Abstract:
A charging method using an alternating current (AC) line voltage for conductive charging of an energy storage system (ESS) coupled to a polyphase motor drive circuit communicated to a polyphase motor, the polyphase motor drive circuit including a plurality M of driver stages, one driver stage for each phase of the polyphase motor with each driver stage coupled across the energy storage system, the method including the steps of: (a) coupling a first connector providing the line voltage to a second connector coupled to the plurality of driver stages; (b) interrupting selectably the line voltage from communication with the plurality of driver stages; (c) measuring both an ESS common mode voltage of the energy storage system with respect to a voltage reference and a line common mode voltage of the line voltage with respect to the voltage reference while the line voltage communication to the plurality of driver stages is interrupted; (d) operating a particular one of the driver stages to power a common mode voltage driver to align the ESS common mode voltage with the line common mode voltage while the line voltage communication to the plurality of driver stages is interrupted; and thereafter (e) suspending the interrupting step (b) when a difference between aid ESS common mode voltage and the line common mode voltage is less than a first predetermined value, the suspending step (e) communicating the line voltage to the plurality of driver stages; and thereafter (f) converting the line voltage to a charging voltage communicated to the energy storage system using a set of the plurality of driver stages not including the particular one driver stage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates generally to charging of rechargeable energy storage systems (e.g., batteries and electric double-layer capacitors and the like) and more particularly to conductive charging systems that counter a possibility of triggering a residual-current device (RCD) when an external charging power source is coupled to an internal, isolated, energy storage system. In the United States, RCDs are referred to as a ground fault circuit interrupter (GFCI), ground fault interrupter (GFI), an appliance leakage current interrupter (ALCI), or the like. 
         [0002]    Energy storage systems used in electric vehicles (EVs) and other industrial applications store significant amounts of energy that is dangerous when improperly handled. Many safety features are adapted and incorporated into these applications to enhance safety. One of those features includes use of an isolated ground for the energy storage system. A person is able to touch both a storage element of the energy storage system and a chassis of the EV without being shocked. 
         [0003]    In the case of rechargeable storage elements in the energy storage system, it is common to use an “external” charging system or external source of charging energy (herein, external charging system includes external sources of charging energy) coupled to the energy storage system. For EVs, it is desirable that these charging systems be available at locations that are convenient for the users and operators of those vehicles. Among these locations, the residence of the user is often a prime location for installation of a charging system. Charging systems for EVs, because they provide high energy, are sometimes special installations with particular design considerations. More commonly, it is desirable to provide for the user to simply plug their EV into an AC line voltage plug at their residence to initiate charging. The charging often requires more than an insignificant amount of time, thus the user will “plug” their vehicle into the wall socket and then leave the charging location to occupy their time until the vehicle is charged and they have desire to use the vehicle. It is common for electrical systems of residences to employ an RCD in general, and particularly at the charging location. 
         [0004]    This charging system has a ground as well. Because the (+) and (−) of the energy storage system is isolated, the actual voltage level of the (+) and (−) of the energy storage system could be different, and in fact is likely different from, the voltage of the AC phases of the charging system. At the moment that the charging system is coupled to the energy storage system, the relative voltages are equalized to the same voltage. It is this equalization that can result in a current flow that the RCD may detect as an unsafe residual current and disable current flow from the charging station into the energy storage system. 
         [0005]    This is inconvenient for the user. The user had left the location of the charging station and is likely unaware that the charging had been suspended. It is the case that returning to the EV after the allotted time with an anticipation of use of the vehicle only to find the state of charge of the energy storage system unchanged may engender a negative reaction in the user. The user must make alternate arrangements to accommodate the situation that the EV is not ready as planned, which can have varying consequences of its own depending upon the nature and availability of alternatives to the EV. 
