Abstract:
A battery management system for managing the voltage of a battery cell, such as a lithium ion battery, is disclosed. The battery management system comprises a semiconductor switch coupled to the battery cell, wherein the semiconductor switch is in an on condition when the voltage across the battery cell exceeds a first threshold voltage, and a microprocessor coupled to the semiconductor switch, wherein the microprocessor monitors the voltage across the battery cell when the semiconductor switch is on, and turns itself off when the when the monitored voltage is less than a second threshold voltage, thereby preventing further current drain from the battery cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/185,809, filed on Jun. 10, 2009, the entirety of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a system for preventing deep discharge of one or more battery cells of a battery pack. 
     BACKGROUND OF THE INVENTION 
     Battery packs, such as for providing electrical power to electrically powered devices, such as electrical vehicles, hybrid vehicles, wheelchairs, e-bikes, and electric scooters, are known. Battery packs typically include a plurality of individual battery cells electrically coupled in series and/or parallel so as to provide a desired output voltage and capacity. Often battery management systems are provided to monitor and control the operation of the battery pack. In general these battery management systems may themselves draw current from the battery pack, even when the vehicle is not drawing power from the battery pack. Over time this can result in the battery pack being drained to what is referred to as a “deep discharge” condition, which can limit the useful life of the battery pack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a system for preventing deep battery discharge, such as might be incorporated in a battery management system for a lithium ion battery pack, in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     While this invention is susceptible of embodiment in many different forms, there will be described herein in detail, a specific embodiment thereof 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 invention to the specific embodiment illustrated. 
     A system for preventing deep battery discharge, or low voltage cut-off circuit, in accordance with the invention, and generally designated  10 , is illustrated in  FIG. 1 . The low voltage cut-off circuit  10  is part of a battery management system  11 . The low voltage cut-off circuit  10  is shown coupled to two lithium ion battery cells,  12 ,  14 , which are coupled in series. Typically battery packs include many more battery cells, but for purposes of discussion, only the two battery cells  12 ,  14  will be discussed. The battery cells  12 ,  14  are preferably lithium iron phosphate battery cells, although they may be of other battery chemistries. 
     In accordance with the present description, the battery cells  12 ,  14  each have a nominal voltage of 3.6-3.7 volts when fully charged, but they typically operate at 3.2-3.3 volts. The battery cells  12 ,  14  provide electrical power to an electrically powered device (not shown). 
     As is known, the battery management system  11  performs various conventional functions. One such function is to monitor the voltage across the battery cells, either individually or collectively. When the battery management system  11  determines that the voltage of the monitored battery cells has dropped below a desirable operating threshold, such as 2.3 volts per battery cell, the battery management system  11  typically turns off battery power to the device. It may be desirable that the battery management system  11  continue to operate, even if power to the device has been turned off, such as to provide an indication that the device was shut down due to a low battery condition. However as noted above, even when the device is turned off, the battery management system  11  itself continues to draw current. The low-voltage cut-off circuit  11  is provided to turn off power to the battery management system  11  when the voltage across the battery cells drops below a cut-off threshold, such as approximately 2.1 volts per cell. This can be accomplished on a battery cell by battery cell basis, or collectively across a series of battery cells. 
     The low voltage cut-off circuit  10  includes a Zener diode  16 , first through fourth resistors  18 , typically having a high impedance in the range of 100 kΩ to 10 MΩ, a complementary transistor switch comprising first and second complementary transistors  26 ,  28 , respectively, and a microprocessor  32  having memory and microprocessor logic stored thereon. The first transistor  26  may be an N-channel MOSFET, and the second transistor  28  may be a complementary P-channel MOSFET. Alternatively the first transistor  26  may be an npn transistor, and the second transistor  28  may be a complementary pnp transistor, or such other combinations of complementary transistor devices. The microprocessor may be a Microchip PIC16F690, a Microchip PIC12F675, a Microchip PIC16F684, a Microchip PIC16F616, or an Atmel ATtiny13A series, or such other conventional microprocessor. 
     In accordance with the invention, the low voltage cut-off circuit  10  obtains zero current draw from the battery cells  12 ,  14 , when the low voltage cut-off circuit  10  is off. 
     When the battery voltage drops below a useable voltage level, typically about 2.1 volts (±0.1 v) per cell for the present battery chemistry, the low voltage cut-off circuit  10  is off, preventing flow of current from the battery cells  12 ,  14 . 
     In accordance with the invention, the low voltage cut-off circuit  10  automatically reactivates when the battery pack voltage rises to a useable value. 
