Abstract:
An electronic circuit for measuring voltage signals in an energy storage device is disclosed. The circuit includes a plurality of battery segments forming the energy storage device. An amplifier circuit is connected across one of the battery segments for converting a differential voltage to a reference current. A sense resistor is associated with the amplifier circuit to convert the reference current to a voltage signal which is proportional to the voltage across the battery segment. A voltage measurement node associated with the sensing resistor may be used for measuring the voltage signal. In one embodiment of the invention, a multiplexing and sampling circuit provides digitized voltage samples to a processor. The voltage level of each cell within the battery pack can then be monitored by the processor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the filing date, and is a continuation-in-part of U.S. non-provisional application Ser. No. 09/224,466 filed Dec. 31, 1998, Pat. No. ______. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field  
           [0003]    The present invention generally relates to an electronic circuit for an energy storage device management system. More particularly, the present invention is directed to an electronic circuit for efficiently and accurately measuring individual voltages in a series connected electrochemical energy storage device which may be utilized with electric and hybrid vehicles.  
           [0004]    2. Discussion  
           [0005]    In order to commercialize electric and hybrid vehicles on a widespread basis, the energy storage devices or batteries, which are the most expensive component of the vehicle, must operate reliably through the life of the vehicle. In the typical configuration the batteries are formed from a stack of series connected electrochemical cells.  
           [0006]    A common requirement for large stacks of electrochemical cells used in electric and hybrid vehicles, particularly in advanced applications such as lead acid, Li-ion or NiMH battery packs, is the need to measure individual or groups of cell voltages almost simultaneously. In practice, this means the measurements should be taken within a time window of a few milliseconds.  
           [0007]    With reference to FIG. 1, a common technique known within the prior art accomplishes voltage measurement through the use of a plurality of resistive divider circuits. More specifically, FIG. 1 shows an exemplary battery pack  10  having forty-eight energy storage cells B 1  through B 48  connected in series. A resistive voltage divider circuit  12  is connected between the positive terminal  16  of battery cells B 2  through B 48  and a common ground node  14 . The discrete resistances R 1 , R 2 , . . . , R n  are selected such that the output potentials V m1 , V m2 , . . . , V mn  fall below a certain voltage limit, for example 4 volts, suitable for input to a multiplexer and A/D converter. The voltage signals from each resistive divider circuit  12  can then be sampled and digitally processed. The actual nodal voltages V 1 , V 2 , V 3 , . . . , V 48  become increasingly higher towards the top of the battery pack  10 , such that in general:  
             V   mn     =         V   n     ·     k   n       =         V   n     ·       R   1         R   1     +     R   n           =         4                 V     ⇒     V   n       =       V   mn       k   n               ;                  ∀   n     =   1       ,   2   ,   …                         
 
           [0008]    The voltage across each cell segment V B1 , V B2 , . . . , V B48  is then computed as the difference between the nodal voltages measured on either side of the cell according to the formula:  
           
         V 
         Bn 
         =V 
         n 
         −V 
         n−1  
       
           [0009]    For example, the voltage V B3  of cell B 3  is measured by taking the difference between V 3  and V 2  provided by the respective voltage divider circuits  12 .  
           [0010]    The principal problem with this technique of voltage measurement is that a small error in measuring the nodal voltages V n  translates into a large relative error in the measurement of segment voltages V Bn . These errors increase as the nodal voltages V n  become increasingly larger towards the top or higher potential cells of the battery pack  10 . For example, suppose:  
           k   48     =     1   48       ,                  k   47     =     1   47                   V   n48     =         V   48     ·     k   48       =     4                 V         ,       ⇒     V   48       =     192                 V       ,     
            V   n47     =         V   47     ·     k   47       =     4                 V         ,       ⇒     V   47       =     188                 V       ,     
            ∴     V   B48       =         V   48     -     V   47       =     4                   V   .                                 
 
           [0011]    If k 48  is in error by =1%, and k 47  is in error by =−1%, measurements of the nodal voltages indicate:  
           V 48 =193.92 V; V 47 =186.12 V  
           V B48 =7.8 V., error=95%  
           [0012]    Thus, the measurement error associated with this network of resistive divider circuits  12  and measurement technique could be in excess of 95%.  