         [0006]    What is needed is a charging apparatus and method that accommodates potential residual current when electrical grounds of the charger and the energy storage system are equalized at the moment of initiating charging. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Disclosed is charging system and method that accommodates/reduces potential residual, also referred to as a leakage, current when electrical grounds of a charger and an energy storage system are equalized at the moment of initiating charging. The charging system using an alternating current (AC) line voltage for conductive charging of an energy storage system (ESS) coupled to a polyphase motor drive circuit communicated to a polyphase motor, the polyphase motor drive circuit including a plurality M of driver stages, one driver stage for each phase of the polyphase motor with each driver stage coupled across the energy storage system, the system including a converter, coupled to the energy storage system and including a number N number of the plurality of driver stages, with N less than M, to convert the line voltage to a charging voltage responsive to a first plurality of drive signals, wherein the charging voltage is communicated to the energy storage system; a switching assembly, coupled between the plurality of driver stages and the line input voltage, controlling communication of the line voltage to the plurality of driver stages, the switching assembly including an on mode that couples the line voltage to the plurality of driver stages and an off mode that decouples the line voltage from the plurality of driver stages; a sensing circuit, coupled to the line voltage and to the energy storage system, measuring an ESS common mode voltage of the energy storage system to a voltage reference and measuring a line common mode voltage of the line voltage to the voltage reference when the switching assembly is in the off mode; a common mode voltage driver, coupled to the energy storage system and to at least one driver stage of the plurality of driver stages, at least one driver stage not including one of the N number of driver stages and responsive to a second set of driver signals to match the ESS common mode voltage with the line common mode voltage when the switching assembly is in the off mode; and a controller, coupled to the plurality of driver stages and responsive to a voltage comparison between the ESS common mode voltage and the line common mode voltage, to provide the second set of driver signals to decrease a difference between the ESS common mode voltage and the line common mode voltage to be within a predetermined value, the controller transitioning the switching assembly from the off mode to the on mode when the difference between the ESS common mode voltage and the line common mode voltage is within the predetermined value, and the controller providing the first set of plurality of drive signals when the switching assembly is in the on mode. 
         [0008]    The charging method using an alternating current (AC) line voltage for conductive charging of an energy storage system (ESS) coupled to a polyphase motor drive circuit communicated to a polyphase motor, the polyphase motor drive circuit including a plurality M of driver stages, one driver stage for each phase of the polyphase motor with each driver stage coupled across the energy storage system, the method including the steps of: (a) coupling a first connector providing the line voltage to a second connector coupled to the plurality of driver stages; (b) interrupting selectably the line voltage from communication with the plurality of driver stages; (c) measuring both an ESS common mode voltage of the energy storage system with respect to a voltage reference and a line common mode voltage of the line voltage with respect to the voltage reference while the line voltage communication to the plurality of driver stages is interrupted; (d) operating a particular one of the driver stages to power a common mode voltage driver to align the ESS common mode voltage with the line common mode voltage while the line voltage communication to the plurality of driver stages is interrupted; and thereafter (e) suspending the interrupting step (b) when a difference between aid ESS common mode voltage and the line common mode voltage is less than a first predetermined value, the suspending step (e) communicating the line voltage to the plurality of driver stages; and thereafter (f) converting the line voltage to a charging voltage communicated to the energy storage system using a set of the plurality of driver stages not including the particular one driver stage. 
