Patent Publication Number: US-2022223937-A1

Title: System and method for resonant heating battery

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to a battery or fuel cell for an electric vehicle. More specifically, the present disclosure relates to a system and method for heating a battery or fuel cell for an electric vehicle. 
     BACKGROUND 
     Electric vehicles (EVs) include battery electric vehicles (BEVs) and fuel cell electric vehicles (FCEVs). In general, the energy storage system (ESS) of EVs is designed to provide optimized performance at a nominal temperature range such as between 20° C. and 90° C. In the winter, ambient temperatures may be significantly lower than the nominal temperature range, in which case the battery or fuel cells may perform differently. 
     SUMMARY 
     A vehicle includes an electric machine, a battery, an inverter coupled between the electric machine and battery, and a controller. The controller switches the inverter at a switching frequency selected to generate an AC current to heat the battery, adjusts a d-axis current of the electric machine to increase a battery heating power without changing the switching frequency selected to generate the AC current to heat the battery, and adjusts a q-axis current of the electric machine according to the adjusted d-axis current. 
     A method includes, responsive to a traction battery temperature being less than a threshold, switching an inverter operatively arranged with the traction battery at a frequency selected to generate heating power for the traction battery, and responsive to the heating power being less than a required heating power, adjusting a d-axis current of an electric machine operatively arranged with the inverter to increase the heating power towards the required heating power, and adjusting a q-axis current according to the adjusted d-axis current. 
     A power system includes a fuel cell, an inverter operatively arranged with the fuel cell, and a controller. The controller switches the inverter at a switching frequency selected to generate heating power for the fuel cell, adjusts a d-axis current to increase the heating power, and adjusts a q-axis current according to the adjusted d-axis current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example block topology of an electrified vehicle illustrating drivetrain and energy storage components; 
         FIG. 2  illustrates an example block diagram of a portion of an electric drive system for a vehicle; 
         FIG. 3  illustrates an example waveform diagram of battery heating power at various frequencies; 
         FIG. 4  illustrates example waveform diagrams for switching signals for switching devices; 
         FIG. 5  illustrates an example waveform diagram for an inverter DC side current ratio; 
         FIG. 6  illustrates an example waveform diagram for a battery current ratio and battery heating power percentage; 
         FIG. 7  illustrates an example flow diagram for a battery heating process. 
         FIGS. 8A, 8B, and 8C  illustrate example waveform diagrams of battery heating power varied by d-axis current; and 
         FIG. 9  illustrates an example waveform diagram for a fuel cell current ratio and fuel cell heating power percentage. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     The present disclosure, among other things, proposes a system and method for heating a battery or fuel cell for an electric vehicle. 
       FIG. 1  illustrates a plug-in hybrid-electric vehicle (PHEV). A plug-in hybrid-electric vehicle  112  may comprise one or more electric machines (electric motors)  114  mechanically coupled to a hybrid transmission  116 . The electric machines  114  may be capable of operating as a motor or a generator. In addition, the hybrid transmission  116  is mechanically coupled to an engine  118 . The hybrid transmission  116  is also mechanically coupled to a drive shaft  120  that is mechanically coupled to the wheels  122 . The electric machines  14  may provide propulsion and deceleration capability when the engine  118  is turned on or off. The electric machines  114  may also act as generators and may provide fuel economy benefits by recovering energy that would be lost as heat in the friction braking system. The electric machines  114  may also reduce vehicle emissions by allowing the engine  118  to operate at more efficient speeds and allowing the hybrid-electric vehicle  112  to be operated in electric mode with the engine  18  off under certain conditions. 
     A traction battery or battery pack  124  stores energy that may be used by the electric machines  114 . A vehicle battery pack  124  may provide a high voltage DC output. The traction battery  124  may be electrically coupled to one or more battery electric control modules (BECM)  125 . The BECM  125  may be provided with one or more processors and software applications configured to monitor and control various operations of the traction battery  124 . The traction battery  124  may be further electrically coupled to one or more power electronics modules  126 . The power electronics module  126  may also be referred to as a power inverter. One or more contactors  127  may isolate the traction battery  124  and the BECM  125  from other components when opened and couple the traction battery  124  and the BECM  125  to other components when closed. The power electronics module  126  may also be electrically coupled to the electric machines  114  and provide the ability to bi-directionally transfer energy between the traction battery  124  and the electric machines  114 . For example, the traction battery  124  may provide a DC voltage while the electric machines  114  may operate using a three-phase AC current. The power electronics module  126  may convert the DC voltage to a three-phase AC current for use by the electric machines  114 . In a regenerative mode, the power electronics module  126  may convert the three-phase AC current from the electric machines  114  acting as generators to the DC voltage compatible with the traction battery  124 . The description herein is equally applicable to a pure electric vehicle. For a pure electric vehicle, the hybrid transmission  116  may be a gear box connected to the electric machine  114  and the engine  118  may not be present. 