           [0013]    Furthermore, this error is nonuniformly distributed between the cell segments varying from a maximum of 2 percent at the bottom to a maximum of 2nX percent at the top of the battery pack  10 . The latter renders this approach useless in applications where comparison of the cell segment voltages are used for diagnostics or corrective actions such as in cell balancing. Lastly, this conventional resistance network continues draining the cells of the battery pack  10  even when the resistance network is not in use.  
           [0014]    While not specifically shown, a matrix of electromechanical relays can also be used for selectively switching across the cell segments of the battery pack. This approach results in slow measurement of cell voltages and is therefore not suitable for modern applications. In addition, such a relay based device also becomes too bulky and heavy for use with an electric or hybrid vehicle. Higher speed and accuracy can be achieved using a separate isolation amplifier for each battery segment, but this approach results in a relatively large and expensive system.  
           [0015]    Accordingly, it is desirable to provide an electronic circuit for overcoming the disadvantages known within the prior art. It is also desirable to provide an electronic circuit which allows for a high degree of accuracy when measuring both the lowest potential cell voltages and the highest potential cell voltages. Moreover, it is desirable to provide a highly efficient electronic circuit which minimizes any loss within the circuit. Finally, it is desirable to provide an electronic circuit with various switched components to prevent the leakage of current from the energy storage device when the circuit is not being used.  
         SUMMARY OF THE INVENTION  
         [0016]    According to the teachings of the present invention, a voltage transfer circuit for measuring the individual segment voltages within an energy storage device is disclosed. The circuit includes a plurality of battery segments forming the energy storage device. An amplifier circuit is connected across one of the battery segments for converting a differential voltage to a reference current. A sense resistor is associated with the amplifier circuit to convert the reference current to a voltage signal which is proportional to the voltage across the battery segment. A voltage measurement node associated with the sensing resistor may be used for measuring the voltage signal. In one embodiment of the invention, a multiplexing and sampling circuit provides digitized voltage samples to a processor. The voltage level of each cell within the battery pack can then be monitored by the processor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    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:  
         [0018]    [0018]FIG. 1 is a schematic diagram of a prior art resistive voltage divider circuit used in conjunction with a series battery pack;  
         [0019]    [0019]FIG. 2 is a schematic diagram of the electronic circuit for a series battery pack in accordance with an embodiment of the invention;  
         [0020]    [0020]FIG. 3 is a schematic diagram of the multiplexing and sampling circuit in accordance with the invention;  
         [0021]    [0021]FIG. 4 is an electronic circuit having on-off control for minimizing leakage current for use with a series battery pack in accordance with an alternate embodiment of the invention;  
         [0022]    [0022]FIG. 5 is a schematic diagram of a voltage transfer circuit for use with a series battery pack constructed in accordance with the principles of the invention; and  
         [0023]    [0023]FIG. 6 is a schematic diagram of a voltage transfer circuit having on-off control for minimizing leakage current for use with a series battery pack in accordance with a presently preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    Turning now to FIG. 2, the electronic voltage measuring circuit of the present invention is shown. The voltage measuring circuit  18  operates in conjunction with a series of five energy storage cells B 1  through B 5  forming battery pack  20 . As shown, node  22  is the common ground node which is also connected to the negative terminal of battery B 1 . Node  24  forms the connection between the positive terminal of battery B 1  and the negative terminal of battery B 2 . Node  26  forms the connection between battery B 2  and battery B 3 . Node  28  forms the connection between battery B 3  and battery B 4 . Node  30  forms the connection between battery B 4  and battery B 5 . Finally, node  32  forms the connection to the positive terminal of battery B 5 .  
         [0025]    A resistive voltage divider circuit  34  is connected between node  26  and the common ground node  22 . The voltage divider circuit  34  is formed by resistor R 1  and resistor R 2  with a voltage measurement node  36  disposed therebetween. The electronic circuit  40  of the present invention is connected across battery B 3  using nodes  26  and  28 . The electronic circuit  40  includes a temperature compensation circuit  42  which is formed by a first pnp transistor  44  and a second pnp transistor  46 . As shown, the bases of transistors  44  and  46  are connected together, and are commonly connected to the collector of transistor  44 . Thus, the temperature compensation circuit  42  functions as a current mirror within electronic circuit  40  and assists in isolating the voltage across its associated battery cell segment B 3 , so that the cell voltage V B3  can be measured with a significantly higher degree of accuracy.  
