolant temperature (see sensor 37 in Fig. 1), a refrigerant pressure (see sensor 21 in Fig.1) and interior cooling requirements from the motor vehicle's HVAC system. The controller 52 is generally configured to provide the cooling required for the traction battery 14 while consuming a minimal amount of vehicle power. To this end, the controller 52 is configured to operate in three different battery cooling modes. In the first mode, which consumes the least amount of vehicle power, the traction battery coolant valve 26 is positioned to circulate the coolant in the coolant circuit 12 through the first loop 30 between the traction battery 14 and the traction battery cooler 15.Ambient air forced through the traction battery cooler 15 during vehicle motion serves to cool the coolant, which is then circulated back through the traction battery 14 by the traction battery coolant pump 36 to maintain a desired operating temperature of the traction battery. If the temperature of the traction battery 14 rises to a certain predetermined temperature, the controller 52 operates in a second cooling mode in which the fan 74 is activated to force cooling air through the cooler, thereby providing additional cooling to the coolant and the traction battery 14 through which the coolant is circulated.

[0026] If the temperature of the traction battery 14 reaches a predetermined higher temperature while operating in the second cooling mode, the controller 52 initiates a third cooling mode by adjusting the traction battery coolant valve 26 to partially or completely direct the coolant through the second loop 34, so that the selected portion of the coolant flow is forced by the traction battery coolant pump 36 to circulate between the traction battery 14 and the cooling device 16. In this mode, the controller 52 is configured to prioritize interior cooling over traction battery cooling during normal vehicle operation.

[0027] To this end, a method is provided for controlling traction battery cooling while limiting temperature fluctuations of the conditioned air delivered to the interior of an electrified motor vehicle. This method includes the following steps: (a) monitoring, by a first device 56, the ambient air temperature; (b) monitoring, by a second device 60, the HVAC fan speed; (c) monitoring, by a third device 64, the evaporator temperature; and (d) prioritizing, by the controller 52, cabin cooling over traction battery cooling based on HVAC load and evaporator fault.

[0028] In particular, as in Fig. 3, upon initiation of the third cooling mode (see box 76), a cooling device request 78 is generated. In response, the controller 52 determines / calculates the evaporator error 80 and the HVAC load 82. As shown in Fig.4, the HVAC load is a function of ambient air temperature and HVAC fan speed and is therefore determined by the controller 52 based on data provided by the devices 56 or devices 60 at the respective data inputs 54, 58. In the Fig. In the embodiment of the traction battery cooling system 10 shown in Figure 4, an ambient temperature of 25 degrees Celsius and a fan speed percentage of 60% result in an HVAC load of 50.

[0029] An evaporator fault is determined by comparing the actual evaporator temperature indicated by the sensor or device 64 at the third data input 62 with a target evaporator temperature stored in the controller 52.

[0030] As in Fig.3, the controller 52 then determines the cooling device conditioning capacity available for traction battery cooling (see box 84) as a function of the determined evaporator fault and the HVAC load. Fig.5 illustrates a performance table for a particular embodiment of the traction battery cooling system 10. In this embodiment, an evaporator error of 3, 4, or 5 indicates that the cooler 16 is not available for any level of traction battery cooling (cooler capacity state 0). In contrast, an evaporator error of 2 and a load of 30 indicate that the cooler 16 is available at a minimum opening for battery cooling (cooler capacity state 1). Further, an evaporator error of 0.5 and a load of 60 indicate that the cooler is available for reduced cooler cooling of the traction battery 14 (cooler capacity state 2). Finally, an evaporator error of 0 and a load of 50 indicate that a full level of the cooler 16 is available for traction battery cooling (cooler capacity state 3).

[0031] As in Fig.3, the operating algorithm returns to the beginning when the cooling device's conditioning capacity is not available for cooling (cooler capacity state 0). In this situation, the battery temperature system continues cooling via the traction battery cooler 15 (with or without operation of the fan 74) and continues to request the cooling device 16. Once cooling device capacity becomes available, the battery temperature system transitions to the cooling device 16. In other words, there are times when the battery cooling mode may request the cooling device 16 but continue to run in loop 30. The current algorithm for determining cooler availability may be a closed-loop algorithm that opens the electronic expansion valve 46 based on the available capacity.

[0032] The steps shown are merely examples of how the expansion valve 46 opens in relation to the available cooling device capacity.

[0033] In contrast, if evaporator cooling is available for the traction battery 14, the level of cooling or the desired opening position of the electronic expansion valve 46 is determined as a function of the chiller capacity state and the compressor speed, as shown in box 90. Thus, for example, for a chiller capacity state of 1 and a compressor speed of 3000 rpm, the electronic expansion valve 46 is opened 20%. For a chiller capacity state of 2 and a compressor speed of 6000 rpm, the electronic expansion valve 46 is opened 30%. The desired position is then relayed to the electronic expansion valve controller, which would ensure that the electronic expansion valve 46 moves to the desired opening amount.

