Abstract:
A battery pack comprising a power cell for providing power to a load or for receiving a charge from a charger, a first protection circuit for protecting from overvoltage and/or overcurrent conditions, and a second protection circuit for protecting from overtemperature conditions. The protection circuits independently control one or more electronic switching devices, through which passes substantially all of the current supplied by the power cell. When overvoltage and/or overcurrent conditions exist, the first protection circuit causes at least one of the switching devices to move to a non-conducting condition. Similarly, when an overtemperature condition exists, the second protection circuit causes at least one of the switching devices to move to a non-conducting condition.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 11/972,214, filed Jan. 10, 2008, which is hereby incorporated by reference. 
    
    
     FIELD 
     The present disclosure relates generally to rechargeable battery packs, and in particular to a reduction of the equivalent series resistance (ESR) of a rechargeable battery pack, such as a Li-Ion battery pack, that does not sacrifice safety. 
     BACKGROUND 
     Rechargeable battery packs, such as Li-Ion battery packs, are commonly used in many consumer electronics such as cell phones and personal digital assistants (PDAs).  FIG. 1  is a schematic diagram of a prior art rechargeable battery pack  5 , such as a Li-Ion battery pack, that may be used to provide power to a passive load  10 , such as a cell phone or a PDA. In some implementations, the passive load  10  is supplanted by an active element such as a battery charger, which can recharge the rechargeable battery pack  5 . As seen in  FIG. 1 , the rechargeable battery pack  5  includes a power cell  15 , such as a Li-Ion cell, a thermal protector  20  and a protection circuit module (PCM)  25  driving the load  10 . The PCM  25  includes an integrated circuit control chip  30  operatively coupled to one or more electronic switching devices  35  and  40 , which in  FIG. 1  are modeled as MOSFETs. As is known in the art, the PCM  25  is essentially a switch that detects abnormal currents and/or voltages and disconnects the cell  15  from the load  10 , or, alternatively, a charger if the rechargeable battery pack  5  is being charged. The thermal protector  20  provides protection for the rechargeable battery pack  5  from overtemperature conditions. Overtemperature conditions can have any of several causes or combinations of causes. Overtemperature conditions can damage or impair electronic components such as those in the load  10 . In the event of an overtemperature condition, it may be desirable to substantially reduce current, and thereby substantially reduce delivered power, to the load  10 , thereby reducing the risk of damage or impairment. The thermal protector  20  may be, for example, a thermal fuse, a thermal breaker or a positive temperature coefficient (PTC) thermistor. Thermal protector  20  may also be either non-resettable or resettable. Non-resettable thermal protectors have lower equivalent series resistance (ESR), but once tripped, a rechargeable battery pack employing the non-resettable thermal protector is essentially no longer of any use. Resettable thermal protectors have higher ESRs, but can be tripped and reset many times. 
     As is known in the art, ESR is one of the main parameters determining the usable energy stored in the cell  15 , and thus the usable energy stored in the rechargeable battery pack  5 . Lower ESR, in general, means longer operation such as longer talk times for a cell phone. In the rechargeable battery pack  5 , the ESR thereof includes the internal resistance of the cell  15 , the resistance of the thermal protector  20 , the resistance of the electronic switching devices  35  and  40 , and the resistance of any connectors and other conductors in the circuit path to the load  10 . In other words, because the thermal protector  20  is in the circuit path coupling the cell  15  to the load  10 , the thermal protector  20  adds to the ESR of the rechargeable battery pack  5 , and the resistance of the thermal protector  20  is not negligible. The circuit control chip  30  is not in the circuit path that includes the cell  15 , the thermal protector  20  and the load  10 , and does not significantly contribute to the ESR. 