         [0009]    For the system and apparatus, additional sensors may be used to measure residual currents during operation allowing the charging controller to regulate the residual current during operation. This system can be very useful with the large size of batteries causing them to have a large value of capacitance to chassis ground and motors also having large values of capacitance causing the charging system to have a large value of RCD current. This is especially important when using the poly phase motor as the boost inductor because the motor case is normally grounded and it typically has a large capacitance to ground. Other features, benefits, and advantages of the present invention will be apparent upon a review of the present disclosure, including the specification, drawings, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic block diagram for a representative electric motor system incorporating a preferred embodiment of the present invention; 
           [0011]      FIG. 2  is a schematic diagram of a conductive charging system for an energy storage system of a multiphase motor including a common mode voltage driver to align ground voltages prior to initiation of charging; and 
           [0012]      FIG. 3  is a schematic diagram of a conductive charging system for an energy storage system of a multiphase motor including a common mode voltage driver to align ground voltages prior to initiation of charging and a residual current sensor to measure and reduce residual currents during charging. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Embodiments of the present invention provide methods and systems for a conductive high-energy charger that accommodates potential leakage current when electrical grounds of the charger and the energy storage system are equalized at the moment of initiating charging, and reduces residual currents during charging. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. In the following text, the terms “energy storage assembly” “battery”, “cell”, “battery cell” and “battery cell pack” “electric double-layer capacitor” and “ultracapacitor” may be used interchangeably (unless the context indicates otherwise” and may refer to any of a variety of different rechargeable configurations and cell chemistries including, but not limited to, lithium ion (e.g., lithium iron phosphate, lithium cobalt oxide, other lithium metal oxides, etc.), lithium ion polymer, nickel metal hydride, nickel cadmium, nickel hydrogen, nickel zinc, silver zinc, or other chargeable high energy storage type/configuration. 
         [0014]    Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0015]    Embodiments of the present invention are applicable to systems that employ electric motors in general, and more specifically to vehicles using multiphase electric induction motors. Electric vehicles (EVs) include vehicles that have one or more sources of stored energy designed to provide electrical energy to the vehicle, wherein the electrical energy is used to at least in part to provide some energy used to propel the vehicle. Electric vehicles may include vehicles designed to carry passengers, to transport goods, or to provide specialty work capabilities. For example, electric vehicles include passenger automobiles, trucks, and recreational watercrafts such as boats. In addition, electric vehicles include specialty vehicles, such as fork trucks used to lift and move cargo, vehicles that incorporate conveyor belts to move objects, such as mobile conveyor belt vehicles used to load and unload cargo such as luggage from airplanes, and specialty equipment used in areas where exhaust fumes from typical gasoline, diesel, or propane powered equipment may present hazards to personnel, such as in underground mining operations. In various instances, electric vehicles are designed and intended to be operated on public highways as licensed automobiles, including both cars and trucks. U.S. Pat. No. 5,341,075 titled Combined motor drive and battery recharge system illustrates a combined battery recharge and motor drive system employs an essentially conventional polyphase pulse width modulated (PWM) inverter and a polyphase motor which may be reconnected to operate in a battery recharge mode. Single phase or three-phase AC power from an external source is applied across the reconfigured circuit, and the inverter switches are then controlled to operate as a boost switching regulator. Electrostatic (Faraday) shielding techniques and construction allow extremely small ground currents for improved safety and GFI outlet compatibility. U.S. Pat. No. 5,341,075, issued 23 Aug. 1994, is hereby expressly incorporated in its entirety by reference hereto for all purposes. 
         [0016]    Generally, an electric vehicle includes some form of a device or devices capable of storing energy and that is operable to provide electrical power to the vehicle. The electrical power may be used to at least in part provide energy for propelling the vehicle. In some instances, the electrical power is used to provide the energy required for all of the vehicle&#39;s functions, including propelling the vehicle. In many instances, the source of the stored energy is a rechargeable battery pack. In various embodiments, a rechargeable battery pack includes a plurality of individual rechargeable battery cells that are electrically coupled to provide a rechargeable battery pack. 
         [0017]      FIG. 1  is a schematic block diagram for a representative electric motor system  100  incorporating a preferred embodiment of the present invention. To simplify further discussion, system  100  will be described below in the context of an electric vehicle. However it is understood that system  100  may be part of another device or system other than an electric vehicle. System  100  includes an energy storage system (ESS)  105  that includes a vehicle propulsion battery or the like and at least one propulsion motor  110  for converting energy into mechanical motion, such as rotary motion. ESS  105  includes various components associated with transmitting energy to and from the vehicle propulsion battery in various examples, including safety components, cooling components, heating components, rectifiers, and the like. ESS  105  may be implemented in many different ways and include many different components, but for purposes of this example, ESS includes a propulsion battery, ultracapacitor, or the like. Thus, the present subject matter should not be construed to be limited to the configurations disclosed herein, as other configurations are possible and within the scope of the present invention. 
