Abstract:
A system configured to actively balance power among power cells such as batteries. The system includes a power module of series-coupled power cells, each exhibiting different charge levels during charging and discharging. A power module includes active cell balancing circuitry configured to substantially balance the charges of the power cells at least during charging. In one embodiment, the active cell balancing circuitry includes: (a) current source circuitry configured to supply extra charging current to a selected power cell; and (b) current source control circuitry configured to control the current source circuitry to supply extra charging current to the power cell with the lowest state of charge. In another embodiment, the system includes multiple power modules, each having multiple power cells coupled in series, and each having an active cell balancing circuit configured to substantially balance the charges of the power cells in an associated one of the power modules.

Description:
CROSS-REFERENCE TO RELATED APPLICATION AND PRIORITY CLAIM 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/882,781, filed Sep. 15, 2010, which claims priority to U.S. Provisional Patent Application No. 61/243,072 filed on Sep. 16, 2009, which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure is generally directed to power supply charging and discharging systems. More specifically, this disclosure is directed to active cell and module balancing for batteries or other power supplies. 
       BACKGROUND 
       [0003]    Modern batteries, such as large lithium ion batteries, often include multiple battery cells connected In series. Unfortunately, the actual output voltage provided by each individual battery cell in a battery may vary slightly. This can cause problems during charging or discharging of the battery cells. In some systems, voltage detection circuitry can be used to determine the output voltage of each battery cell, and a voltage balancing system can be used to compensate for variations in the output voltages of the battery cells. 
         [0004]    Consider battery cells connected in series, where each battery cell is ideally designed to provide an output voltage of 3v. Voltage detection circuitry may determine that one of the battery cells actually has an output voltage of 3.9V. A conventional passive voltage balancing system typically includes resistors that dissipate electrical energy from battery cells having excessive output voltages. In this example, the dissipation of electrical energy causes the 3.9V output voltage to drop to the desired level of 3.8V. However, since electrical energy is dissipated using the resistors, this can result in significant energy being lost from the battery cell, which shortens the operational life of the battery. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0005]    For a more complete understanding of this disclosure and its features, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
           [0006]      FIG. 1  illustrates an example active cell balancing circuit in accordance with this disclosure; 
           [0007]      FIG. 2  illustrates another example active cell balancing circuit in accordance with this disclosure; 
           [0008]      FIG. 3  illustrates an example active cell balancing circuit incorporating switch driving circuits in accordance with this disclosure; 
           [0009]      FIG. 4  illustrates an example algorithm that can be used during active cell balancing according to this disclosure; 
           [0010]      FIG. 5  illustrates an example power pack with multiple modules each having multiple power cells according to this disclosure; 
           [0011]      FIG. 6  illustrates example safe operating regions of various batteries according to this disclosure; 
           [0012]      FIG. 7  illustrates example uneven voltage levels on power cells in modules according to this disclosure; 
           [0013]      FIG. 8  illustrates an example active module balancing system in accordance with this disclosure; and 
           [0014]      FIG. 9  illustrates an example bi-directional active cell balancing circuit that supports active cell balancing within a module according to this disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1 through 9 , described below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any type of suitably arranged device or system. 
         [0016]    Active Cell Balancing 
         [0017]    In one aspect of this disclosure, various active cell balancing circuits are disclosed that can balance multiple power cells connected in series within a single module, such as multiple battery cells in a single battery. In some embodiments, a monitor receives information related to the power cells, such as voltage, current, and temperature. Using that information, an active balancing circuit can operate a system of switches to connect an electrical source to one or more power cells with lower voltage(s) to charge those power cells to a desired higher voltage. An active balancing circuit can also operate the system of switches to drain power from one or more power cells with excessive voltage(s) to bring the power cells to a desired lower voltage. 
         [0018]      FIG. 1  illustrates an example active cell balancing circuit  100  in accordance with this disclosure. In this example, the circuit  100  employs forward-based active cell balancing. The circuit  100  includes or is coupled to multiple power cells  102   a - 102   n  connected in series. Each power cell  102   a - 102   n  is coupled to two switches  104   a   1 - 104   a   2 ,  104   b   1 - 104   b   2 ,  104   n   1 - 104   n   2 , respectively. The power cells  102   a - 102   n  represent any suitable sources of power within a module, such as battery cells within a battery. The switches  104   a   1 - 104   n   2  represent any suitable switching devices, such as transistors. 
         [0019]    A monitor circuit  106  receives information about the power cells  102   a - 102   n , such as information concerning voltage, current, and temperature associated with the power cells  102   a - 102   n . In this example, the information includes voltage values V1-Vn from the power cells  102   a - 102   n , respectively. The information also includes a total current I flowing through the power cells  102   a - 102   n  and one or more temperatures TEMP of the power cells  102   a - 102   n.    