       FIG. 2  is a schematic diagram of an equivalent conceptual circuit of the rechargeable battery pack  5  that shows each of the resistance components which add to the ESR of the rechargeable battery pack  5 . In particular, the equivalent circuit includes a resistor  50  that represents the resistance of the cell  15 , a resistor  55  that represents the resistance of the thermal protector  20 , resistors  56  and  57  that represent the resistance of the electronic switching devices  35  and  40 , and a resistor  58  that represents the resistance of the connectors and other conductors in the circuit path to the load  10  (which is represented by the resistor  59 ). 
     Since, as described above, ESR is one of the main parameters determining the usable energy in a rechargeable battery pack, it would be advantageous to be able to reduce the ESR of a rechargeable battery pack in a manner that does not adversely affect the safety of the rechargeable battery pack. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures. 
         FIG. 1  is a schematic diagram of a prior art rechargeable battery pack, such as a Li-Ion battery pack, that may be used to provide power to a load, such as a cell phone or a PDA. 
         FIG. 2  is a schematic diagram of an equivalent conceptual circuit of the rechargeable battery pack shown in  FIG. 1 . 
         FIG. 3  is a schematic diagram of a rechargeable battery pack. 
         FIG. 4  is a schematic diagram of an alternate embodiment of a rechargeable battery pack. 
         FIG. 5  is a schematic diagram of a rechargeable battery pack showing a switching configuration in more detail. 
         FIG. 6  is a schematic diagram of a rechargeable battery pack showing in further detail one embodiment of the rechargeable battery pack. 
         FIG. 7  is a schematic illustration of an equivalent conceptual circuit for the rechargeable battery pack shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a schematic diagram of a rechargeable battery pack  60  according to one aspect of the disclosed embodiments. The rechargeable battery pack  60  provides power to a load  68  (between the upper and lower power supply rails, denoted as +Ve and −Ve) and includes a power cell  64 , such as one or more Li-Ion cells. (Use of the term “battery” pack is not intended to indicate that more than one power cell  64  is necessarily employed, although in various embodiments, more than one power cell may be employed.) The rechargeable battery pack  60  further includes circuitry  61  that monitors for overvoltage and/or overcurrent conditions, and thermal protection circuitry  62  that monitors for overtemperature conditions. The rechargeable battery pack  60  also includes an electronic switch  66 . The electronic switch is coupled to the power cell  64  such that substantially all of the current passing through the power cell  64  also passes through the electronic switch  66 . An arrow  69  shows the direction of positive current flow in a loop that includes the power cell  64 , the load  68  and the electronic switch  66 . When the load  68  is supplanted by a charger, the direction of positive current flow  69  in the loop is reversed. 
     Currents flow through circuit loops that include the overvoltage/overcurrent circuitry  61  and the thermal protection circuitry  62 . These currents, however, are negligible in comparison to the current in the loop that includes the power cell  64 , the load  68  and the electronic switch  66 . Under normal operation, the electronic switch  66  is “ON,” that is, in a conducting condition in which it can conduct current. 
     The overvoltage/overcurrent circuitry  61  and the thermal protection circuitry  62  are operatively coupled to the electronic switch  66  such that the overvoltage/overcurrent circuitry  61  and the thermal protection circuitry  62  can each independently cause the electronic switch  66  to assume an “OFF” or non-conducting condition, in which the electronic switch  66  will not conduct current. When the electronic switch  66  is OFF, the loop that includes the power cell  64 , the load  68  and the electronic switch  66  is effectively made open, thereby preventing the power cell  64  from supplying current to the load  68 . 
       FIG. 4  is a schematic diagram similar to  FIG. 3 . In  FIG. 4 , the overvoltage/overcurrent circuitry  61  and the thermal protection circuitry  62  each controls an electronic switch. The overvoltage/overcurrent circuitry  61  controls electronic switch  66 A and the thermal protection circuitry  62  controls electronic switch  66 B. The electronic switches  66 A and  66 B can independently be controlled to become OFF and assume a non-conducting condition. When either electronic switch  66 A or  66 B is in a non-conducting condition, the power cell  64  is prevented from supplying current to the load  68 . 