     In addition to providing energy for propulsion, the traction battery  124  may provide energy for other vehicle electrical systems. The vehicle  112  may include a DC/DC converter module  128  that converts the high voltage DC output of the traction battery  124  to a low voltage DC supply that is compatible with other low-voltage vehicle loads. An output of the DC/DC converter module  128  may be electrically coupled to an auxiliary battery  130  (e.g., 12V battery). Other high-voltage loads  146 , such as compressors and electric heaters, may be coupled to the high-voltage output of the traction battery  124 . 
     The vehicle  112  may be battery electric vehicle (BEV) or a plug-in hybrid electric vehicle (PHEV) in which the traction battery  124  may be recharged by an external power source  136 . Alternatively, the vehicle  112  may be fuel cell electric vehicle (FCEV) propelled by energy stored and/or converted in one or more fuel cells. The external power source  136  may be a connection to an electrical outlet. The external power source  136  may be an electrical power distribution network or grid as provided by an electric utility company. The external power source  136  may be electrically coupled to electric vehicle supply equipment (EVSE)  138 . The EVSE  138  may provide circuitry and controls to regulate and manage the transfer of energy between the power source  136  and the vehicle  112 . The external power source  136  may provide DC or AC electric power to the EVSE  138 . The EVSE  138  may have a charge connector  140  for plugging into a charge port  134  of the vehicle  112 . The charge port  134  may be any type of port configured to transfer power from the EVSE  138  to the vehicle  112 . The charge port  134  may be electrically coupled to a charger or on-board power conversion module  132 . The power conversion module  132  may condition the power supplied from the EVSE  138  to provide the proper voltage and current levels to the traction battery  124 . The power conversion module  132  may interface with the EVSE  138  to coordinate the delivery of power to the vehicle  112 . The EVSE connector  140  may have pins that mate with corresponding recesses of the charge port  134 . Alternatively, various components described as being electrically coupled may transfer power using wireless inductive coupling. 
     One or more wheel brakes  144  may be provided for decelerating the vehicle  112  and preventing motion of the vehicle  112 . The wheel brakes  144  may be hydraulically actuated, electrically actuated, or some combination thereof. The wheel brakes  144  may be a part of a brake system  146 . The brake system  146  may include other components to operate the wheel brakes  144 . For simplicity, the figure depicts a single connection between the brake system  146  and one of the wheel brakes  144 . A connection between the brake system  146  and the other wheel brakes  144  is implied. The brake system  146  may include a controller to monitor and coordinate the brake system  146 . The brake system  146  may monitor the brake components and control the wheel brakes  144  for vehicle deceleration. The brake system  146  may respond to driver commands and may also operate autonomously to implement features such as stability control. The controller of the brake system  146  may implement a method of applying a requested brake force when requested by another controller or sub-function. 
     One or more electrical loads  148  may be coupled to the high-voltage bus. The electrical loads  148  may have an associated controller that operates and controls the electrical loads  148  when appropriate. Examples of electrical loads  148  may be a heating module, an air-conditioning module or the like. 
     The various components discussed may have one or more associated controllers to control and monitor the operation of the components. The controllers may communicate via a serial bus (e.g., Controller Area Network (CAN)) or via discrete conductors. A vehicle system controller (VSC)  150  may be present to coordinate the operation of the various components. 
     The electric machines  114  may be coupled to the power electronics module  126  via one or more conductors that are associated with each of the phase windings.  FIG. 2  depicts a block diagram of a portion of an electric drive system for a vehicle. The vehicle  112  may include one or more power electronics controllers  200  configured to monitor and control the components of the power electronics module  126 . The power electronics controllers  200  may be under a global control or coordination of the VSC  150 . 
     The conductors may be part of a wiring harness between the electric machine  114  and the power electronics module  126 . A three-phase electric machine  114  may have three conductors coupled to the power electronics module  126 . The power electronics module  126  may be configured to switch positive and negative terminals of the high-voltage (HV) bus (HV rail)  204  to phase terminals of the electric machines  114 . The power electronics module  126  may be controlled to provide a pulse-width modulated (PWM) voltage and sinusoidal current signals to the electric machine  114 . The duty ratio of the signals may be proportional to the rotational speed of the electric machine  114 . The controller  200  may be configured to adjust the voltage and current output of the power electronics module  126  at one or more predetermined switching frequencies. The switching frequency may be the rate at which the states of switching devices within the power electronics module  126  are changed. 