     Referring to  FIG. 1 , the low voltage cut-off circuit  10  is off until the battery voltage collectively across first and second battery cells  12 ,  14  rises above the threshold voltage of the Zener diode  16 . The threshold voltage of a Zener diode (i.e., the voltage at which a Zener diode begins to conduct) is typically slightly lower than its nominally rated Zener voltage. The rated voltage of the Zener diode  16  is selected depending upon the number of battery cells being measured. In the present disclosure with respect to two battery cells, the rated voltage of the Zener diode  16  is 4.7 v, but could also be 5.1 v or 5.7 v, depending upon the desired circuit operation, i.e., when one wants the circuit to turn on. 
     Once the voltage across the first and second battery cells  12 ,  14  reaches the threshold voltage of the Zener diode  16 , a “first” threshold voltage, the Zener diode  16  begins to conduct, and the current flows through the first resistor  18 . This raises the gate voltage on the first transistor  26 , causing the first transistor  26  to conduct. When the first transistor  26  conducts, current flows through the second resistor  20 . This causes the gate voltage of the second transistor  28  to fall (i.e., to increase in a negative direction), and current begins to flow through the second transistor  28 . Thus the complementary switch is in an ON, or conducting, condition 
     The third resistor  22  is a feedback resistor, which conducts some of the current flowing through the second transistor  28  to flow back through the first transistor  18 . This helps to maintain the first transistor  26 , and thus the second transistor  28 , conducting, and provides some hysteresis in the turn-on voltage of the first transistor  26 . In other words, the turn-on voltage (the “first” threshold voltage) is slightly higher than the turn-off voltage, a “second” threshold voltage, preventing oscillation due to voltage fluctuations of the battery cells  12 ,  14 . 
     The microprocessor  32  includes a battery voltage sensing input  32   a , a ground reference input  32   b  and a hold logic signal output  32   c.    
     The fourth resistor  24  is coupled to hold logic signal output  32   c , which is controlled by microcontroller logic resident in the memory of the microprocessor  32 . When the second transistor  28  conducts, the battery sensing input  32   a  senses the voltage across the first and second battery cells  12 ,  14 , permitting the microprocessor  32  to monitor the voltage across the first and second battery cells  12 ,  14 . As long as the sensed battery voltage of the battery cells  12 ,  14  remains above a third threshold voltage, which set by the microprocessor  32  and which is less than the second threshold voltage, the hold logic signal from the hold logic output  32   c  is high. This supplies additional current back to the first transistor  26 , keeping the low voltage cut-off circuit  10  active. 
     When the voltage level of the battery cells  12 ,  14 , falls below the threshold voltage of the Zener diode  16 , i.e., the first threshold voltage, the Zener diode  16  will stop conducting. However the current through the third resister  22  and the hold logic signal through the fourth resister  24  will cause the first and second transistors  26 ,  28  to continue conducting, and the microprocessor  32  will continue to monitor the voltage across the battery cells  12 ,  14 . When the voltage across the first and second battery cells  12 ,  14  falls below the second threshold voltage, the current through the third resister will no longer be sufficient to cause the first and second transistors  26 ,  28  to continue conducting, but the hold logic signal will cause the first and second transistors  26 ,  28  to continue conducting, and the microprocessor  32  will continue to monitor the voltage across the battery cells  12 ,  14 . However when the voltage across the first and second battery cells  12 ,  14  falls below the third threshold voltage, the hold output signal will go low, causing the first transistor  26 , and thus the second transistor  28 , to stop conducting. This causes the input to the voltage sensing input  32   a  of the microprocessor to go to zero. In response to the input to the voltage sensing input  32   a  of the microprocessor going to zero, the microprocessor  32  shuts itself down. This causes the hold logic signal to go low and the low voltage cut-off circuit  10  will shut off completely. 
     Because the value of the third threshold voltage is set by the microprocessor logic of the microprocessor  23 , the value can be readily adjusted and the voltage at which the cut-off circuit  10  turns off can be tightly regulated. 
     Later when the battery cells  13 ,  14  are replaced, or recharged, and the voltage exceeds the first threshold voltage (i.e., the threshold voltage of the Zener diode  16 ), the Zener diode will begin again to conduct, the first transistor  26  will conduct, causing the second transistor  28  to conduct. The microprocessor  32  will then sense the battery voltage at the battery voltage sensing input  32   a , causing the microprocessor  32  to turn on, and the process repeats itself. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claim all such modifications as fall within the scope of the claim.