         [0018]    The propulsion battery of ESS  105  of this example includes one or more lithium ion batteries. In some examples, the battery includes a plurality of lithium ion batteries coupled in parallel and/or series. Some examples include cylindrical lithium ion batteries. In some cases, ESS  105  includes one or more batteries compatible with the 18650 battery standard, but the present subject matter is not so limited. Some examples include approximately 2981 batteries which are interconnected. The vehicle propulsion battery used in ESS  105 , in some examples, provides approximately 390 volts. 
         [0019]    Additionally system  100  includes an energy converter  115 . Energy converter  115  converts energy from ESS  105  into energy useable by motor  110 . In some instances, there is energy flow from motor  110  into ESS  105  through energy converter  115 . ESS  105  transmits energy to energy converter  115 , which converts the transmitted energy into energy usable by motor  110  to propel the electric vehicle. Motor  110  may also generate energy that is transmitted to energy converter  115 . In these instances, energy converter  115  converts the transmitted energy from motor  110  into energy which may be stored in ESS  105 . As shown below in connection with an exemplary  FIG. 2  and  FIG. 3 , energy converter  115  includes semiconductor power devices such as transistors. These transistors may include one or more field effect transistors. Some examples include metal oxide semiconductor field effect transistors. Some examples include one or more insulated gate bipolar transistors. As such, in various examples, the energy converter  115  includes switching elements which are configured to receive direct current (DC) power from ESS  105  and to output multiphase (e.g., three-phase) alternating current (AC) to power motor  110 . As noted above, it is sometimes the case that energy converter  115  is configured to convert a three-phase output from motor  110  to DC power to be stored in ESS  105 . Some configurations of energy converter  115  convert energy from ESS  105  into energy usable by electrical loads other than motor  110 . Some of these examples switch energy from approximately 390 Volts of ESS  105  to 14 Volts (DC). 
         [0020]    In this example, motor  110  is a three phase AC motor. Sometimes system  100  may include a plurality of such motors. The vehicle optionally includes a transmission, such as a 2-speed transmission, though other examples are possible. Manually clutched transmissions are contemplated, as are those with hydraulic, electric, or electrohydraulic clutch actuation. Some examples employ a dual-clutch system that, during shifting, phases from one clutch coupled to a first gear to another coupled to a second gear. Rotary motion is transmitted from the transmission to the wheels via one or more axles. 
         [0021]    A management system  120  is optionally provided which provides control for one or more of ESS  105  and energy converter  115 . In some cases, management system  120  is coupled to a vehicle system which monitors safety (such as a crash sensor). In some examples management system  120  is coupled to one or more driver inputs (such as a speed adjuster, colloquially termed a throttle, although the present subject matter is not limited to examples having an actual throttle). Management system  120  is configured to control power to one or more of ESS  105  and energy converter  115 . 
         [0022]    A power connector  125  accesses an external power source  130 , e.g., a charging station, to receive energy and communicate it with ESS  105  through energy converter  115 . In some examples, the charging station converts power from a one or more phase 110V AC power source into power storable by ESS  105 . In additional examples, the charging station converts power from a 220V AC power source into power storable by ESS  105 . Some implementations include single-phase line voltages while others employ polyphase line voltages.  FIG. 1  illustrates an implementation in which energy converter  115  converts power from energy source  130  to energy storable by ESS  105 . For purposes of this discussion, power connector  125  is integrated with the EV and external power  130  is external to the EV and provides the input line voltage described herein. 
