Abstract:
An apparatus connected to an energy storage device for powering an electric motor and optionally providing a warming function for the energy storage device is disclosed. The apparatus includes a circuit connected to the electric motor and the energy storage device for generating a current. The apparatus also includes a switching device operably associated with the circuit for selectively directing the current to one of the electric motor and the energy storage device.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under NREL subcontract number ZAN-6-16334-01, prime contract number DE-AC36-83CH10093 issued by the Department of Energy. The government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention generally relates to a device for heating batteries. More particularly, the present invention is directed to an electrical circuit and mechanism using an inverter circuit and the battery main disconnect circuitry to heat the battery or a group of batteries. 
     2. Discussion 
     It is well known that most electrochemical batteries including lead acid, NiCd, NiMH, or Li-Ion, Li-Polymer, etc. with potential use in electric, hybrid electric, or in conventional vehicles typically need some form of heating at cold, particularly extreme cold temperatures before they can deliver their full power capability. Traditional methods of warming up starter batteries in the cars have included leaving the headlights on for a few moments or even applying a short at the battery terminals momentarily to warm up the battery. These methods, including those using an external energy source, such as a heated jacket, to warm up the battery, tend to waste a substantial amount of energy outside the battery in order to obtain a proportionately small increase in the battery internal temperature. 
     Research and investigation has shown that the most efficient way of self-heating a battery is through exchanging energy back and forth between the battery and an external energy storage device such as an inductor or a capacitor, or a combination thereof. An exemplary circuit for performing this energy exchange for battery self heating is disclosed in U.S. application Ser. No. 09/070,331, filed Apr. 30, 1998, which is commonly owned and expressly incorporated herein by reference. An even simpler way of implementing this concept is by passing an alternating current through the battery, which is effectively a constant voltage source. 
     Laboratory experiments have shown that, depending on the battery chemistry, the internal impedance of the battery at −20° C. (below freezing) drops to as much as half when subjected to a 10 Amp 60 Hz current in less than 50 seconds for a 6-10 Ah battery. This means that the power delivery capability of the battery doubles in less than 50 seconds. Increasing the frequency as well as the magnitude of the current applied can substantially reduce this time, but the impact on the battery life may be adversely affected. 
     In hybrid electric vehicles (HEV), the most effective way of warming the battery in subfreezing temperatures is through charging. For example, the vehicle&#39;s engine (gas, diesel, etc.) will propel the electric motor in the “generator mode” and the motor inverter in the “rectifier” mode to effectively charge the vehicle&#39;s battery using the engine&#39;s power. Since the impedance of the battery rises considerably in the extreme cold, the charging or flow of current through the battery automatically warms up the battery internally much more effectively than in above-freezing temperatures. 
     This option is not available in pure electric vehicles (EV) due to the absence of the supplemental engine or power source. In view of this limitation with electric vehicles, it is desirable to provide a circuit which is capable of warming the battery used for powering the electric vehicle during cold environmental conditions. It is also desirable to provide a circuit which can be easily incorporated with the existing power circuitry for the electric vehicle. Finally, it is desirable to provide a circuit which provides a dual function within the electric vehicle for minimizing the number of components within the electric vehicle&#39;s power circuit. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, an apparatus connected to an energy storage device for powering an electric motor and optionally providing a warming function for the energy storage device is disclosed. The apparatus includes a circuit connected to the electric motor and the energy storage device for generating a current. The apparatus also includes a switching device operably associated with the circuit for selectively directing the current to one of the electric motor and the energy storage device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a dual function inverter circuit and battery warming circuit in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a graph of the waveforms which can be generated by the circuit of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of the dual function power circuit for an electric vehicle in accordance with a preferred embodiment of the present invention; 
         FIG. 4  is a schematic diagram of the dual function power circuit, also in accordance with a preferred embodiment of the present invention; 
         FIG. 5  is a waveform diagram showing the current waveforms associated with a preferred embodiment of the present invention; 
         FIG. 6  is a graphical representation of typical current waveforms produced by the dual function power circuit; and 
         FIG. 7  is also a graphical representation of typical current waveforms produced by the dual function power circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its applications or uses. Referring now to  FIG. 1 , the circuit  10  for warming a multi-cell battery pack  12  according to the teachings of the present invention is shown. As shown, the battery pack  12  includes a first battery cell  14  and a second battery cell  16 , which are connected in series. A common node  26  is disposed between battery  14  and battery  16 . A first transistor  18  is connected between the positive terminal of battery  14  and the common node  26 . A second transistor  20  is connected between the common node  26  and the negative terminal of battery  16 . A first anti-parallel diode  22  has its cathode connected to the positive terminal of battery  14  and its anode connected to common node  26 . A second anti-parallel diode  24  has its cathode connected to common node  26  and its anode connected to the negative terminal of battery  16 . 