[0034] Once the controller 52 determines that the chiller capacity is fully available (chiller capacity state 3), the position of the electronic expansion valve 46 is determined by the box 92 (see Fig. 3 and Fig.3a). Consequently, the controller 52 performs temperature and superheat control in parallel. Superheat is the temperature of a substance above the saturation temperature at a constant pressure. The controller 52 implements a PI control to control the coolant temperature in the battery based on a calibratable target coolant temperature. The electronic expansion valve 46 makes position adjustments based on the error in these two temperatures. If the actual refrigerant superheat between the chiller 16 and the air conditioning compressor 20 drops below a minimum superheat value, the controller 52 activates a superheat PI control, along with the temperature PI control (see Box 92 in Fig.3a). The refrigerant superheat control makes position adjustments to the electronic expansion valve 46 based on the current superheat and a calibratable target superheat. These controls are performed continuously in the controller 52; however, in chiller states 1 and 2, the maximum limit on the position of the electronic expansion valve 46 is applied based on the compressor speed, see box 90, thereby limiting the opening position of the electronic expansion valve 46. The controller 52 also adjusts the speed of the air conditioning compressor 20 as needed to provide the desired cooling function for the vehicle interior and the traction battery 14 (see box 94).

[0035] In summary, numerous advantages and benefits are provided by the traction battery cooling system 10. The controller 52 actuates the electronic expansion valve 46 to directly throttle the flow of refrigerant to the cooling device 16. To perform this task, the controller 52 actively controls the electronic expansion valve 46 based on the refrigerant temperature and pressure, as indicated by the devices 68 or devices 72 at the respective data inputs 66, 70 of the controller. As shown in Fig. As shown in Figure 1, these devices 68 or devices 72 are provided in the refrigerant circuit 18 at the cooler outlet to ensure that the refrigerant is properly evaporated by the cooler. This is done to prevent the possibility of liquid refrigerant being sent to the air conditioning compressor 20, as this could lead to compressor failure.

[0036] As should further be understood from the foregoing description, the controller 52 monitors the ambient conditions of the motor vehicle interior to determine when to start the cooling device 16 and how to manage the total air conditioning capacity thereafter. Consequently, the availability of cooling device capacity for traction battery cooling is a function of the air conditioning load. If the interior is not cool or the capacity to cool the interior is at its limit, the start of traction battery cooling via the cooling device 16 is delayed. Specifically, the circulation of refrigerant to the cooling device 16 may be delayed by the controller 52 until the air conditioning load for the interior has stabilized and is below the total air conditioning capacity. In the meantime, the traction battery 14 is cooled by the battery cooler 32 in the first or second mode.

[0037] Advantageously, the controller 52 can regulate the flow of refrigerant to the cooling device 16 at any time. This allows traction battery cooling via the cooling device 16 to be started smoothly and carried out continuously with minimal impact on the interior. As a result, adverse fluctuations in the temperature of the conditioned air passing through the evaporators 241, 242 and later released into the interior are minimized. The traction battery cooling system 10 and associated method prioritize interior cooling by delaying the cooling device start-up and also provide a method for operating at reduced cooling capacity to maintain interior comfort under changing air conditioning capacity conditions.Only under extreme operating conditions where traction battery temperatures rise to predetermined critical levels does the controller 52 prioritize battery cooling over interior cooling by first providing the cooling device 16 with full air conditioning capacity for traction battery cooling and the remaining air conditioning capacity for the interior.

[0038] The foregoing is for illustrative and descriptive purposes. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. For example, the interior temperature could be a system input to be monitored and utilized in controlling the traction battery cooling system 10. All such modifications and variations are within the scope of the appended claims when interpreted with the breadth to which they are fairly, legally, and equitably entitled. List of reference symbols 10 Cooling system, traction battery cooling system 12 Coolant circuit 14 Traction battery 15 Battery coolers, traction battery coolers 16 Cooling device 18 Refrigerant circuit 20 compressors, air conditioning compressors 22 Capacitor 241 first evaporator, front evaporator 242 second evaporator, rear evaporator 26 Traction battery coolant valve 30 first loop 32 battery coolers 34 second loop 36 Traction battery coolant pump 37 Sensor 38 Shut-off valve 40 Shut-off valve 42 first expansion device 44 second expansion device 46 Expansion valve 50 Tax system 52 Control 54 first data input 56 Ambient temperature sensor, device 58 Data input 60 Sensor, device 62 third data input 64 Evaporator temperature sensor, device 66 fourth data input 68 Sensor, device 70 fifth data input 72 Sensor, device 74 fans 76 boxes 78 Cooling device request 80 Evaporator error 82 HVAC load 84 boxes 90 boxes 92 boxes 94 boxes Key to symbols Fig. 3 76 start 78 Cooling device requirement true? 80 Calculate evaporator errors 82 Calculate load 84 Available air conditioning capacity of the cooling device = Funct. (Load, Evaporator Error) 86 Available air conditioning capacity state of the cooling device > 0 88 Translating the available air conditioning capacity of the cooling device into EXV control 92 Position control logic EXV 94 Fixed compressor speed Fig. 3a SH: Super Heat Min_SH_Value: Minimum Super Heat value at which AC refrigerator is approaching saturation temperature. Decreasing or running the AC system below this value could lead to liquid returning to the AC compressor. Superheat Min_Superheat_Value: Minimum superheat value at which the HVAC refrigerant approaches its saturation temperature. Lowering or operating the HVAC system below this value could result in fluid backflow into the HVAC compressor. box Crate If Actual SH below Min_SH_Value If current ÜH is below Min ÜH value Yes Yes No No Pressure and temperature Pressure and temperature Actual SH Current ÜH Calibratable Target SH Calibrable target ÜH Error Mistake Error=Actual-Target Error = Actual - Target PI controller based on Super Heat PI control based on superheat EXV Adjustment Setting EXV Sum Sum EXV position Max Clip Max. EXV position limit EXV position Position of the EXV Temperature Controller Temperature control Battery Coolant Temperature Battery coolant temperature Target Battery Coolant Temperature Target battery coolant temperature PI controller based on Battery Coolant Temperature PI control based on battery coolant temperature

