Abstract:
A system and method for charging an undercharged cell of a bank of series connected cells utilizes a charging circuit. The charging circuit includes an inductor that receives current from the entire bank of cells and then provides current to the undercharged cell. Pulse width modulation is utilized to turn the switches on and off to regulate the current that flows to the inductor and thus is provided to the undercharged cell.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/895,768 filed Mar. 20, 2007, which is incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention relates generally to a system and method of balancing a state of charge (SOC) of a plurality of cells connected in series. 
     2. Description of the Related Art 
     Electric vehicles and hybrid-electric vehicles typically utilize numerous cells (i.e., batteries) for powering electric loads such as drive motors and other electric equipment. These cells are often connected together in a series relationship, as is well known to those skilled in the art, to provide higher voltages. 
     Due to variations between individual cells, such series-connected cells require periodic balancing, i.e., charge equalization, to maintain a steady voltage. prevent premature failure, and provide maximum power to the load. Various systems and techniques have been developed to address this necessity. For example, active cell balancing methods include charge shunting, where current is shunted around fully charged cells; charge shuttling, where charge is removed from one cell and delivered to another cell; energy converters, where inductors or transformers move energy from a cell or group of cells to another cell or group of cells; and energy dissipation, where charge is removed from the cells with the highest charge. 
     Particularly, the use of energy converters, such as inductive charge shuttling, to shuttle current between individual cells is commonly used. However, inductive charge shuttling techniques are typically “open loop” techniques, i.e., the current delivered to the cell to be charged is based on the tolerance of the inductor and generally not variable. 
     As it as advantageous to vary the amount of current provided to the cell based on the current charge of the cell, there remains an opportunity for a method and system for balancing a state of charge of a cell based on the current charge of the cell. 
     SUMMARY OF THE INVENTION AND ADVANTAGES 
     The subject invention includes a system for balancing a state of charge (SOC) of a plurality of cells connected in series. The system includes a positive bus and a negative bus. The system also includes a plurality of positive bus switches with each positive bus switch electrically connected to the positive bus and to one of the cells for electrically connecting the positive bus to a positive side of the one of the cells. The system further includes a plurality of negative bus switches with each negative bus switch electrically connected to the negative bus and to one of the cells for electrically connecting the negative bus to a negative side of the one of the cells. A charging circuit is electrically connected to a voltage source and the busses for charging an undercharged cell of the plurality of cells. The charging circuit includes an inductor having a first side electrically connected to the negative bus and a second side electrically connected to the positive bus. A first switch is electrically connected to the first side of the inductor and a positive terminal of a voltage source. A second switch is electrically connected to the second side of the inductor and a negative terminal of the voltage source. The system also includes a controller in communication with the first and second switches for activating the first and second switches to transfer energy from the voltage source to the inductor and deactivating the first and second switches to transfer energy from the inductor to the undercharged cell of the plurality of cells. 
     The subject invention also provides a method of balancing a state of charge (SOC) of a plurality of cells connected in series. The method includes the step of identifying an undercharged cell from the plurality of cells to charge. The method also includes the steps of electrically connecting a positive side of the undercharged cell to a positive bus and electrically connecting a negative side of the undercharged cell to a negative bus. An inductor is electrically connected to a voltage source during a first time period. The inductor is electrically disconnected from the voltage source in response to an elapse of the first time period. The method also includes the step of maintaining the disconnection of the inductor from the plurality of cells for a second time period corresponding generally to the time for the inductor to discharge energy to the undercharged cell. 
     By selectively connecting and disconnecting the inductor from the voltage source, the system and method of the subject invention provides a controlled amount of current to charge the undercharged cell. This controlled amount of current may be varied based on the amount of charge on the undercharged cell. Accordingly, the speed at which the undercharged cell is charged is increased over that of the prior art. Furthermore, the system and method of the subject invention does not waste energy that occurs in charge shunting or energy dissipation techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is a block electrical schematic diagram of the system showing a plurality of cells, a positive bus, a negative bus, a plurality of bus switches, and a plurality of bus diodes; 
         FIG. 2  is a block electrical schematic diagram of the system showing a controller and a charging circuit; and 
         FIG. 3  is a flowchart diagram showing a method of the subject invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a system  10  and method  100  for balancing a state of charge (SOC) in a plurality of cells  12  connected in series is shown. Those skilled in the art realize that a “cell” is commonly referred to as a “battery”. However, for purposes of consistency, the term cell  12  shall be used throughout and should not be regarded as limiting in any way. 