         [0020]    Note that the number of temperature sensors used and their locations may depend upon the nature of the particular application. A single power cell could be associated with one or multiple temperature sensors, and/or a single temperature sensor could measure the temperature of one or multiple power cells. The monitor circuit  106  represents any suitable structure for monitoring power cells, such as an integrated circuit or “IC.” 
         [0021]    As shown in  FIG. 1 , the switches  104   a   1 - 104   a   2  couple opposite ends of the power cell  102   a  to opposite ends of a transformer  108 . The switches  104   b   1 - 104   b   2  through  104   n   1 - 104   n   2  couple opposite ends of the power cells  102   b - 102   n , respectively, to the opposite ends of the transformer  108 . A diode  110  is coupled between one end of the transformer  108  and the switches  104   a   1 ,  104   b   11 ,  104   n   1 . A capacitor  112  is coupled to the diode  110  and to the other end of the transformer  108 . 
         [0022]    An output of the monitor circuit  106  is connected via a signal line  114  to a module controller  116 . The signal line  114  provides voltage, current, and temperature information or other information from the monitor circuit  106  to the module controller  116 . The signal line  114  represents any suitable signal trace or other communication path. The module controller  116  operates to control the charging of the power cells  102   a - 102   n  based on that information. 
         [0023]    In this example, the module controller  116  includes a state of charge (SOC) estimation module  118 , which estimates the state of charge for each of the power cells  102   a - 102   n . A communications module  120  facilitates communication with a central controller, which could support module balancing (described below). The communications could occur over an isolated communication link. The module controller  116  further includes an internal power management module  122 , which can control the overall operation of the module controller  116 . In addition, the module controller  116  includes an active cell balance module  124 . The active cell balance module  124  controls the operation of the switches  104   a   1 - 104   n   2 . A voltage sensor  126  is connected in parallel with the capacitor  112 , and the active cell balance module  124  receives voltage information from the voltage sensor  126 . The active cell balance module  124  also controls the operation of a transistor  128 , which can be opened to interrupt the operation of the transformer  108 . The module controller  116  represents any suitable structure for controlling active cell balancing. The voltage sensor  126  represents any suitable structure for sensing voltage. The transistor  128  represents any suitable transistor device. 
         [0024]    In one aspect of operation, the monitor circuit  106  may continually, near-continually, or intermittently monitor the voltage, current, and temperature information from the power cells  102   a - 102   n . The monitor circuit  106  can send various information to the module controller  116 . If the module controller  116  determines that the first power cell  102   a  is the weakest cell (has the lowest output voltage), the active cell balance module  124  can cause the switches  104   a   1 - 104   a   2  to close and cause the other switches  104   b   1 - 104   n   2  to open. This causes current from the secondary side of the transformer  108  to flow through the diode  110 , the switch  104   a   1 , the power cell  102   a , and the switch  104   a   2  back to the secondary side of the transformer  108 . This provides an extra charge to charge up the power cell  102   a . The module controller  116  can determine when the power cell  102   a  has been sufficiently charged (such as when it reaches an average charge of the power cells  102   a - 102   n ) and cause the active cell balance module  124  to open the switches  104   a   1 - 104   a   2 . This process could be repeated any number of times to charge any of the power cells  102   a - 102   n.    
         [0025]    The transformer  108 , diode  110 , and switches  104   a   1 - 104   n   2  effectively function as controllable current sources coupled to the power cells  102   a - 102   n . These controllable current sources can be used to charge up any of the power cells  102   a - 102   n  individually or in groups (as described below). Because of this, the active cell balancing circuit  100  can help to keep the output voltages of the power cells  102   a - 102   n  all at or near a desired level. Any other suitable controllable current sources could be used here. 
         [0026]      FIG. 2  illustrates another example active cell balancing circuit  200  in accordance with this disclosure. In this example, the circuit  200  employs flyback-based active cell balancing. The circuit  200  uses a flyback (boost type) converter to draw current from power cells that have undesirable higher voltages. The circuit  200  identifies a power cell that has more voltage and then causes that power cell to transfer a portion of its voltage back to the entire string of power cells. 