       FIG. 5  is a schematic diagram of a rechargeable battery pack  70  according to another aspect of the disclosed embodiments. The rechargeable battery pack  70  provides power to a load  78  between the upper and lower power supply rails +Ve and −Ve, and includes a power cell  74 , such as one or more Li-Ion cells. The rechargeable battery pack  70  further includes circuitry  75  that monitors for overvoltage and/or overcurrent conditions, and thermal protection circuitry  72  that monitors for overtemperature conditions. In operation, when the temperature within the rechargeable battery pack  70  exceeds some predetermined threshold level, one or more circuit elements in the thermal protection circuitry  72  will be caused to trip, thereby triggering a change in operation of the thermal protection circuitry  72 . 
     In the embodiment depicted in  FIG. 5 , the overvoltage/overcurrent circuitry  75  is similar to PCM  25  in  FIG. 1 , and includes an integrated circuit control chip  73  operatively coupled to one or more electronic switching devices  76 A and  76 B, which in  FIG. 5  are modeled as MOSFETs. In the embodiment depicted in  FIG. 5 , the integrated circuit control chip  73  is configured to supply a voltage to a controlling terminal of the electronic switching devices  76 A and  76 B, particularly, the gates of electronic switching devices  76 A and  76 B, causing the electronic switching devices  76 A and  76 B to be ON during normal operation. Either the integrated circuit control chip  73  or the thermal protection circuitry  72  can independently cause the electronic switching devices  76 A or  76 B, or both, to turn OFF. An arrow  79  shows the direction of positive current flow in a loop that includes the power cell  74 , the load  78  and the electronic switches  76 A and  76 B. When either the electronic switch  76 A is OFF or the electronic switch  76 B is OFF or both are OFF, the loop current  79  effectively goes to zero. 
     In  FIG. 5 , the electronic switch  76 A is directed to charge control and the electronic switch  76 B is directed to discharge control. Separate electronic switches to control discharge (in which the direction of positive current flow is in the direction shown by arrow  79 ) and charge (in which the direction of positive current flow is in the direction opposite that shown by arrow  79 ) are advantageous in that some transistors are more effective in cutting off current flow in particular directions. As a result, two electronic switches are better able to cut off current flow regardless of the direction of current flow  79 . This disclosure does not require, however, that separate electronic switches be provided for charge and discharge. 
       FIG. 6  is a schematic diagram of a rechargeable battery pack  80 , such as a Li-Ion battery pack, according to one aspect of the disclosed embodiments, which has a reduced ESR without sacrificing safety. The rechargeable battery pack  80  provides power to a load  85  between the power supply rails +Ve and −Ve and includes a power cell  90 , such as one or more Li-Ion cells, and a PCM  95 . The PCM  95  includes an integrated circuit control chip  100  operatively coupled to electronic switching devices  105  and  110 , which are modeled in  FIG. 6  as MOSFETs but which may comprise any electronic switching component or circuitry under the control of the integrated circuit control chip  100 . The power cell  90  is coupled to the electronic switching devices  105  and  110  such that substantially all current passing through the power cell  90  also passes through the electronic switching devices  105  and  110 . When the electronic switching devices  105  and  110  are ON, the electronic switching devices  105  and  110  are in a conducting condition, and current passes through the power cell  90 . When the electronic switching devices  105  and  110  are OFF, the electronic switching devices  105  and  110  are in a non-conducting condition, and effectively no current passes through the power cell  90 , thereby cutting off the power cell  90  as a power source for load  85 . 
     As shown in  FIG. 6 , the power cell  90  may continue to deliver current to other electronic components, such as the PCM  95 , when the electronic switching devices  105  and  110  are OFF. These other currents, however, are negligible in comparison to currents passing through the load  85  under normal operating conditions. In a typical implementation, over ninety-nine percent of the current passing through the power cell  90  would pass through the load  85 , and less than one percent would pass through other components of rechargeable battery pack  80 . In other words, substantially all of the current passing through the power cell  90  passes through the load  85 , and substantially all of the current passing through the power cell  90  also passes through the electronic switching devices  105  and  110 . 