     Each cell  12  includes a positive side (i.e., a cathode) and a negative side (i.e., an anode) as is known to those skilled in the art. Referring to  FIG. 1 , the plurality of cells  12  are electrically connected together in series. That is, the negative side of a first cell  12 A is electrically connected to the positive side of a second cell  12 B, the negative side of the second cell  12 B is electrically connected to the positive side of a third cell  12 C, and so on. Furthermore, each cell  12  may actually be multiple physical cells  12  connected in series and/or parallel. The cells  12  may be implemented in a vehicle (not shown), such as an electrically driven vehicle or a hybrid-electric vehicle. However, those skilled in the art realize that the cells  12  may be implemented in other environments including non-vehicle applications. 
     In the illustrated embodiments of the invention, five cells  12  are connected in series, and labeled sequentially from the first cell  12 A through a fifth cell  12 E. Of course, any number, type, or capacity of cells  12  may be utilized with the subject invention, and the use of five cells  12  in the illustrated embodiment should not be regarded as limiting. The system  10  may include one or more voltage sensors (not shown) electrically connected to the cells  12  for sensing the voltage of each cell  12 . Alternatively, the voltage sensors may be a part of an external device (not shown) that is in communication with the system  10  of the illustrated embodiment. 
     In the illustrated embodiment, the system  10  includes a positive bus  14  and a negative bus  16 . A plurality of positive bus switches  18  are electrically connected to the positive bus  14  and plurality of negative bus switches  20  are electrically connected to the negative bus  16 . Accordingly, the positive bus  14  may be implemented as the electrical connections between the positive bus switches  18  and the negative bus  16  may be implemented as the electrical connections between the negative bus switches  20 . 
     The bus switches  18 ,  20  serve to electrically connect the busses  14 ,  16  to the cells. Specifically, in the illustrated embodiments, each positive bus switch  18  electrically connects the positive bus  14  to a positive side of one of the cells  12  and each negative bus switch  20  electrically connects the negative bus  16  to a negative side of one of the cells  12 . 
     The bus switches  18 ,  20  are shown in  FIG. 1  with a generic electrical switch symbol. Preferably, the bus switches  18 ,  20  are implemented with transistors, such as metal-oxide-semiconductor field-effect transistors (MOSFETs). Of course, other alternative techniques for implementing the bus switches  18 ,  20  are realized by those skilled in the art, such as, but not limited to, mechanical-type switches. 
     In the illustrated embodiment, the system  10  also includes a plurality of positive bus diodes  22  and a plurality of negative bus diodes  24 . Each positive bus diode  22  is electrically connected between each positive bus switch  18  and each cell  12  to allow current flow from the positive bus  14  to the cell  12 . Each negative bus diode  24  is electrically connected between each negative bus switch  20  and each cell  12  to allow current flow from the cell  12  to the negative bus  16 . For purposes of illustration, the positive bus switches  18  are labeled sequentially from a first positive bus switch  18 A through a sixth positive bus switch  18 F and the negative bus switches  20  are labeled sequentially from a first negative bus switch  20 A through a sixth negative bus switch  20 F. The positive bus diodes  22  are labeled sequentially from a first positive bus diode  22 A to a sixth positive bus diode  22 F and the negative bus diodes  24  are labeled sequentially from a first negative bus diode  24 A to a sixth negative bus diode  24 F. In  FIG. 1 , the bus diodes  22 ,  24  are shown as Zener diodes. However, other suitable diode types may also be acceptable. 
     In an alternative embodiment (not shown), the system  10  includes a plurality of field-effect transistors (FETs) (not shown) instead of the diodes  22 ,  24  of the illustrated embodiment. Specifically, the FETs are electrically connected between each bus switch  18 ,  20  and each cell  12 . This alternative embodiment configuration allows bidirectional flow of current between the cells  12  and the busses  14 ,  16 . This allows the shuttling of current between the cells  12 . Preferably, the FETs are high-speed FETs, which are well known by those skilled in the art. 