         [0023]    The present example employs conductive charging (as opposed to inductive charging) using the SAE J1772-2001 standard and subsequent revisions, hereby expressly incorporated herein by reference for all purposes, as the charging interface. Most relevant to the present invention are two levels from this standard; Level 1 and Level 2. Level 1 includes 120 V AC and peak current of 16 Amps and Level 2 includes 240 V AC and peak current of 40 Amps, both are single phase. Other implementations may use multiphase input line voltage. 
         [0024]    Part of the need for the present invention arises from providing ESS  105  with an isolated ground for improving safety. A user is able to contact a terminal of a storage element of ESS  105  and vehicle chassis without electric shock. This is desirable during operation, but can cause the difficulties described herein when external power  130 , having a ground at a different voltage level from that of ESS  105 , is coupled to power connector  125 . 
         [0025]    The present example further includes a Level 1 charging interface for an EV having a nominal 400 VDC energy storage system. Therefore the peak-to-peak voltage of the line input is +/−180 V. The 400 VDC for the ESS results in a +/−200 VDC. Before being coupled together, these two voltages do not have the same reference voltage. 
         [0026]    Because the ESS is isolated from the chassis, a largest potential difference between the ESS and the chassis can exist when either of the positive or negative terminals of ESS  105  are close to chassis ground. The voltage of the line input voltage varies sinusoidally and has a largest potential at the peak of the cycle. At the moment that external power  130  is coupled to power connector  125 , the ground reference of the line input voltage and the ground reference of the ESS are almost instantly synchronized. In that moment of synchronization, large residual currents may be generated that may trip the RCD of external power  130 . 
         [0027]    As further explained below, system  100  includes, mostly as part of an input stage of energy converter  115  in the preferred embodiment, actuators that decouple external power  130  from ESS  105 . Management  120  includes sensors to measure a common mode voltage between the line input voltage and a reference voltage (e.g., chassis ground) and a common mode voltage between ESS  115  and the reference voltage. 
         [0028]    Energy converter  115  includes a plurality of motor driver stages, one driver stage for each phase of polyphase motor  110  (e.g., three phases and three motor driver stages), for operation of motor  110 . These driver stages are also used during charging to rectify and convert the voltage of external power  130  to energy storable in ESS  105 . Converter  115  requires only two of the driver stages for actual charging, leaving one driver stage “unused” and available. 
         [0029]    Embodiments of the present invention use this previously unused driver stage to operate a common mode voltage driver. The common mode voltage driver matches the common mode voltages of external power  130  and ESS  105  to each other (within a predetermined tolerance), and then, the actuators couple external power  130  to ESS  105 . The common mode voltage driver may be deactivated after the actual charging has started. 
         [0030]    In other embodiments of the present invention, energy converter  115  is provided with residual current sensor, in addition to or in lieu of, the common mode voltage driver. The residual current sensor measures residual currents at power connector  125 , these currents are used by management  120  after charging has commenced. The driver stages that actually perform the voltage conversion are controlled to reduce the residual currents to within a desired tolerance. These residual currents can be significant and are preferably measured in real-time because the input voltage is an AC voltage and the average voltage changes sinusoidally (i.e., it is not zero). This produces a 60 Hz varying charging voltage to ESS  115  that can generate residual currents. The charging-related driver stages are controlled to reduce these residual currents to be within a predetermined tolerance. These residual currents may exist during operation because the ESS is isolated from chassis ground and the average of the battery voltage is about equal to the average of the AC line voltage (which is not zero). 
         [0031]      FIG. 2  is a schematic diagram of a conductive charging system  200  for an energy storage system ESS of a multiphase motor  205  including a common mode voltage driver  210  to align ground voltages prior to initiation of charging. System  200  will be described in the context of an electric vehicle using a 3-phase electric induction motor  205 , though other implementations are possible. System  200  includes a plurality of semiconductor motor driver stages, one motor driver stage for each phase of motor  205 . 
