Abstract:
A method and device for balancing the charges of a plurality of series-connected battery cells, the device comprising a current dissipative loop connected across the terminals of each battery cell, the voltage across each battery cell being measured, the average value of the measured voltages being determined, the measured voltage being then compared to the average value, and the measured voltage of each battery cell being adjusted to the average value so as to equalize charges of all battery cells.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and a device for balancing the charges of a plurality of series-connected battery cells. 
     2. Description of the Prior Art 
     The devices known for balancing the charges of series connected battery cells are based either on a comparison of the individual voltage of each battery cell with an overcharge voltage, either upon actuation of an overcurrent relay, connecting the different battery cells together. These techniques are not adapted for lithium battery cells because current or voltage overcharge can cause the destruction of battery cells. 
     EP-0 767 524 (MOTOROLA) describes a method for balancing the charge distributed among two series connected cells. This method consists in comparing the currents through each of the cells and discharging the cell whose charging voltage is the highest when the difference between the detected currents is greater then a predetermined value. One drawback of this device that, on the one hand, currents can reach high values and damage the cells, and on the other hand, that the arrangement required for carrying out this comparison is complex. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and device for balancing the charges of series-connected battery cells while avoiding the destruction of such battery cells due to a voltage or current overcharge. 
     This object is achieved by a method wherein a current dissipative loop is connected across each battery cell, the voltage across each battery cell is measured, an average value of the measured voltages is determined, the measured voltage is compared to said average value, and for a given battery cell, the measured voltage is adjusted to said average value so as to equalize charges of all battery cells. 
     For this purpose, the dissipative loop is closed if the voltage measured across the relevant battery cell is greater than the average of the measured voltages, and said dissipative loop is opened if the voltage measured across said battery cell is lower than the average of the measured voltages. 
     The method according to this invention is carried out by a device comprising means for measuring the individual voltage of each battery cell, means for determining the average voltage of all series-connected battery cells and control means for equalizing the measured voltage with said determined average voltage. 
    
    
     Other features and advantages of the invention will become apparent from the following description of preferred embodiments of the present invention, provided by way of non-limiting examples, in reference to the appended Figures. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows a block diagram of a device for carrying-out the method according to the present invention; 
     FIG. 2 schematically shows a first preferred embodiment of the device shown in FIG. 1 
     FIG. 3 schematically shows a second preferred embodiment of the device shown in FIG. 1; 
     FIG. 4 shows a modification of the measurement and control circuit in one cell, according to the second more specific embodiment shown in FIG. 3; 
     FIG. 5 shows another modification of the measurement and control circuit in one cell, according to the second specific embodiment shown in FIG. 3; 
     FIG. 6 shows still another modification of the cell measurement and control circuit, according to the second specific embodiment shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The device shown in FIG. 1 comprises a first means  2  for measuring the individual voltage of each battery cell  4 , a second means  6  for determining the average voltage of all series-connected battery cells  4 , and a control unit  8  for the equalization of the measured voltage with said determined average voltage. 
     Each battery cell  4  comprises a current dissipative loop  10 , connected to said control unit  8 , which causes either the opening of loop  10 , if the individual voltage of battery cell  4  is lower than the average voltage, or the closing of said loop if the individual voltage of battery cell  4  is greater than the average voltage. 
     In a preferred embodiment of the present invention shown in FIG. 2, said dissipative loop  10  comprises a comparator  12  and a resistor Rc  14 , a first terminal  16  of which, of electric potential V r+ , is connected to the positive terminal  18  of comparator  12  through a first resistor R S , and a second terminal  20  of which, of electric potential V r− , is connected to the negative terminal  22  of said comparator  12  via a second resistor R S , which is identical to the first one, each of said positive  18  and negative  22  terminals of comparator  12  being additionally connected, respectively, to the negative terminal  24  and to the positive terminal  26  of battery cell  4  of electric potentials V c−  and V c+ , respectively, via a third and a fourth resistors R S , which are identical to the first one. The output of comparator  12  is connected to a dissipating resistor Rb. The dissipative loop  10  therefore comprises, in the drawing of FIG. 2, a resistor Rb, the comparator  12 , which controls the opening and closing of the loop, and fourth resistor R S . 
     With such a device, the dissipative loop  10  is controlled so as to be closed when the following condition is fulfilled: 
     
       
         V c +V r− &gt;V r+ +V c−   (1)  
       