     The power electronics module  126  may interface with a position/speed feedback device  202  that is coupled to the rotor of the electric machine  114 . For example, the position/speed feedback device  202  may be a resolver or an encoder. The position/speed feedback device  202  may provide signals indicative of a position and/or speed of the rotor of the electric machine  114 . The power electronics  126  may include the power electronics controller  200  that interfaces to the speed feedback device  202  and processes signals from the speed feedback device  202 . The power electronics controller  200  may be programmed to utilize the speed and position feedback to control the power electronics module  126  to operate the electric machine  114 . 
     The traction inverter or power electronics module  126  may include power switching circuitry  206  that includes a plurality of switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220 . The switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may be Insulated Gate Bipolar Transistors (IGBT), Metal Oxide Semiconductor Field Effect Transistors (MOSFET), or other solid-state switching devices. The switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may be configured to selectively couple a positive terminal and a negative terminal of the high-voltage bus  204  to each phase terminal or leg (e.g., labeled U, V, W) of the electric machine  114 . The power electronics  126  may be configured to provide a U-phase voltage, a V-phase voltage and a W-phase voltage to the electric machine  114 . Each of the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  within the power switching circuitry  240  may have an associated diode  222 ,  224 ,  226 ,  228   230 ,  232  connected in parallel to provide a path for inductive current when the switching device is in a non-conducting state. Each of the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may have a control terminal for controlling operation of the associated switching device. The control terminals may be electrically coupled to the power electronics controller  200 . The power electronics controller  200  may include associated circuitry to drive and monitor the control terminals. For example, the control terminals may be coupled to the gate input of the solid-state switching devices. 
     A phase leg of the power electronics module  126  may include switching devices and circuitry configured to selectively connect a phase terminal of the electric machine  114  to each terminal of the high-voltage bus  204 . A first switching device S 1   210  may selectively couple the HV bus positive terminal  204   a  to a first phase terminal (e.g., U) of the electric machine  114 . A first diode  222  may be coupled in parallel to the first switching device S 1   210 . A second switching device S 2   212  may selectively couple the HV bus negative terminal  204   b  to the first phase terminal (e.g., U) of the electric machine  114 . A second diode  224  may be coupled in parallel to the second switching device S 2   212 . A first inverter phase leg may include the first switching device S 1   210 , the first diode  222 , the second switching device S 2   212 , and the second diode  224 . 
     A third switching device S 3   214  may selectively couple the HV bus positive terminal  204   a  to a second phase terminal (e.g., V) of the electric machine  114 . A third diode  226  may be coupled in parallel to the third switching device S 3   214 . A fourth switching device S 4   216  may selectively couple the HV bus negative terminal  204   b  to the second phase terminal (e.g., V) of the electric machine  114 . A fourth diode  228  may be coupled in parallel to the fourth switching device S 4   216 . A second inverter phase leg may include the third switching device S 3   214 , the third diode  226 , the fourth switching device S 4   216 , and the fourth diode  228 . 
     A fifth switching device S 5   218  may selectively couple the HV bus positive terminal  204   a  to a third phase terminal (e.g., W) of the electric machine  114 . A fifth diode  230  may be coupled in parallel to the fifth switching device S 5   218 . A sixth switching device S 6   220  may selectively couple the HV bus negative terminal  204   b  to the third phase terminal (e.g., W) of the electric machine  114 . A sixth diode  232  may be coupled in parallel to the sixth switching device S 6   220 . A third inverter phase leg may include the fifth switching device S 5   218 , the fifth diode  230 , the sixth switching device S 6   220 , and the sixth diode  232 . 
     The power switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may include two terminals that handle the high-power current through the power switching device. For example, an IGBT includes a collector (C) terminal and an emitter (E) terminal and a MOSFET includes a drain terminal (D) and a source (S) terminal. The power switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may further include one or more control inputs. For example, the power switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may include a gate terminal (G) and a Kelvin source/emitter (K) terminal. The G and K terminals may comprise a gate loop to control the power switching device. 