         [0032]    Therefore in this example there are three motor driver stages. Each driver stage includes a pair of power transistors (for example transistor M 1  and transistor M 2 , shown as NPN insulated gate bipolar transistors but other transistors may be used) with the emitter of M 1  coupled to both the collector of M 2  and to one of the power inputs of motor  205 . The collector of M 1  is coupled to a first terminal (e.g., the positive terminal) of an energy storage system (ESS) and the emitter of M 2  is coupled to a second terminal (e.g., the negative terminal) of the ESS. The ESS may include, for example, one or more batteries or ultracapacitors or the like). Each transistor is coupled to a diode, an anode of the diode coupled to the emitter of the transistor and a cathode of the diode coupled to the collector of the transistor. Thus a diode D 3  is coupled to M 1  and a diode D 4  is coupled to M 2 . The other driver stages include M 3  coupled to M 5  (with D 2  and D 5  respectively) and M 4 /D 1  coupled to M 6 /D 6 . For operating motor  205 , all three driver stages are controlled by a motor/charge control  215  to drive motor  205  using energy from the ESS. The motor driver stages convert the DC voltage of the ESS to the AC voltage required by motor  205 . 
         [0033]    As noted above, it is desirable to meet reduced budgets for weight, size, cost, and component count, particularly when system  200  is part of an electric vehicle. System  200  uses the semiconductors of the motor driver stages during charging. In some configurations, it is sufficient to provide use the driver stages for charging only. In this configuration, it is sufficient to use two driver stages, for example M 4 /M 6  as a first driver stage and M 3 /M 5  as a second driver stage. 
         [0034]    In the charging mode, the line input voltage is 110 V single phase provided from an external power source having a ground reference. A power coupler, shown as a connector  220 , communicates external power to system  200 . A first node of an inductance L 1  is coupled through connector  220  to V 3  and a first node of an inductance L 2  is coupled through connector  220  to V 2 . A second node of L 1  is coupled to a throw of a switch/actuator W 1  and a second node of L 2  is coupled to a throw of a switch/actuator W 2 . A pole of W 1  and a pole of W 2  are coupled to an input of an EMI filter including a pair of inductances (L 5  and L 6 ) and a capacitance C 4 . A first output node of the EMI filter is coupled to the emitter of M 4  and a second output node of the EMI filter is coupled to the emitter of M 3 . 
         [0035]    Common mode voltage driver  210  includes a switch/actuator W 3  and a resistance R 1 . A pole of W 3  is coupled to an emitter of M 1 , a collector of M 2 , and a winding of motor  305 , a throw of W 3  is coupled to a first node of R 1 , and a second node of R 1  is coupled to a ground reference voltage, e.g., chassis ground. The value of resistance for R 1  used to control the battery voltage relative to chassis reduces the isolation but just during charging when the battery bus is electrically tied to the AC line and the battery is not considered isolated from ground. 
         [0036]    Motor/Charge control  215  includes a DC voltage sense coupling to the emitters of M 2 , M 5 , and M 6  and to the negative terminal of the ESS. Control  215  also includes an AC voltage sense coupling to V 2  and V 3 , through connector  220 . 
         [0037]    Prior to initiation of charging, with V 2 /V 3  decoupled from connector  220 , switches W 1 , W 2 , and W 3  are all open. When V 2 /V 3  is coupled to connector  220 , control  215  measures a common mode voltage of the ESS bus to chassis ground, and a common mode voltage of the AC line voltage relative to chassis ground. When they are not within a predetermined difference (determined by application) from each other, control  215  closes switch W 3  and operates the driver stage of M 1 /M 2  to produce a voltage drop across R 1  that controls the ESS voltage relative to the chassis ground, moving the measured common mode voltages to the predetermined difference. 
         [0038]    When the measured common mode voltages are close enough, control  215  closes W 1  and W 2  and couples V 2 /V 3  to the charging driver stages. Control  215  then operates the driver stages to initiate charging. Connecting the connector to the energy storage system thus does not produce any residual currents that could trip an RCD coupled to the power source used in charging. 