         [0026]    The emitter of transistor  44  is connected to node  26 , and the collector of transistor  44  is connected to biasing resistor Ry, which is then connected to the common ground node  22 . The emitter of transistor  46  is connected to resistor R X , which is in turn connected to node  28 , and the collector of transistor  46  is connected to resistor R 3 . The collector of transistor  46  also forms the voltage measurement node  48 . As will be appreciated, resistor R X  and resistor R 3  form the primary measurement components of the electronic circuit  40 . Additionally, identical electronic circuits  40  are also connected across battery cells B 4  and B 5 , and function in a substantially similar manner.  
         [0027]    With brief reference to FIG. 3, the processing circuit  50  associated with the voltage measuring circuit  18  of the present invention is shown. The processing circuit  50  includes a multiplexer  52  which receives the individual cell segment voltage signals V m1  through V m5  from the individual electronic voltage measurement circuits  40 , the resistive voltage divider circuit  34 , and voltage node  24 . The output of multiplexer  52  is provided to an A/D converter  54  so that the individual voltage signals can be digitally sampled and communicated to a suitable processor  56 . The processor  56  is then able to directly monitor the individual cell segment voltages, and use this information for functions such as cell diagnostics and cell equalization.  
         [0028]    The present invention involves a modification to the resistive voltage divider circuit, disclosed in FIG. 2, that creates a voltage signal across the measuring resistances R 3 , R 4 , R 5 , . . . , R n  which is directly proportional to the actual battery cell segment voltages V B3 , V B4 , V B5 , . . . , V Bn  that are being measured.  
         [0029]    In operation, the electronic circuit  40  of the present invention is described in conjunction with a battery pack of five v lithium ion cells as shown in FIG. 2. Assuming the A/D converter  54  can measure voltages up to +5 V DC, V B1  can be measured directly from node  24  which produces voltage signal V m1 , and V B2  can be measured using a ±1% resistive divider circuit  34  from node  36  which produces voltage signal V m2  and then subtracting the V B1  measurement. For V B3 , note that  
         
       V 
       B3 
       =I 
       E2 
       R 
       X 
       +V 
       EB2 
       −V 
       EB1  
     
         [0030]    If R x  and R y  are so selected and Q 1  and Q 2  are operated so that, V EB2 =V EB1 , then from above:  
         
       V 
       B3 
       ≡I 
       E2 
       R 
       X  
     
         [0031]    [0031]             Since                     I   E2       ≻   ≻     I   B2       ,                    then                     I   E2       =         I   B2     +     I   C2       ≅     I   C2         ,                and                 V   B3       V   m3       =           I   E2          R   X           I   C2          R   3         =       R   X       R   3                                 
         [0032]    In this circuit a direct measurement of V m3  will be proportional to the voltage across the cell segment V B3  and the measurement error will be % uniformly across the stack provided that V EB1  and V EB2  are approximately equal. As will be appreciated, the electronic circuit  40  of the present invention allows for the measurement of the voltage across each battery cell segment with a significantly higher degree of accuracy. Accordingly, the higher potential battery cell segments can be measured with nearly the same degree of accuracy as the lower potential battery cell segments because the electronic circuit  40  serves to measure only the voltage across an isolated battery cell segment, rather than measure the voltage potential of the cell segment with respect to ground.  
         [0033]    In the actual implementation, V EB1  and V EB2  cannot be matched perfectly, but if transistors  44  and  46  are mounted or formed in the same package, they can easily be matched within a few millivolts with respect to both initial tolerance and wide temperature ranges. This provides a very small and inexpensive measurement system which has about the same tolerance as the components. As will be appreciated by one skilled in the art, the remaining voltage measurements V B4 , V B5 , . . . , V Bn  are performed in the same manner as V B3 . As part of the present invention, it should be noted that the resistance values are chosen such that R 2 =R 3 =R 4 =R 5  and R 1 =R X .  