     The power electronics module  126  may be configured to flow a rated current and have an associated power rating. Some applications may demand higher power and/or current ratings for proper operation of the electric machine  114 . The power switching circuitry  206  may be designed to include power switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  that can handle the desired power/current rating. The desired power/current rating may also be achieved by using power switching devices that are electrically coupled in parallel. Power switching devices may be electrically coupled in parallel and controlled with a common control signal so that each power switching device flows a portion of the total current flowing to/from the load. 
     The power electronics controller  200  may be programmed to operate the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  to control the voltage and current applied to the phase windings of the electric machine  114 . The power electronics controller  200  may operate the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  so that each phase terminal is coupled to only one of the HV bus positive terminal  204   a  or the HV bus negative terminal  204  at a particular time. 
     Various motor control algorithms and strategies are available to be implemented in the power electronics controller  200 . The power electronics module  126  may also include current sensors  234 . The current sensors  234  may be inductive or Hall-effect devices configured to generate a signal indicative of the current passing through the associated circuit. In some configurations, two current sensors  234  may be utilized and the third phase current may be calculated from the two measured currents. The controller  200  may sample the current sensors  234  at a predetermined sampling rate. Measured values of the phase currents of the electric machine  114  may be stored in controller memory for later computations. 
     The power electronics module  126  may include one or more voltage sensors. The voltage sensors may be configured to measure an input voltage to the power electronics module  126  and/or one or more of the output voltages of the power electronics module  126 . The power electronics module  126  may include a line voltage sensor  236  that is configured to measure a line voltage across the V and W phase outputs. The voltage may be a voltage difference between the V-phase voltage and the W-phase voltage. The voltage sensors may be resistive networks and include isolation elements to separate high-voltage levels from the low-voltage system. In addition, the power electronics module  126  may include associated circuitry for scaling and filtering the signals from the current sensors  234  and the voltage sensors. In some configurations, each phase leg of the inverter may have corresponding voltage and current sensors. 
     Under normal/discharge operating conditions, the power electronics controller  200  controls operation of the electric machine  114 . For example, in response to torque and/or speed setpoints, the power electronics controller  200  may operate the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  to control the torque and speed of the electric machine  114  to achieve the setpoints. The torque and/or speed setpoints may be processed to generate a desired switching pattern for the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220 . The control terminals of the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may be driven with PWM signals to control the torque and speed of the electric machine  114 . The power electronics controller  200  may implement various well-known control strategies to control the electric machine  114  using the switching devices such as vector control and/or six-step control. During discharge operating conditions, the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  are actively controlled to achieve a desired current through each phase of the electric machine  114 . 
     Under regenerative/charge operating conditions (e.g. regenerative mode), the power electronics controller  200  may control the power electronics module  126  to accommodate power generated by the electric machine  114 . For example, the power electronics controller  200  may operate the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  to convert AC power generated by the electric machine  114  to DC current to charge the traction battery  124  via the high-voltage rail  152 . The power electronics controller  200  may implement various well-known control strategies to perform the regenerative operation. 
     The power electronics module  126  may also include one or more bus capacitors  240  that are coupled across the positive and negative terminals of the HV bus  204 . The bus capacitors  260  may smooth the voltage of the positive terminal  204   a  as well as the voltage of the negative terminal  204   b  of the HV bus. As illustrated in  FIG. 2 , the bus capacitor  240  may be integrated with the power electronics module  126 . Alternatively, the bus capacitor  240  may be independent components outside the power electronics module  126 . The traction battery  124  may include a stray/parasitic inductance L b    242  caused by various electronic components of the traction battery  124 . Alternatively, an extra inductor may be used to add inductance L b  to the battery circuit depending on design needs. With the inductance  242  and the capacitor  240 , an LC resonant circuit may be achieved with appropriate switching frequency on the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220 . The resonant frequency of the circuit may be calculated by the following equation: 
     