     In the preferred embodiment, transistors  18  and  20  are insulated gate bipolar transistors (IGBTs). However, the principles of the invention may be extended to the use of other switching devices such as MOSFET&#39;s, BJT&#39;s, and MCT&#39;s. Circuit  10  may be used as a simple DC to AC inverter for generating an alternating current by appropriately switching transistors  18  and  20  between a conducting and nonconducting state with a suitable control circuit. When this alternating current passes through the battery cells,  14 ,  16 , heat is generated for warming the battery pack  12 . As will be appreciated, circuit  10  is suitable for use as a DC to AC inverter for use with the electric motor of an electric vehicle. 
     The graph of  FIG. 2  illustrates three separate waveforms versus time. Waveform  28  represents the constant DC voltage source provided by either of battery cells  14 ,  16 . Waveform  30  represents an alternating current source which can be produced by appropriately switching the transistors  18 ,  20  of circuit  10  at alternate time intervals. Waveform  32  represents the oscillating power profile which results when the AC current I is coupled with the DC battery voltage V. The power profile of waveform  32  includes positive energy (+w) represented by area  34  and negative energy (−w) represented by area  36 , and when ignoring losses, the oscillating power profile V*I has a net zero effective energy exchange. 
     Turning now to  FIG. 3 , the power circuit  40  in accordance with a preferred embodiment of the present invention is shown. The power circuit  40  is particularly suited for controlling the power distribution between a multi-cell battery pack  42  and a three-phase motor and generator unit  70 . A four-contact electrical disconnect  50  controls the flow of current between battery pack  42  and a three-phase inverter circuit  60 . 
     As disclosed, battery pack  42  includes a first battery cell  44  and a second battery cell  46  having a common node  48  formed therebetween. Each battery cell  44 ,  46  within battery pack  42  may include one or more individual energy storage devices or cells, and may also be chosen from a wide variety of battery technologies. 
     The power circuit disconnect  50  includes four separate switching elements. As shown, switching element  52  is connected between the positive terminal of battery  44  and a first node  102  of the inverter circuit  60 . An inductor  62 , preferably contained within the power circuit disconnect  50 , is connected between common node  48  and one terminal of the second switching element  54 . The other terminal of switching element  54  is connected to a second node  104  of the inverter. A third switching element  56  is connected to second node  104 , and its other terminal is connected to line or node  72  of the motor  70 . The fourth switching element  58  is connected between the negative terminal of the battery cell  46  and a third node  106  of the inverter circuit  60 . 
     The inverter circuit  60  is preferably a three-phase DC to AC inverter having a first inverter branch  64 , a second inverter branch  66 , and a third inverter branch  68 . A capacitor  63  associated with power circuit  40  is connected between the first node  102  and the third node  106 . Each branch  64 ,  66 ,  68  of the inverter circuit  60  includes electronic components which are substantially similar to those shown with the inverter circuit  10  of FIG.  1 . More specifically, the first inverter branch  64  includes a first transistor  78  and a second transistor  80  which are connected between the first node  102  and the third node  106 . An anti-parallel diode  82  is connected in parallel with transistor  78 , and an anti-parallel diode  84  is connected in parallel with transistor  80 . The second node  104  is also disposed between transistors  78  and  80 . 