         [0039]      FIG. 3  is a schematic diagram of a conductive charging system  300  for an energy storage system (ESS) of a multiphase motor  305  including a common mode voltage driver  310  to align ground voltages prior to initiation of charging and a residual current sensor to measure and reduce residual currents during charging. Charging system is configured and operates similarly to system  200  shown in  FIG. 2  and described above. System  300  further includes a residual current sensor, e.g., an inductance L 3  magnetically coupled to L 1  and L 2  for measuring residual currents during operation. Control  315 , coupled to inductance L 3 , operates the charging motor driver stages to maintain the residual currents within predetermined target values. The added residual current sensor reduces a possibility of errant tripping of an RCD during operation of charging system  300 . 
         [0040]    System  300  will be described in the context of an electric vehicle using a 3-phase electric induction motor  305 , though other implementations are possible. System  300  includes a plurality of semiconductor motor driver stages, one motor driver stage for each phase of motor  305 . 
         [0041]    Therefore in this example there are three motor driver stages. Each driver stage includes a pair of power transistors (for example transistor M 1  and transistor M 2 , shown as NPN insulated gate bipolar transistors but other transistors may be used) with the emitter of M 1  coupled to both the collector of M 2  and to one of the power inputs of motor  305 . The collector of M 1  is coupled to a first terminal (e.g., the positive terminal) of an energy storage system (ESS) and the emitter of M 2  is coupled to a second terminal (e.g., the negative terminal) of the ESS. The ESS may include, for example, one or more batteries or ultracapacitors or the like). Each transistor is coupled to a diode, an anode of the diode coupled to the emitter of the transistor and a cathode of the diode coupled to the collector of the transistor. Thus a diode D 3  is coupled to M 1  and a diode D 4  is coupled to M 2 . The other driver stages include M 3  coupled to M 5  (with D 2  and D 5  respectively) and M 4 /D 1  coupled to M 6 /D 6 . For operating motor  305 , all three driver stages are controlled by a motor/charge control  315  to drive motor  305  using energy from the ESS. The motor driver stages convert the DC voltage of the ESS to the AC voltage required by motor  305 . 
         [0042]    As noted above, it is desirable to meet reduced budgets for weight, size, cost, and component count, particularly when system  300  is part of an electric vehicle. System  300  uses the semiconductors of the motor driver stages during charging. In some configurations, it is sufficient to provide only a boost-mode for charging. In this boost-mode-only configuration, it is sufficient to use two driver stages, for example M 4 /M 6  as a first driver stage and M 3 /M 5  as a second driver stage. 
         [0043]    In the charging mode, the line input voltage is 110 V single phase provided from an external power source having a ground reference. A power coupler, shown as a connector  320 , communicates external power to system  300 . A first node of an inductance L 1  is coupled through connector  320  to V 3  and a first node of an inductance L 2  is coupled through connector  320  to V 2 . A second node of L 1  is coupled to a throw of a switch/actuator W 1  and a second node of L 2  is coupled to a throw of a switch/actuator W 2 . A pole of W 1  and a pole of W 2  are coupled to an input of an EMI filter including a pair of inductances (L 5  and L 6 ) and a capacitance C 4 . A first output node of the EMI filter is coupled to the emitter of M 4  and a second output node of the EMI filter is coupled to the emitter of M 3 . 
         [0044]    Common mode voltage driver  310  includes a switch/actuator W 3  and a resistance R 1 . A pole of W 3  is coupled to an emitter of M 1 , a collector of M 2 , and the winding of motor  305  contactor is opened to disconnect motor  305  during charging. A throw of W 3  is coupled to a first node of R 1  and a second node of R 1  is coupled to a ground reference voltage, e.g., chassis ground. The value of resistance for R 1  used to control the battery voltage relative to chassis reduces the isolation but just during charging when the battery bus is electrically tied to the AC line and the battery is not considered isolated from ground. 