       
         
           
             
               
                 
                   
                     f 
                     
                       resonan 
                       ⁢ 
                       t 
                     
                   
                   = 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                       ⁢ 
                       
                         
                           
                             L 
                             b 
                           
                           ⁢ 
                           C 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     When the LC resonant current flows through an internal resistance  244  of the traction battery  124 , heat may be generated to warm the battery cells. The internal resistance  244  may be inherently formed by various components such as wires and cells within the traction battery  124 . Alternatively, one or more resistors having a predefined value may be added to the traction battery  124  to provide a desired heating effect depending the specific design need. Since the switching frequency may be actively controlled by the power electronics controller  200 , the current on the inverter side I inv  may have a major harmonic component at or near the resonant frequency of the circuit. The heating power may be manipulated by adjusting the switching frequency. The current flowing through the battery resistor I R  may be calculated using the following equation: 
     
       
         
           
             
               
                 
                   
                     I 
                     R 
                   
                   = 
                   
                     
                       I 
                       
                         i 
                         ⁢ 
                         n 
                         ⁢ 
                         v 
                       
                     
                     * 
                     
                       
                         1 
                         / 
                         
                           ( 
                           
                             j 
                             * 
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             fC 
                           
                           ) 
                         
                       
                       
                         R 
                         + 
                         
                           j 
                           * 
                           2 
                           ⁢ 
                           π 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           fL 
                         