     The second inverter branch  66  also includes a first transistor  86  and a second transistor  88  which are connected as shown between the first node  102  and the third node  106 . Line or node  74  is disposed between the first and second transistors  86 ,  88 . A first anti-parallel diode  90  is connected in parallel with transistor  86  and an anti-parallel diode  92  is connected in parallel with transistor  88 . 
     The third inverter branch  68  also includes a first transistor  94  and a second transistor  96  which are similarly connected between the first node  102  and the third node  106 . Line or node  76  is disposed between first and second transistors  94 ,  96 . An anti-parallel diode  98  is connected in parallel with transistor  94  and an anti-parallel diode  100  is connected in parallel with transistor  96 . 
     In operation, the DC to AC inverter  60  produces three-phase power on lines  72 ,  74 ,  76  for powering the electric motor  70  for driving the electric vehicle.  FIG. 3  shows how the circuitry in these existing vehicle components can be used to accomplish the dual function of the present invention. As shown, the four-contact power circuit disconnect  50 , upon activation in subzero temperatures, will first make contacts  52 ,  54  and  58  while contact  56  is kept open. During this time period, the first branch or pole  64  of the inverter circuit  60  operates as described above to generate an alternating current for warming up the battery by injecting the AC current through the center tap or common node  48  of the battery pack  42 . Within approximately 50 seconds, or whatever time the control algorithm determines, (e.g., based on temperature feedback), contact  54  of the disconnect circuit  50  opens and contact  56  closes at which point the inverter circuit  60  will be ready to operate the electric motor  70 . 
     Turning now to  FIG. 4 , the power circuit  110  in accordance with an alternate preferred embodiment of the present invention is shown. The power circuit  110  is also suited for controlling the power distribution between a multi-cell battery pack  112  and a three-phase motor and generator unit  70 . In the embodiment of  FIG. 4 , a two-position switch  128  is connected between the battery pack  112  and the electric motor  70 . The switch  128  controls the flow of current to the electric motor  70  or to the battery pack  112  depending upon the switch position. 
     As disclosed, battery pack  112  includes a first battery cell  114  and a second battery cell  116  having a common node  118  formed there between. Each battery cell  114 ,  116  within battery pack  112  may include one or more individual energy storage or devices or cells, and may also be chosen from a wide variety of battery technologies. 
     The inverter circuit  130  is preferably a three-phase DC to AC inverter having a first inverter branch  132 , a second inverter branch  134 , and a third inverter branch  136 . A capacitor  138  associated with power circuit  110  is connected between the first node  140  and the second node  142 . Each branch  132 ,  134 ,  136  of the inverter circuit  130  includes electronic components which are substantially similar to those shown with the inverter circuit of FIG.  1 . More specifically, the first inverter branch  132  includes a first transistor  150  and a second transistor  152  which are connected between the first node  140  and the second node  142 . An anti-parallel diode  154  is connected in parallel with transistor  150 , and an anti-parallel diode  156  is connected in parallel with transistor  152 . The first phase node  158  is connected between transistors  150  and  152 . 
     The second inverter branch  134  also includes a first transistor  160  and a second transistor  162  which are connected as shown between the first node  140  and the second node  142 . A second phase line  168  is connected between the first and second transistors  160 ,  162 . A first anti-parallel diode  164  is connected in parallel with transistor  160  and a second anti-parallel diode  166  is connected in parallel with transistor  162 . 
     The third inverter branch  136  also includes a first transistor  170  and a second transistor  172  which are similarly connected between the first node  140  and the second node  142 . A third switchable line  178  is connected between first and second transistors  170  and  172 . A first anti-parallel diode  174  is connected in parallel with transistor  170  and a second anti-parallel diode  176  is connected in parallel with transistor  172 . 