     The system  10  of the illustrated embodiment also includes a controller  28  for controlling the SOC balancing, i.e., the charging and discharging, of the cells  12 . The controller  28  may be implemented with multiple electronic devices or may be packaged in one microchip, as is well known to those skilled in the art. 
     The system  10  also includes a charging circuit  30 . The charging circuit  30  facilitates the charging of the various cells  12 . The charging circuit  30  is electrically connected to a voltage source  32  for receiving electric current. The voltage source  32  includes a positive terminal  34  and a negative terminal  36 . In the illustrated embodiments, the series-connected cells  12  act as the voltage source  32 . However, in other embodiments, the voltage source  32  may be an external source (not shown) such as other battery packs or power supplies. The charging circuit  30  is also electrically connected to the busses  14 ,  16 . The charging circuit  30  supplies electric current to the busses  14 ,  16  for charging an undercharged cell  12 , as described in detail below. 
     The charging circuit  30  includes an inductor  38  having a first side  40  and a second side  42 . The first side  40  is electrically connected to the negative bus  16  and the second side  42  is electrically connected to the positive bus  14 . The inductor  38  may be implemented as a pair of inductors connected in series. One suitable inductor  38  has an inductance of about 47 uH, such as the DR125-470 manufactured by Cooper-Bussman. 
     The charging circuit  30  also includes a first switch  44  and a second switch  46 . The first switch  44  is electrically connected between the first side  40  of the inductor  38  and the positive terminal  34  of the voltage source  32 . As such, the first switch  44  electrically connects and disconnects the inductor  38  from the voltage source  32 . The second switch  46  is electrically connected between the second side  42  of the inductor  38  and the negative terminal  36  of the voltage source  32 . Accordingly, the second switch  36  also electrically connects and disconnects the inductor  38  from the voltage source  32 . The controller  28  is in communication with the first and second switches  44 ,  46  for activating the first and second switches  44 ,  46  to transfer energy from the voltage source  32  to the inductor  38  The controller  28  also controls deactivation of the first and second switches  44 ,  46  to transfer energy from the inductor  38  to the undercharged cell  12  of the plurality of cells  12 , as is further described below. 
     Preferably, the first and second switches  44 ,  46  are implemented with transistors, such as MOSFETs having a gate, a source, and a drain. With respect to the first switch  44 , the gate is electrically connected to the controller  28 , the source is electrically connected to the positive terminal  34  of the voltage source  32 , and the drain is electrically connected to the first side  40  of the inductor  38 . With respect to the second switch  46 , the gate is electrically connected to the controller  28 , the source is electrically connected to the second side  42  of the inductor  38 , and the drain is electrically connected to the negative terminal  36  of the voltage source  32 . 
     The charging circuit  30  further includes a current sensor  48  electrically connected between the second switch  46  and the negative terminal  36  of the voltage source  32 . The current sensor  48  is in communication with the controller  28  for sensing an amount of current flowing through the inductor  38 . As such, the amount of current that is to be delivered to the undercharged cell  12  may be regulated. 
     In the illustrated embodiment, the current sensor  48  is implemented as a resistor  50 . Those skilled in the art refer to this resistor  50  as a “current sense resistor”. The voltage across the resistor  50  is communicated to the controller  28 . The controller then computes the current flowing through the resistor  50 , and thus generally the current flowing through the inductor  38 . 
     In the illustrated embodiments, the charging circuit  30  also includes a charging diode  52 . The charging diode  52  has an anode (not numbered) electrically connected to the second side  42  of the inductor  38  and a cathode (not numbered) electrically connected to the positive bus  14 . The charging diode  52  allows current to flow from the inductor  38  to the positive bus  14 , but generally prevents current from flowing from the positive bus  14  to the inductor  38 . The charging diode  52  is preferably implemented as a Zener diode; however, other types of diodes may also be suitable. Also in the illustrated embodiments, the charging circuit  30  includes a capacitor  54  electrically connected between the positive bus  14  and the negative bus  16 . 