                         + 
                         
                           1 
                           / 
                           
                             ( 
                             
                               j 
                               * 
                               2 
                               ⁢ 
                               π 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               fC 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
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     wherein j represents a unit imaginary number which is equal to √{square root over (−)}1. 
     Referring to  FIG. 3 , an example waveform diagram  300  illustrating battery heating power adjusted by the switching frequency at multiple vehicle speeds is illustrated. With continuing reference to  FIGS. 1 and 2 , in the present example the battery heating power may be adjustable by varying the switching frequency on the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220 . The horizontal axis of the waveform diagram  300  represents a ratio between the switching frequency and the resonant frequency of the circuit (i.e., f switching /f resonant ). In the present example, given that the dominant content frequency may occur at the second harmonic of the switching frequency, the present example may purposely operate at a ratio of 0.5 in order to excite dominant content at the resonant frequency. The vertical axis represents a percentage of battery heating power as compared with a reference heating power applied to the battery  124  via an external heater (not shown). For instance, the external heater may have a heating power of 4 kW in the present example for demonstrative purposes although other reference heating power may be used in other examples under substantially the same principle of the present disclosure. The waveform diagram  300  includes 3 waveforms  302 ,  304 ,  306 , each corresponds to a different vehicle speed. The first waveform  302  illustrates the heating power percentage at substantially zero vehicle speed; the second waveform  304  illustrates the heating power percentage when the vehicle  112  is operating at 5% of the maximum vehicle speed; the third waveform  306  illustrates the heating power percentage when the vehicle  112  is operating at 10% of the maximum vehicle speed. Taking the third waveform  306  at 10% of maximum speed for instance, the battery heating power is the maximum at around 40% (e.g. point C) when the switching frequency is around half of the resonant frequency. The battery heating power reduces significantly when the switching frequency is above resonant heating power (e.g. point A). The battery heating power may be substantially adjustable between 0.25 and 0.75 of the resonant frequency. For instance, the battery heating power may be reduced to 30% by adjusting the switching frequency to 0.45 or 0.55 of the resonant frequency (points B and D). The second waveform  304  at 5% of the maximum speed generally follows the same trend although the specific heating power number is lower due to a lower vehicle speed. Since the duty ratio of the switch signals for the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  is substantially zero at zero speed, as illustrated on the waveform  302 , the heating power may be limited. The waveforms illustrated in  FIG. 3  are merely examples of the present disclosure and the maximum heating power may be achieved at other switching frequencies. For instance, the maximum heating power may be achieved at k*(f resonant /2) frequency, wherein k is an integer equal to or greater than 1. 
     Referring to  FIG. 4 , example waveform diagrams  400  for the switching signals (gate signals) for the switching devices S 1   210 , S 3   214  and S 5   218  are illustrated. Although the waveform diagrams  400  only illustrate the switching devices S 1   210 , S 3   214  and S 5   218  connecting the phase terminals U, V, W of the power electronics module  126  to the positive HV bus  204   a , the corresponding switching devices S 2   212 , S 4   216 , S 6   220  connecting the phase terminals U, V, W to the negative HV bus  204   b  are controlled in a complementary manner because only one terminal of the HV bus  204  may be connected at a particular time. In the present example, the switching devices are being switched at around half of the resonant frequency of the circuit. The switching signals illustrated in  FIG. 4  may cause a DC current on the power electronics module  126 .  FIG. 5  illustrates a waveform  500  for a current on the DC side of the power electronics module  126  normalized to a motor-rated reference current. The DC current  502  illustrated in  FIG. 5  may cause an AC current  602  illustrated in  FIG. 6  in a normalized manner. As the AC current  602  flows through the battery resistor  244  of the battery  124 , the heating effect may be achieved. As illustrated in  FIG. 6 , the battery heating power is presented in waveform  604  normalized to the external reference heater of 4 kW power. 
     Referring to  FIG. 7 , a flow diagram for a battery heating process  700  is illustrated. With continuing reference to  FIGS. 1-6 , at operation  702 , responsive to receiving a torque demand T*, the power electronics controller  200  determines a current command I d  on a d-axis and a current command I q  on a q-axis of the electric machine  114 . The torque demand T* may be obtained by the power electronics controller  200  responsive to a user input to an accelerator pedal (not shown) of the vehicle. For instance, the torque demand T* may be substantially proportional to a force/depth of depression applied to the accelerator pedal. With the torque demand T* obtained, the power electronics controller  200  may determine the current command I d  and I q  on the d-axis and q-axis using one or more lookup tables. For instance, a one-dimensional lookup table for the torque demand T* and the current command I d  on the d-axis may be used to determine the d current command I d , and another one-dimensional lookup table for the torque demand T* and the current command I q  on the q-axis may be used to determine the q current command I q . Alternatively, current command may further vary by the rotor speed ω rm . Therefore, the d-axis current command may be determined using a two-dimensional lookup table corresponding to the torque command T* and rotor speed ω rm , and the q-axis current command may be determined using another two-dimensional lookup table corresponding to the torque command T* and rotor speed ω rm . At operation  704 , the power electronics controller  200  determines the maximum heating power that is available under the dq current command as determined. The maximum heating power may be determined using a lookup table that records a corresponding relationship between the current commands and the maximum heating power. The lookup table may further record other factors such as frequency, torque and speed. For instance, a waveform of the lookup table for a given d-axis current and q-axis current is illustrated in  FIG. 3  as described above. The power electronics controller  200  may determine the maximum heating power is point C at half of the switching frequency in this example. 
     At operation  706 , the power electronics controller  200  measures the temperature of the traction battery  124  and compares it to a desired temperature to determine a temperature difference. Based on the temperature difference, the power electronics controller  200  determines the power that is required to heat up the battery. As an example, the required power may be proportional to the temperature difference. In other words, a greater temperature difference (e.g., extremely cold ambient temperature) may result in a higher required power to heat up the battery  124 . With the required power determined, at operation  708 , the power electronics controller  200  determines the switching frequency for the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  as illustrated in  FIG. 3  for instance. If the required power does not exceed the maximum available power determined at operation  704 , the power electronics controller  200  may clip heating power output by adjusting the switching frequency away from half of the resonant frequency at which the heating power is at the maximum in the present example. Otherwise, if the required power exceeds the maximum available power, the process proceeds from operation  710  to operation  712  and the power electronics controller  200  adjusts the d-axis current command I d  to increase the maximum heating power. 
     Referring to  FIG. 8 , example waveform diagrams of battery heating power varied by d-axis current I d  at different speed conditions with a fixed switching frequency of the resonant frequency are illustrated.  FIG. 8A  illustrates the waveform for zero vehicle speed,  FIG. 8B  illustrates the waveform for 5% maximum vehicle speed, and the  FIG. 8C  illustrates the waveform for 10% maximum vehicle speed. The horizontal axis of the waveform diagrams represent a ratio between the d-axis current command I d  and a maximum reference current (I d /I max ), and the vertical axis of the diagrams represent a percentage of battery heating power as compared with a reference heating power applied to the battery  124  via an external heater (4 kW for instance). As illustrated in the diagrams, a first waveform  802  illustrates the battery heating power at zero torque from the electric machine  114 , a second waveform  804  illustrates the battery heating power at 25% of the maximum torque, and a third waveform  808  illustrates the battery heating power at 50% of the maximum torque. Points  808  represent the original d-axis current command Ia. The maximum heating power represented by the vertical axis of the diagrams may be adjusted by varying the d-axis current command I d  (represented by the horizontal axis). Taking the present figure for example, the original current command I d  is around −0.5 of the maximum reference current. To increase the battery heating power, the power electronics controller  200  may adjust the d-axis current I d  to the left toward the −1 end of the diagram. In other situations, the power electronics controller  200  may adjust the d-axis current I d  to the right toward the +1 end of the diagram to achieve the battery heating power increase. 
     After adjusting the d-axis current command at operation  712 , the power electronics controller  200  adjusts the q-axis current command accordingly at operation  714 . The q-axis current command may be calculated using the following equation: 
     