         [0027]    As shown in  FIG. 2 , the circuit  200  includes power cells  202   a - 202   n , which is coupled to two switches  204   a   1 - 204   a   2 ,  204   b   1 - 204   b   2 ,  204   n   1 - 204   an   2 . The power cells  202   a - 202   n  are also coupled to a monitor circuit  206 . The active cell balancing circuit  200  also includes a transformer  208 , a diode  210 , and a capacitor  212 . The active cell balancing circuit  200  further includes a signal line  214  that provides voltage, current, and temperature information or other information from the monitor circuit  206  to a module controller  216 . The module controller  216  includes an SOC estimation module  218 , a communication module  220 , an internal power management module  222 , and an active cell balance module  224 . A transistor  228  is coupled to the secondary side of the transformer  208 . Many of these components may be structurally the same as or similar to corresponding components in  FIG. 1 . forward-based active cell balancing circuit  100 . However, the flow of current is from the primary side of the transformer  208  through the diode  210  to the top of the power cell string (starting at the power ce11  202   a ). Also, the active cell balance module  224  receives a voltage signal from the secondary side of the transformer  208 . 
         [0028]    In one aspect of operation, the monitor circuit  206  may continually, near-continually, or intermittently monitor the power cells  202   a - 202   n . The module controller  216  can determine which power cell has the highest voltage. The module controller  216  then causes that power cell to be discharged somewhat to a lower voltage. Pulse charging and discharging can be used to speed up the charging/discharging process in this example. 
         [0029]      FIG. 3  illustrates an example active cell balancing circuit  300  incorporating switch driving circuits in accordance with this disclosure. In particular, the circuit  300  of  FIG. 3  is similar in structure to the circuit  100  of  FIG. 1 . Note that the switch driving circuits could be used in other active balancing circuits, such as the circuit  200  of  FIG. 2 . 
         [0030]    In this example, the circuit  300  includes power cells  302   a - 302   n , a transformer  308 , a diode  310 , a capacitor  312 , an SOC estimation module  318  with a micro-controller interface, and a transistor  328 . In particular embodiments, the monitor circuit  306  could represent an LMP8631 analog front end from NATIONAL SEMICONDUCTOR CORPORATION. The circuit  300  also includes an inductor  311  coupled between the diode  310  and the capacitor  312 , as well as a diode  313  coupled to the diode  310  and inductor  311  and to the capacitor  312 . 
         [0031]    Rather than using a single switch to couple one end of a power cell  302   a - 302   n  to the transformer  308 , the circuit  300  uses a pair of switches to couple one end of a power cell to the transformer  308 . For example, transistors  304  and  304 ′ can be used to couple one end of the power cell  302   a  to the transformer  308 . Diodes  305  and  305 ′ represent the body diodes of the transistors  304  and  304 ′, respectively. Driver circuits  330  and  330 ′ drive the transistors  304  and  304 ′ and have boost capacitors  332  and  332 ′, respectively, which could represent off-chip capacitors. 
         [0032]    In this example, each driver circuit  330  and  330 ′ includes a diode  334  that receives a supply voltage VDD. An under-voltage lockout (UVLO) unit  336  detects when the supply voltage VDD falls below a threshold level. A Schmitt trigger  338  receives an input drive signal (Din_R or Din_L) and generates an output signal for a level shifter  340 , which shifts the voltage level of the output signal. An AND gate  342  receives outputs of the UVLO unit  336  and the level shifter  340  and provides an input to a driver  344 . The driver  344  generates the drive signal for one of the transistors  304  and  304 ′. In particular embodiments, the driver circuits  330  and  330 ′ could represent LM5101A high-voltage high-side and low-side gate drivers from NATIONAL SEMICONDUCTOR CORPORATION. 
         [0033]    In  FIG. 3 , each boost capacitor  332  or  332 ′ can have a charge path from its associated driver  334 , through that boost capacitor, and through the body diode  305  or  305 ′ of its associated left transistor  304 . Each left transistor  304  effectively has a floating current source on its left side. As a result, each boost capacitor  332  or  332 ′ can be charged since the floating current source node is periodically pulling to ground. Various driver circuits can also be disabled or enabled using a transistor  346  coupled to an input of that driver circuit. 
         [0034]    In some embodiments as described above, an active cell balancing circuit can charge or discharge individual power cells within a single module. It is also possible to charge or discharge groups of power cells within a single module.  FIG. 4  illustrates an example algorithm that can be used during active cell balancing according to this disclosure. 
         [0035]    In this example, an active cell balancing circuit may initially charge three cells coupled in series at a time, rather than charging just one cell at a time. For example, the active cell balancing circuit could charge cells  5 - 7  (Group 1) together for a certain time until cell  7  reaches the voltage of the maximum-voltage cell (cell  4  in this case). Then, cells  1 - 3  (Group 2) can be charged until cell  2  reaches the voltage of cell  4 . After that, cells  10 - 12  (Group 3) can be charged until cell  10  reaches the voltage of cell  4 . At this point, cells can be charged individually rather than three at a time. 