     The system  10  may also include a positive bus transfer switch  56  and a negative bus transfer switch  58 . The positive bus transfer switch  56  is electrically connected to the positive bus  14  and the negative bus transfer switch  58  is electrically connected to the negative bus  16 . These transfer switches  56 ,  58  may be utilized to electrically connect the busses  14 ,  16  to an external bank of cells (not shown). Therefore, the cells  12  may be used to charge other, external cells. The transfer switches  56 ,  58  may also be utilized to transfer energy, i.e., electric current, to an external system (not shown). The transfer switches  56 ,  68  may further be utilized to gather energy from an external system (not shown). 
     The method  100  of the subject invention may utilize the system  10  described above. However, those skilled in the art realize alternative techniques for implementing the method  100 . Therefore, the method  100  described herein should not be read as limited only to use with the system  10  described herein. 
     The method  100  includes the step  102  of identifying an undercharged cell  12  from the plurality of cells  12  to charge. That is, determining which of the cells  12  is most in need of charging. Those skilled in the art comprehend multiple techniques for performing this step. One technique includes the utilization of voltage detection circuits (not shown) which are electrically connected to each cell  12 . These voltage detection circuits determine a voltage of each cell  12  and are in communication with the controller. 
     Once it is determined which of the cells  12  is to be charged, the method continues with the step of  104  of electrically connecting a positive side of the undercharged cell  12  to the positive bus  14 . This step  104  may be achieved by closing the positive bus switch  18  electrically connected to the positive side of the undercharged cell  12  such that current may flow from the positive bus  14  to the undercharged cell  12 . 
     The method also includes the step  106  of electrically connecting a negative side of the undercharged cell  12  to the negative bus  16 . Again, this step  106  may be achieved by closing the negative bus switch  20  electrically connected to the negative side of the undercharged cell  12 . 
     The method further includes the step  108  of electrically connecting the inductor  38  to the voltage source  32  during a first time period. As such, the inductor  38  receives electric current from the voltage source  32 . Specifically, current flows from the first side  40  to the second side  42  of the inductor  38 . This step  108  is accomplished by closing the first and second switches  44 ,  46  to allow current to flow through them, i.e., activating the MOSFETs. 
     The method also includes the step  110  of electrically disconnecting the inductor  38  from the voltage source  32  in response to an elapse of the first time period. This is accomplished by opening the first and second switches  44 ,  46 , i.e., deactivating the MOSFETs, thus preventing current from flowing. 
     The first time period is determined by the amount of current flowing through the charging circuit  30  in general and the inductor  38  in particular. Said another way, the first and second switches  44 ,  46  permit the flow of current until the current reaches a predetermined level. The predetermined level corresponds to the desired rate of current flow to charge the undercharged cell  12 . To determine the amount of current flowing through the inductor, the method  100  also includes the step  112  of sensing the current flowing through the inductor  38 . In the illustrated embodiment, this step  112  is implemented with the current sensor  48  disposed between the second switch  46  and the second terminal  36  of the voltage source  32 . 
     The method  100  further includes the step  114  of maintaining the disconnection of the inductor  38  from the plurality of cells  12  for a second time period. The second time period corresponds generally to the time for the inductor  38  to discharge energy to the undercharged cell  12 . Preferably, the second time period may be adjusted based on the voltage of the undercharged cell  12  in comparison to the other cells  12 . 
     The steps  104 ,  106 ,  108 ,  110 ,  112 ,  114  of electrically connecting the inductor  38 , sensing the current, electrically disconnecting the inductor  38 , and maintaining the disconnection in response to the undercharged cell  12  remaining undercharged with respect to the plurality of cells  12  are repeated as necessary to fully charge the undercharged cell  12 . The first and second time periods may be varied based on the changing charge of the undercharged cell  12 . As such, a pulse-width modulation (PWM), based on the charged of the undercharged cell  12 , is performed by the charging circuit  30 . 
     The present invention has been described herein in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the invention are possible in light of the above teachings. The invention may be practiced otherwise than as specifically described within the scope of the appended claims.