         [0045]    Motor/Charge control  315  includes a DC voltage sense coupling to the emitters of M 2 , M 5 , and M 6  and to the negative terminal of the ESS. Control  315  also includes an AC voltage sense coupling to V 2  and V 3 , through connector  320 . 
         [0046]    Prior to initiation of charging, with V 2 /V 3  decoupled from connector  320 , switches W 1 , W 2 , and W 3  are all open. When V 2 /V 3  is coupled to connector  320 , control  315  measures a common mode voltage of the ESS bus to chassis ground, and a common mode voltage of the AC line voltage relative to chassis ground. When they are not within a predetermined difference (determined by application) from each other, control  315  closes switch W 3  and operates the driver stage of M 1 /M 2  to produce a voltage drop across R 1  that controls the ESS voltage relative to the chassis ground, moving the measured common mode voltages to the predetermined difference. 
         [0047]    When the measured common mode voltages are close enough, control  315  closes W 1  and W 2  and couples V 2 /V 3  to the charging driver stages. Control  315  then operates the driver stages to initiate charging. Connecting the connector to the energy storage system thus does not produce any residual currents that could trip an RCD coupled to the power source used in charging. 
         [0048]    It may be desirable in some instances to combine embodiments of the present invention with a ground detect mechanism to improve safety of operation and to reduce false trips of an RCD. Further, as noted herein, in typical operation the polyphase motor is disconnected from the driver/inverter stages) in the case when separate boost inductances are used from the inductances of the polyphase motor. Contactors in subsystem  205 / 305  are used to switch the motor (and the inductances) as necessary or desirable. As also noted, it is possible in some cases to use inductances of the polyphase motor as the boost inductance(s) and minimizing/eliminating separate inductances. In such cases, the polyphase motor will not be completely disconnected from the inverters during the disclosed operations. The phase of the polyphase motor that is used to reduce the RCD current is always disconnected from the polyphase motor using contactors in subsystem  205 / 305 . 
         [0049]    The system above has been described in the preferred embodiment of charging a multicell energy storage module used in electric vehicle (EV) systems. In the description herein, numerous specific details are provided, such as examples of components and/or methods, to provide a thorough understanding of embodiments of the present invention. One skilled in the relevant art will recognize, however, that an embodiment of the invention can be practiced without one or more of the specific details, or with other apparatus, systems, assemblies, methods, components, materials, parts, and/or the like. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of embodiments of the present invention. 
         [0050]    Reference throughout this specification to “one embodiment”, “an embodiment”, or “a specific embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention and not necessarily in all embodiments. Thus, respective appearances of the phrases “in one embodiment”, “in an embodiment”, or “in a specific embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any specific embodiment of the present invention may be combined in any suitable manner with one or more other embodiments. It is to be understood that other variations and modifications of the embodiments of the present invention described and illustrated herein are possible in light of the teachings herein and are to be considered as part of the spirit and scope of the present invention. 
         [0051]    It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application. 
         [0052]    Additionally, any signal arrows in the drawings/Figures should be considered only as exemplary, and not limiting, unless otherwise specifically noted. Furthermore, the term “or” as used herein is generally intended to mean “and/or” unless otherwise indicated. Combinations of components or steps will also be considered as being noted, where terminology is foreseen as rendering the ability to separate or combine is unclear. 
         [0053]    As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
         [0054]    The foregoing description of illustrated embodiments of the present invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed herein. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes only, various equivalent modifications are possible within the spirit and scope of the present invention, as those skilled in the relevant art will recognize and appreciate. As indicated, these modifications may be made to the present invention in light of the foregoing description of illustrated embodiments of the present invention and are to be included within the spirit and scope of the present invention. 
         [0055]    Thus, while the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of embodiments of the invention will be employed without a corresponding use of other features without departing from the scope and spirit of the invention as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular terms used in following claims and/or to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include any and all embodiments and equivalents falling within the scope of the appended claims. Thus, the scope of the invention is to be determined solely by the appended claims.