Claims

[1] Cooling system (10) for an electrified motor vehicle, comprising: a coolant circuit (12) which supplies coolant between a traction battery (14) and a battery cooler (15) or a cooling device (16); a refrigerant circuit (18) which circulates refrigerant between a compressor (20), a condenser (22) and a first evaporator (241) or the cooling device (16); a plurality of flow control valves in the coolant circuit (12) and the refrigerant circuit (18); and a control system (50) comprising a controller (52) configured to (a) control operation of the plurality of flow control valves, and (b) prioritize interior cooling over traction battery cooling, wherein the plurality of flow control valves comprise an electronic expansion valve (46) in the refrigerant circuit (18) between the condenser (22) and the cooling device (16), wherein the controller (52) is configured to include a first data input (54) for an ambient air temperature, a second data input (58) for an HVAC fan speed, a third data input (62) for an evaporator temperature, and a fourth data input (66) for refrigerant temperature monitored in the refrigerant circuit (18) between the cooling device (16) and the compressor (20), wherein the control system (50) further comprises an ambient temperature sensor (56) and an evaporator temperature sensor (64). [2] The refrigeration system (10) of claim 1, wherein the controller (52) is further configured to include a fifth data input (70) for refrigerant pressure monitored in the refrigerant circuit (18) between the cooling device (16) and the compressor (20). [3] The refrigeration system (10) of claim 2, wherein the control system (50) further comprises a refrigerant temperature sensor (68) and a refrigerant pressure sensor (72) in the refrigerant circuit (18) between the cooling device (16) and the compressor (20). [4] Cooling system (10) according to claim 3, wherein the refrigerant circuit (18) comprises a second evaporator (242) parallel to the first evaporator (241). [5] A method for controlling traction battery cooling while limiting temperature fluctuations of conditioned air delivered into an interior of an electrified motor vehicle, comprising: Monitoring, by a first device (56), an ambient air temperature; Monitoring, by a second device (60), an HVAC fan speed; Monitoring, by a third device (64), an evaporator temperature; and Prioritizing, by a controller (52), interior cooling over traction battery cooling based on an HVAC load and an evaporator fault. [6] The method of claim 5, further comprising determining, by the controller (52), an HVAC load based on an indicated HVAC fan speed and an indicated ambient air temperature. [7] The method of claim 6, further comprising: (a) determining, by the controller (52), an evaporator fault by comparing an indicated evaporator temperature with a desired evaporator temperature; and (b) determining, by the controller (52), an air conditioning capacity state of the refrigeration device (16) as a function of evaporator fault and HVAC load. [8] The method of claim 7, further comprising monitoring, by a fourth device (68), a refrigerant temperature. [9] The method of claim 8, further comprising monitoring, by a fifth device (72), a refrigerant pressure. [10] The method of claim 9, comprising circulating coolant between the traction battery (14) and a cooling device (16) and circulating refrigerant between a compressor (20), a condenser (22) and the cooling device (16). [11] A method according to claim 10, comprising controlling the flow of refrigerant through the cooling device (16) by means of an electronic expansion valve (46) in a refrigerant circuit (18) between the condenser (22) and the cooling device (16). [12] The method of claim 11, comprising controlling, by the controller (52), the operation of the electronic expansion valve (46) based on (a) the available chiller capacity, (b) the coolant temperature upstream of the traction battery (14), and (c) the refrigerant superheat temperature between the chiller (16) and the compressor (20). [13] A method according to any one of claims 7 to 12, comprising determining, by the controller (52), a maximum opening position of the electronic expansion valve (46) as a function of the air conditioning capacity state of the refrigeration device (16) and the compressor speed of the air conditioning system. [14] The method of claim 13, comprising determining, by the controller (52), a desired opening position of the electronic expansion valve (46) by summing an output of a coolant temperature PI controller and an output of a superheat PI controller, wherein a final opening position of the electronic expansion valve (46) is determined as a function of the maximum opening position of the electronic expansion valve (46) and the desired opening position of the electronic expansion valve (46).

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