         [0034]    An alternate less preferred approach employs discrete transistors rather than a matched pair of transistors. Using discrete devices reduces the cost of the circuit and improves manufacturability, but increases the error associated with the voltage measurement. The increased error is caused by using separate pieces of silicon to fabricate the transistors and the differences in the operating temperature of each discrete device. The increased error associated with employing discrete devices is a function of the amplitude of the segment voltage that is being measured. Larger valued segment voltages result in a decreased error associated with mismatching of the transistor V EB &#39;s. For example, assuming a V EB  mismatch of 0.2 volts and a nominal segment voltage of 4 volts, the error due to V EB  mismatch is 5%.  
         [0035]    An alternate embodiment of the electronic circuit of the present invention is disclosed in FIG. 4. The components of the electronic circuit  40 ′ are substantially similar to those of the circuit shown in FIG. 2. As an additional feature, a switch  60  is connected between the resistor Ry and the common ground node  22 . According to this embodiment of the electronic circuit  40 ′, no current will flow through either side of the temperature compensation circuit  42  until switch  60  is closed. As part of the present invention, the switch  60  can be implemented with a semiconductor switch.  
         [0036]    The anode of a diode D 1  is connected to node  26 , or the negative terminal of the battery cell B 3 , and the cathode is connected to transistor  44 . The diode D 1  prevents reverse V EB2  avalanche and the resulting battery leakage current if V B3  is above approximately 5-6 V. The anode of a diode D 2  is connected to node  28 , or the positive terminal of the battery cell B 3 , and the cathode is connected to resistor R x . The diode D 2  is required for temperature compensation of diode D 1 .  
         [0037]    Referring to FIG. 5, a voltage transfer circuit  100  for use with a battery pack  102  is shown. The voltage transfer circuit  100  is particularly suitable for operation in conjunction with battery packs that are formed of relatively low voltage segments of about 1.0 volt to 5.0 volts such as with Li-Ion batteries. However, the scope of the invention includes using higher voltage battery segments such as are typical with NiCad, NiMH, and lead acid battery backs. Battery segments typically are formed from one or more battery cells having a characteristic voltage generally ranging from 0.8 volts to 4.5 volts. The battery pack  102  associated with the voltage transfer circuit  10  comprises series connected battery segments B 1  through Bn each of which consists of a single Li-ion battery cell.  
         [0038]    Node  104  forms the connection between a positive terminal of the battery pack  102  and the Vcc input of an amplifier quad pack  108 . Node  106  is the common ground node which connects to a negative terminal of the battery pack  102 . Node  110  forms the connection between battery segment Bn and battery segment Bn- 1 . Node  112  forms the connection between battery segment Bn- 1  and battery segment Bn- 2 . Node  114  forms the connection between battery segment Bn- 2  and battery segment Bn- 3 . Node  116  forms the connection between battery segment Bn- 3  and battery segment B 2 . Node  118  forms the connection between battery segment Bn- 4  and battery segment Bn- 5  (not shown). Node  119  forms the connection between battery segment B 2  and battery segment B 1 .  
         [0039]    A resistive voltage divider circuit  120  is connected between node  116  and common ground node  106 . Voltage divider circuit  120  is formed by resistors R 1  and R 3  with voltage measurement node Vm 2  disposed therebetween.  
         [0040]    Connected across each of the battery segments Bn through Bn- 3  is a corresponding amplifier circuit  122   a  through  122   d . Each amplifier circuit  122  includes an input resistor  124  R 101 , connected between the positive terminal of the battery segment, Bn, and the negative input of a corresponding amplifier, An. An input resistor  126  R 104 , is connected between the negative terminal of the battery segment, Bn, and the positive input of the corresponding amplifier, An. The negative input and an output  128  of the amplifier, An, respectively connect to the source and gate of a buffer transistor Q 101   130 . The drain of Q 101  connects to sense resistor R 102   132  with voltage measuring node Vmn disposed therebetween. The buffer transistor  130  is preferably a PMOS FET, however the scope of the invention includes other transistors such as PNP transistors. The other terminal of sense resistor R 102  connects to common ground node  106 . Input resistors R 101  and R 104 , and sense resistor R 102  are preferably selected so that each has the same value within each amplifier circuit  122 , thus maintaining consistent voltage translation ratios corresponding to each battery segment. However, it is within the scope of the invention to select differing voltage translation ratios and resistor values.  