         [0036]    As shown here, rather than simply charging one power cell at a time, multiple power cells (such as three cells) can be charged simultaneously. Once the groups of cells have been charged adequately, the algorithm can switch and begin charging cells individually. A similar algorithm could be used to discharge groups of cells together. This algorithm could allow for faster charging or discharging times. A combination of approaches could also be used, such as where groups of cells are charged to an average charge of the cells and groups of cells are discharged to the average charge of the cells before individual cells are charged/discharged. 
         [0037]    Active cell balancing can be useful in a number of situations. As a particular example, active cell balancing (such as shown in  FIGS. 1 through 3 ) can be useful in situations where some (but not all) cells in a module are being replaced. In that case, active cell balancing may be needed since there can be a large difference between the charge levels of the older cells and the charge levels of the newer cells. Without balancing, it may not be possible to charge the older and newer cells to a relatively equal level. This could significantly interfere with the operation of the module and may force replacement of all battery cells in the module, even battery cells that can still hold an adequate charge. Also, the group charging/discharging algorithm described with respect to  FIG. 4  could be used to increase the speed at which the balancing of the older and newer cells occurs. 
         [0038]    Active Module Balancing 
         [0039]    In another aspect of this disclosure, various module balancing circuits are provided that can regulate multiple modules (such as multiple batteries), each of which may contain multiple battery cells or other power cells. In some embodiments, the multiple modules could form one or multiple packs, such as one or multiple battery packs. 
         [0040]      FIG. 5  illustrates an example power pack  500  with multiple modules  502  each having multiple power cells  504  according to this disclosure. In this example, the modules  502  are coupled in series and provide an output voltage Pack+/Pack−. Also, groups of cells  504  are arranged in parallel, and parallel groups of cells  504  are coupled serially to form each module  502 . Each module  502  could represent a battery formed by multiple battery cells. 
         [0041]      FIG. 6  illustrates example safe operating regions of various batteries according to this disclosure. As shown in  FIG. 6 , all of the cells  504  in each module  502  often must operate within a specified safe operating region under all charging and discharging conditions. In  FIG. 6 , the lines represent the safe operating regions for different batteries. In general, the safe operating regions for these batteries is between 2.0-3.5V. 
         [0042]      FIG. 7  illustrates example uneven voltage levels on power cells in modules according to this disclosure. As shown in  FIG. 7 , mismatch issues can affect charging of the cells  504 . In  FIG. 7 , a line  702  represents the charges on the cells  504  in various modules before charging, and a I ne  704  represents the charges on the cells  504  in various modules after charging. As can be seen here, mismatch issues can prevent many cells  504  from being charged and can possibly force some of the cells  504  to operate outside the 2.0-3.5V range. Any module balancing approach can take this safe operating region into account. 
         [0043]      FIG. 8  illustrates an example active module balancing system BOO in accordance with this disclosure. In this example, the active module balancing system BOO includes multiple modules B02a-B02n, each of which includes multiple power cells B04 coupled in series. Each of the modules B02a-B02n has a corresponding module controller B06a-B06n, each of which includes an active cell balancing circuit used to perform active cell balancing within the corresponding module. Each module controller B06a-B06n could, for instance, include any of the active cell balancing circuits described above or below. 
         [0044]    The active module balancing system  800  further includes multiple module balancing circuits  800   a - 8   n . The module balancing circuits  800   a - 800   n  can control the power provided to or removed from the modules  802   a - 802   n , which can help to control the charging or discharging of the modules  802   a - 802   n . The module balancing circuits  808   a - 808   n  are coupled to an internal direct current (DC) bus B10, which is used to route DC power to and between the module balancing circuits  808   a - 808   n.    
         [0045]    A central control unit B12 monitors the current provided by the modules  802   a - 802   n . The central control unit  812  here includes a resistor  814  through which the current provided by the modules  802   a - 802   n  flows. The central control unit  812  also includes a difference amplifier  816  that amplifies a voltage difference across the resistor  814 . An analog-to-digital converter (ADC) 818 digitizes an output of the difference amplifier  814  using a reference voltage (VREF) provided by a precision reference  820 . The ADC  818  could represent a 16-bit ADC, and the precision reference  820  could represent any suitable source of a reference voltage. A central controller  822  uses the digitized output of the ADC  818 . 
         [0046]    The central control unit  822  can also communicate with the module controllers  806   a - 806   n  over a bus  824 . The central control unit  822  can further operate to control the balancing performed by the module balancing circuits  808   a - 808   n  and the module controllers  806   a - 806   n.    