       
         
           
             
               
                 
                   
                     I 
                     q 
                     * 
                   
                   = 
                   
                     
                       T 
                       * 
                     
                     
                       
                         
                           3 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           P 
                         
                         22 
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             λ 
                             
                               P 
                               ⁢ 
                               M 
                             
                           
                           + 
                           
                             
                               ( 
                               
                                 
                                   L 
                                   d 
                                 
                                 - 
                                 
                                   L 
                                   q 
                                 
                               
                               ) 
                             
                             ⁢ 
                             
                               I 
                               d 
                               * 
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     wherein T* represents the desired torque command, P represents the pole pair parameter, λ PM  represents the permanent magnetic flux, L d  represents the inductance on the d-axis, L q  represents the inductance on the q-axis, and I* d  represents the d-axis current command as adjusted. Alternatively, the adjusted q-axis current command I* q  may be determined using a lookup table previously calibrated. In this way, the maximum battery heating power may be manipulated by the switching frequency, the d-axis current command I d  and the q-axis current command I q . The process may return to operation  702  to continue to monitor and adjust the parameters until the desired battery temperature is achieved. 
     The process  700  may be applied to a fuel-cell electric vehicle under substantially the same concept. Referring to  FIG. 9 , an example waveform diagram of fuel cell current ratio and fuel cell heating power percentage is illustrated. An AC current  902  may flow through one or more resistors of a fuel cell to achieve the heating effect. Since fuel cells cannot operate at negative instantaneous current, no negative current is permitted in the present example. The fuel cell heating power is presented in waveform  904  normalized to the external reference heater of 4 kW power. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.