Abstract:
A circuit for preventing overcharging a battery cell is disclosed. The circuit comprises a voltage detector for monitoring a voltage across a battery cell. The voltage detector is configured to determine when the voltage across the battery cell is greater than a first threshold voltage. The circuit further comprises an electronic switch responsive to the voltage detector and is configured to shunt current around the battery cell when the voltage detector has determined the battery cell voltage is greater than the first threshold voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/253,651, filed on Oct. 21, 2009, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to battery charging circuitry to balance the charging of series connected battery cells, such as lithium ion battery cells. 
         [0003]    The invention further prevents damage to battery cells if the cells are connected to the battery charging circuitry in reverse polarity. 
       BACKGROUND OF THE INVENTION 
       [0004]    The nominal voltage of a fully charged lithium iron phosphate (LFP) battery cell is approximately 3.65 volts. This voltage is battery chemistry dependent and can vary for battery cells having other lithium ion chemistries. 
         [0005]    When charging series connected lithium ion battery cells, the cells do not necessarily charge at the same rate. Thus one of the battery cells may begin to exceed its nominal voltage before the other battery cell, or cells, reach their nominal voltage(s). As battery chargers typically do not stop charging the series connected cells until the cumulative series voltage of the cells reaches a threshold voltage, the cell which first reaches its nominal voltage may end up being over-charged, which can damage the particular battery cell. 
         [0006]    There can also be a problem if the battery charger is connected to one or more battery cells in reverse polarity. 
         [0007]    The present invention is provided to address these and other problems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram of a battery charger coupled to two series connected battery cells, in accordance with the present invention; 
           [0009]      FIG. 2  is a schematic of a first embodiment of a circuit contained in the battery charger of  FIG. 1 , the circuit for balancing the charging of the battery cells; and 
           [0010]      FIG. 3  is a schematic of a second embodiment of a circuit contained in the battery charger of  FIG. 1 , the circuit for balancing the charging of the battery cells. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail, preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. 
         [0012]    A battery charger  10  coupled to two battery cells  12 , is illustrated in  FIG. 1 . The battery cells  12  may be conventional lithium iron phosphate battery cells, of the type having a nominal fully charged voltage of 3.65 volts, or may be another type of rechargeable battery cell. 
         [0013]    Associated with the battery charger  10  are a plurality of battery charging circuits  18 , one associated with each of the battery cells  12 . 
         [0014]    A schematic of a first embodiment of the battery charging circuit  18  is illustrated in  FIG. 2 . The battery charging circuit  18  includes a first electrode  20  coupled to a positive terminal its respective battery cell  12  and a second electrode  22  coupled to a negative terminal of its respective battery cell  12 . 
         [0015]    The battery charging circuit  18  includes a voltage detector  24 , such as an S-80835 IC voltage detector having a 3.5 volt detection voltage value, sold by Seiko Instruments, Inc., Japan. The battery charging circuit  18  further includes an electronic switch  28 , such as a Zetex N-channel MOSFET, model ZXMN6A07FTA, sold by Diodes Incorporated of Dallas, Tex. The battery charging circuit  18  also includes a conventional yellow LED  36 . 
         [0016]    In normal operation, the battery charger  18  provides current through the serially connected cells  12 . The voltage detector  24  associated with each of the cells  12  monitors the voltage across its respective one of the cells  12 . As long as the voltage is sufficiently low, the output of the voltage detector  24  maintains the electronic switch  28  in an open position. Thus the charging current will flow through the respective cell  12 . However when the voltage detector  24  detects the voltage across its respective cell  12  has reached a voltage in the range of 3.6 to 3.68 volts, the output of the voltage detector  24  causes the electronic switch  28  to close, thus shunting charging current around that particular cell  12 , through resistors  30 ,  32  and  34 , and effectively stopping the charging of that particular cell  12 . Also this shunted current also illuminates the LED  36 . Multiple resistors are utilized in parallel to reduce the current through any particular one of the resistors, thus permitting lower wattage resistors to be utilized. 
         [0017]    Should the voltage across that particular cell  12  subsequently drop below 3.5 volts, the voltage detector will cause the electronic switch  28  to again open, causing the full amount of charging current to flow through the particular cell  12 . 
         [0018]    A second embodiment of the present invention is illustrated in  FIG. 3 . The second embodiment protects the battery cell should the battery cell be connected to the the charger in reverse polarity. If in reverse polarity, typical battery chargers are fine, as typically they are current limited to allow only a limited amount of current to charge the battery cells. Further the battery cells would typically not be damaged until the conventional charger depletes the stored charge of the battery cells being charged. At that point, the conventional battery cell charger would typically attempt to charge the battery cells in the reverse direction. An Ideal capacitor, rather than a battery cell, would charge to negative polarity. However battery cells would tend to develop internal shorts due to the chemistry involved. 
         [0019]    In the embodiment illustrated in  FIG. 2 , the voltage detector used determines when to do the balance current bleed, or overflow, and this item could be damaged due to a reversed input polarity. 
         [0020]    A second embodiment of the battery charging circuit  18 ′ is illustrated in  FIG. 3 . This circuit  18 ′ is provided to protect the voltage detector  24  should the battery cell  12  be connected in reverse polarity. 
         [0021]    The circuit  18 ′ includes a diode-like device in the form of a PNP transistor  40  in the circuit path to shunt, and thereby eliminate, any current from reverse polarity connections for the voltage detector  24 . 
         [0022]    The battery charging circuit  18 ′ according to the second embodiment includes a voltage detector  24 ′ having a lower detection voltage value than the voltage detector  24  discussed above, to compensate for the approximately 0.2v voltage drop across the transistor  40 . The voltage detector  24 ′ may be an NCP300LSNT1G voltage detector, sold by ON Semiconductor, Phoenix, Ariz., or an S-80834CLMC-B6TT2G voltage detector, sold by Seiko Instruments, Japan. 
         [0023]    The second embodiment  18 ′ further includes a second electronic switch  28 ′ and a second pair of parallel resistors, in parallel with the first electronic switch and corresponding parallel pair of resistors, to reduce the amount of shunted current through the first electronic switch  28 . The first and second electronic switches  28 ,  28 ′ include an inherent diode characteristic which permits current flow should the battery cell  12  be installed in a reverse-biased manner. This current flow illuminates a Red LED  46  to indicate when current is flowing in the wrong direction. The electronic switches  18 ,  18 ′ and the LED&#39;s  36 ,  46  are not adversely affected by a reverse battery cell connection. 
         [0024]    It should be noted that even with the circuit  18 ′, it is possible to connect the charger to the battery cell  12  in reverse polarity and deplete the charge of the battery cell. The polarity protection is for the balancing circuit when connecting to the battery cells. 
         [0025]    While specific embodiments have been illustrated and described, numerous modifications may come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claim.