         [0047]    In some embodiments, the central control unit  822  performs current sensing using the resistor  814 . The central control unit  822  also performs state of charge or state of health (SOH) estimation for the modules  802   a - 802   n  and their cells  804 . The central control unit  822  further performs module balance control to determine how to balance the modules  802   a - 802   n  and communicates the necessary data to the modules  802   a - 802   n  and the module controllers  806   a - 806   n.    
         [0048]    In particular embodiments, during module balancing, the internal DC bus  810  can be used for energy buffering and transfers between the modules  802   a - 802   n . The module controllers  806   a - 806   n  and module balancing circuits  808   a - 808   n  can receive SOC information from the central control unit  812 . The module with highest SOC can charge the module with lowest SOC directly through the internal DC bus  810 . The module balancing circuits  808   a - 808   n  can operate in voltage mode when in a discharging status and in current mode when in a charging status (although other modes could be used when in the charging and discharging statuses, such as current mode when in the discharging status and in voltage mode when In the charging status). 
         [0049]    Bi-Directional Active Balancing 
         [0050]    In yet another aspect of this disclosure, various bi-directional active balancing circuits are disclosed that can balance multiple power cells in one or more modules. In these embodiments, it is possible for the active balancing circuits to transfer power from one or more power cells (such as a power cell with a higher charge) to one or more other power cells (such as a power cell with a lower charge). Note that the module balancing circuits described above already indicated that the power transfer on the internal DC bus  810  could be bi-directional, meaning the active module balancing system  800  can support bi-directional power transfer on the bus  810 . 
         [0051]    Referring back to  FIG. 7 , the cells represented by the lowest charges in the line  702  may represent cells that require charging (compared to other cells). Similarly, the cells represented by the highest charges in the line  704  may represent cells that require discharging (compared to other cells). Bi-directional active balancing would allow an individual cell to be charged or discharged, depending on its charge level relative to other cells. As shown in  FIG. 7 , bi-directional active balancing would allow the cells having excessive charge to be used to charge the cells having lower charge. 
         [0052]      FIG. 9  illustrates an example bi-directional active cell balancing circuit  900  that supports active cell balancing within a module according to this disclosure. The active balancing circuit  900  includes multiple power cells  902   a - 902   n  and switches  904   a   1 - 904   a   2 ,  904   b   1 - 904   b   2 ,  904   n   1 - 904   n   2 . The active balancing circuit  900  also includes a monitor circuit  906 . Here, the output of the monitor circuit  906  is provided to an SOC estimation module  918 , which can identify the power cells  902   a - 902   n  that need charging and discharging. An active cell balance control module  924  controls the switches  904   a   1 - 904   n   2  in order to charge or discharge the appropriate power cell(s)  902   a - 902   n.    
         [0053]    A bi-directional isolated DC-to-DC converter  950  is used to provide a balancing current to or from the power cells  902   a - 902   n  in order to support the active balancing. Current flowing into or out of the module (I w) and current flowing into or out of the cells  902   a - 902   n  (IcELL) can be measured and used by the active cell balance control module  924 . If used in the active module balancing system  800 , the DC-to-DC converter  950  could form part of the module balancing circuits  808   a - 808   n  and transfer power over the DC bus  810 . 
         [0054]    In some embodiments, voltage, temperature, and/or current sensing can be done for each cell  902   a - 902   n  to estimate its state of charge. Current or charge can be injected from the module into the cell(s) with the least SOC, and the cell(s) with the most SOC can be discharged back to the module. Balancing current (charge and discharge) injection can be performed in a way that is superimposed on the main module charging/discharging current (used to balance the modules). Balancing current (both directions) can be handled by the bi-directional DC-DC converter  950 , and the switch matrix can handle which cell is charged or discharged. 
         [0055]    Once again, as a particular example, active module balancing and bi-directional balancing can be useful in situations where some but not all power cells in a pack (formed from multiple modules) are being replaced. The active balancing may be needed since there can be a large difference between the charge levels of the older modules and the charge levels of the newer modules. 
         [0056]    Although the figures have illustrated various embodiments for active balancing as described above, any number of changes can be made to these figures. For example, any number of power supplies in any number of modules could be balanced using these circuits. Also, note that other power supplies could be used in place of or in addition to battery cells in batteries, such as super-capacitors. 
         [0057]    It may be advantageous to set forth definitions of certain words and phrases that have been used within this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more components, whether or not those components are in physical contact with one another. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. 
         [0058]    It may be advantageous to set forth definitions of certain words and phrases that have been used within this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more components, whether or not those components are in physical contact with one another. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. 
         [0059]    While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this invention. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this invention as defined by the following claims.