     In operation, the DC to AC inverter  130  also produces three-phase power on lines  158 ,  168 ,  178  for powering the electric motor  70  for driving the electric vehicle.  FIG. 4  further illustrates the power circuit  110  also provides an AC battery heater that is derived from one branch or phase  132  of a three phase inverter circuit  130  for an AC motor  70 . When switch  128  is in position M, the power circuit  110  functions as a motor drive circuit for powering the electric vehicle. When the switch  128  is in position B, the circuit  110  functions as an AC battery heater as described herein. One advantage of this system is that the same power circuit  110  can provide both a motor drive and a battery heater for applications such as electric vehicles. 
     When the switch  128  is in position B for warming the battery pack  112 , a control circuit (not shown) will alternately switch transistor  150  (Q 1 ) and transistor  152  (Q 2 ) to create the alternating currents i 1  and i 2  ( FIG. 5 ) in the two batteries  114  and  116 . It should be understood that while in the battery warming mode transistors  160 ,  162 ,  170 , and  172  as well as diodes  164 ,  166 ,  174 , and  176  are inactive. In an ideal circuit, inductances L 1    120 , L 2    124 , L 3    122  and capacitor C  138  would not be present, and i 1  and i 2  would be much different from the waveforms shown in FIG.  5 .  FIG. 5  shows the modeled current waveforms for currents i O , i 1 , i 2 , and i C . However, L 1    120 , L 2    124 , and L 3    122  will always be present due to the parasitic inductance of the cables that connect the circuit  130  to batteries  114  and  116 . Also shown is inductance L X    126  which is modeled as an external inductance associated with the power circuit  110 . Because of the energy stored in L 1    120  and L 2    124 , capacitor C  138  is highly preferred to prevent excessive voltages across the transistors  150  Q 1  and  152  Q 2  when they turn off. 
     Another goal of the present invention is to minimize the current due to resonance between the capacitor C  138  and the inductances L 1    120  and L 2    124 . If the switching frequency of transistors Q 1  and Q 2 , f=1/T, is sufficiently close to the natural resonant frequency f o , where 
               f   o     =     1     2   ⁢   π   ⁢         (       L   1     +     L   2       )     ⁢   C                                 
 
then the currents i 1  and i 2  will contain excessive resonant components. A relatively simple control system can be achieved if resonance is avoided. This may be achieved if C is large enough so that,
 
 f&gt;&gt;f   o .
 
     When f&gt;&gt;f o , the circuit will operate in the following manner in the steady state. The following description is also depicted graphically in FIG.  5 .
     Q 1  on: i 0 =i 1 +i c , i c =i 2  and all currents reamp towards their peak until Q 1  turns (0≦t≦t 1 ) off at t 1 .   D 2  on: i 0 =−i c +i 2 , i 1 =−i c  and all currents ramp towards 0 until D 2  turns off (t 1 ≦t≦t 2 ) at t 2 .   Q 2  on: −i 0 =−i 2 +ic, i c =−i 1  and all currents ramp towards their peak until Q 2  (t 2 ≦t≦t 3 ) turns off at t 3 .   D 1  on: −i 0 =−i 1 −i c , −i c =−i 2  and all currents ramp towards 0 until D 1  turns (t 3 ≦t≦t 4 ) off at T.
 
The resulting battery currents, i 1  and i 2 , are almost exactly half of i 0 .
   

     If the condition of f&gt;&gt;f o  is not met, current regulation can become more complex, but higher I 1RMS /I ORMS  and I 2RMS /I ORMS  ratios can be achieved. The advantage of this is that the same RMS battery currents, I 1RMS  and I 2RMS , can be achieved with lower losses in Q 1 , Q 2 , D 1 , D 2  and the total inductance, L 0 , seen by current i 0 —i.e., L 0 =L a +L b +L x +L 3 . Typical −i 0 , i 1  and i 2  waveforms are also shown in  FIGS. 6 and 7 . 
     The foregoing discussion discloses and describes exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications, and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.