         [0041]    In operation, amplifier circuit  122  senses the voltage across the corresponding battery segment, Bn, and translates the sensed voltage to a proportional voltage that is referenced to common ground node  106 . To achieve equilibrium the differential voltage across the inputs of the amplifier An must be approximately zero volts. Therefore, VBn=i1*R 101   
         [0042]    and VBn−1+VBn−2+VBn−3+VBn−4+=VSG+Vo  
         [0043]    where; i1 is the current through R 101 ,  
         [0044]    VSG is the transistor source-gate voltage, and  
         [0045]    Vo is the amplifier output voltage referenced to Vss.  
         [0046]    Vo will adjust so that VSG maintains equilibrium, and  
         [0047]    Vmn=l1*R 102 =(R 102 /R 101 )*VBn  
         [0048]    The voltage transfer circuit  100  eliminates current gain (beta) induced error associated with PNP transistor circuits. In addition, an inexpensive amplifier such as an LM224 may be used in the voltage transfer circuit  100  since low input voltage offset drift is not required.  
         [0049]    For example, an LM224 (typical offset drift of +/−7 uV/C) produces the following results for a temperature change of 50 C.  
         Δ V =±7 μV/°Cx 50° C.=±35 mV.  
         [0050]    This shows that l1*R 101  would have to change by only 0.35 mV to compensate 50 degrees of temperature change. For a battery segment voltage of 4 volts, this represents an error of only 0.009%, whereas the +/−1 bit error of a conventional A/D is approximately +/−0.125% when using a 5 Vdc reference. This shows that temperature variation is primarily dependent only on the temperature induced error of the R 102 /R 101  ratio. The calibration procedure to reduce the initial tolerance is the same as described above for FIGS. 3 and 4.  
         [0051]    [0051]FIG. 6 is a schematic diagram of a presently preferred embodiment of a voltage transfer circuit  200  in accordance with the principles of the invention. The voltage transfer circuit  200  is similar to voltage transfer circuit  100  in function with corresponding elements numbered in the range  200 - 299 , except that voltage transfer circuit  200  includes on-off control circuitry for minimizing leakage current. Optical switch circuit  234  is connected between the positive terminal of the battery pack  202  and Vcc of the quad amplifier pack  208 . Optical switch circuit  236  is connected between node  218  and Vss of the quad amplifier pack  208 . Each amplifier circuit  222  additionally includes a control switch Q 202  connected to node  240  in series with the sense resistor  232 . The divider circuit additionally includes a control switch Q 203  connected in series with R 201  and R 203 . The optical switch circuits  234  and  236 , and control switches Q 202  and Q 203  are controlled by the application of a control voltage  242 . Preferably, 15 volt is applied as the control voltage  242  to turn-on the voltage transfer circuit  200 . An open or 0 volts applied as the control voltage  242  causes the voltage transfer circuit  200  to turn-off. The on-off control circuitry advantageously mitigates the flow of leakage currents drawn from the battery pack during periods when the voltage transfer circuit  200  is off. Leakage currents can add up to a significant loss in battery energy when the system remains inactive or in storage for several weeks. During storage or inactive periods, the optical switch circuit  234  disconnects the battery pack from Vcc of the quad amplifier pack  208 . However, in spite of the operation of optical switch  234 , the amplifier inputs remain connected to the battery pack  202  providing a path for leakage currents. To open the paths to the amplifier inputs, it is desirable to disconnect Vss using optical switch  236  and to also disconnect the amplifier outputs using the Q 202  transistors. Preferably, FETs are used instead of BJTs for Q 201  in the amplifiers  222  to further reduce the flow of leakage current. A zener diode, D 201 ,  244  is connected in parallel with the gate-source junction of transistor Q 201  to protect the junction from damaging voltages during the off-state.  
         [0052]    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.