     
     In an embodiment which is not shown, the operation of the device according to the present invention is improved by the addition of a positive feedback resistor connecting the output of the comparator  12  to the positive input  18  of said comparator. This causes a hysteresis that prevents oscillations when the voltages of battery cells are close to each other. 
     In another embodiment, the number of opening cycles of dissipative loop  10  can be substantially reduced by a slight modification in the input resistances R S  of comparator  12  so as that the dissipative loop is closed when the individual voltage of a battery cell  4  exceeds the average voltage by a predefined value. This allows for small variations in the voltages of battery cell  4  without closing the dissipative loop  10  at the same time. 
     According to another embodiment, the value of both summing resistors at the input of comparator C can be slightly changed so as to derive from the cell current only when their charge exceeds the predetermined average value by a given value. This causes a reduction in the number of switching cycles and allows small variations in the cell voltages to be obtained without at the same time deriving therefrom. 
     According to yet another embodiment shown in FIG. 3, the resistor network Rc was eliminated from the circuit shown in FIG. 2, and resistors R S  connecting the positive and negative inputs of comparator  12 , respectively, to voltages V r+  and V r−  are replaced by resistors of value n*R S , where n is the number of battery cells  4 , and which are related to the battery voltages V B+  and V B− , respectively. Therefore, the voltage comparison which is thus carried-out by comparator  12  is equivalent to a voltage comparison performed by the corresponding cell with the average cell voltage (battery voltage/n). 
     By means of this arrangement, the input voltages of the comparator are always ranging between its supply voltages V CC  and V DE , whatever the imbalances between the respective charges of cells  4 . The battery power which is dissipated within the resistor network Rc becomes zero. In addition, it is easier to adapt the circuit to the cell number when one of them is detected to be faulty and is disconnected. The latter feature is essential for space applications, since there are normally other devices in the battery which will permanently disconnect any cell detected as faulty. 
     In the case of the circuit shown in FIG. 2, resistor Rc corresponding to a faulty cell should be short-circuited, which requires the addition of n switching circuits, where n is the number of battery cells. On the contrary, in the circuit shown in FIG. 3, only one switch is necessary for the entire battery. In this respect, according to a first aspect shown in FIG. 4, each cell is associated with two additional resistors of resistance (n−1)*R S , which are thus dimensioned for a battery comprising n−1 cells. Before cell failure, the first set of resistors n*R S  is connected in parallel with the connection terminals of the battery. When detecting a cell failure, the first resistor set is disconnected, whereas the second resistor set (n−1)*R S  is connected, by means of a switch  25 . 
     In a specific embodiment of the present invention shown in FIG. 4, the output of comparator  12  is connected to the base of a pnp transistor T connected in parallel across the positive  26  and negative  24  terminals of each cell  4 , the collector of transistor T being connected to terminal  24  of the cell through a resistor R 1 , which is series-connected to a Zener diode Z 1  or to a potential barrier, so as to avoid cell discharging if its charge voltage has decreased below a predetermined voltage threshold. In this circuit, the dissipative loop comprises resistor R 1 , Zener diode Z 1  and the collector-emitter junction of transistor T. Of course, Zener diode Z 1  can be series-connected to other Zener diodes so that the redetermined voltage threshold can be achieved. 
     Transistor T is useful in the case where the output current of comparator  12  is insufficient for efficiently discharging the corresponding cell  4  when its voltage is greater than the average voltage of the battery cells. 
     According to another preferred embodiment of the present invention, shown in FIG. 5, adjustable resistors R 2  are series-connected with resistors Rs for adjusting the resistance of the latter. Moreover, in this embodiment, transistor T is controlled by the output of comparator  12 , which is connected to two Zener diodes Z 1 , Z 2 , and a resistor R 3 , which are series-connected, resistor R 3  being connected to the base of transistor T. In this circuit, the dissipative loop comprises the collector-emitter junction of transistor T and a resistor R 4 , which is connected between the emitter of transistor T and terminal  24  of the corresponding cell  4 . Zener diodes Z 1  and Z 2  allow the threshold voltage at which the collector-emitter junction of transistor T is turned on, and therefore below which cell  4  is not discharged, to be increased. 
     According to a second aspect shown in FIG. 6, each circuit in the cell is provided with a set of resistors R S (n−1)*R S , which is adapted for a battery of n−1 cells. Before cell failure, this resistor set is connected through a switch  30  to the junction of two series-connected resistors Rd and (n−1)*Rd that comprise a resistor divider bridge, connected to terminals  28 ,  29  of the battery so as to achieve a voltage equal to (n−1)/n of the voltage (V B+ −V B− ) provided by the battery. After cell failure, switch  30  is toggled so as to apply the entire voltage provided by the battery, across both resistors (n−1)*Rd. 
     With respect to the first aspect shown in FIGS. 4 and 5, this aspect uses more energy due to the presence of the divider bridge, but requires less additional components. 
     Of course, any programmable device such as a computer or a custom-made circuit can be used for controlling cell charging and discharging according to the invention, while remaining within the scope of this invention.