     Whether the electronic switching devices  105  and  110  are ON or OFF is under the control of integrated circuit control chip  100 , which is responsive to overvoltage and over current conditions. As discussed below, however, whether the electronic switching devices  105  and  110  are OFF can also be controlled by thermal protection circuitry that is responsive to overtemperature conditions. Concerns about overvoltages, overcurrents and overtemperature conditions are similar whether cell  90  is supplying power during ordinary operation or consuming power during recharging (although overvoltage is generally not a problem when the power cell  90  is coupled to a passive load  85 , but overvoltage can be a concern during recharging). For simplicity, the focus below will assume that the cell  90  is supplying power to the passive load  85 . 
     The rechargeable battery pack  80  further includes a thermal protector  115  for providing overtemperature protection for the rechargeable battery pack  80 . However, unlike the thermal protector  20  in the rechargeable battery pack  5  shown in  FIG. 1 , the thermal protector  115  is not provided in the circuit path that includes the cell  90  and the switching devices  105  and  110  for providing power to the load  85 . Rather, the thermal protector  115  is removed from that circuit path and instead is, as shown in  FIG. 6 , provided in a separate circuit path. The thermal protector  115  may be, for example, a thermal fuse, a thermal breaker or a positive temperature coefficient (PTC) thermistor. Because the thermal protector  115  is not provided in the circuit path to the load  85 , it does not add to the ESR of the rechargeable battery pack  80 , thereby reducing the ESR as compared to the prior art rechargeable battery pack  5  shown in  FIG. 1 . 
     Nearly all of the current passing through the thermal protector  115  passes through a resistor  120 . The resistor  120  is provided to bias the electronic switching device  1250 N when the thermal protector  115  trips and the impedance of the thermal protector  115  goes from low to high. The resistor  120  is further sized to have a large impedance in comparison to the load  85 , such that most current passing through the cell  90  will pass through the load  85 , and the current flowing through the thermal protector  115  will be comparatively low. In ordinary operation, the impedance of the load  85  would be in the range of a few ohms. The resistor, by comparison, might be sized in the range of tens of thousands of ohms, such as fifty thousand ohms. Current flowing through the load  85  is in the range of several amperes, and current flowing through the resistor  120  is in the range of millionths of an ampere. Current flowing through the integrated circuit control chip  100  is also very small in comparison to current flowing through the load  85  under normal operating conditions. As a result, currents flowing through other loops in rechargeable battery pack  80  are negligible in comparison to current flowing through the load  85 . During recharging, currents passing through the cell  90  may not be of the same magnitudes as in the case when the cell  90  is supplying power to a passive load, but currents supplied to the overvoltage/overcurrent circuitry and the thermal protection circuitry generally remain negligible, and substantially all of the current supplied by the charger passes through the cell  90  and the electronic switching devices  105  and  110 . 
     The electronic switching device  125  in  FIG. 6  is modeled as an NPN bi-polar junction transistor, but electronic switching device  125  can also be realized as a PNP bi-polar junction transistor, or other transistor or combination of transistors. When the electronic switching device  125  turns ON, it robs the drive to the switching devices  105  and  110 , and they turn OFF, thereby cutting off the load  85  from the cell  90 . When the thermal protector  115  in ON (operating normally), the electronic switching device  125  cannot be biased ON since the thermal protector  115  itself is robbing the bias from the base of the electronic switching device  125 . 
     As seen in  FIG. 6 , the thermal protector  115  is operatively coupled to the electronic switching devices  105  and  110  through the electronic switching device  125  and the diodes  130  and  135 . In operation, when the temperature within the rechargeable battery pack  80  exceeds some predetermined threshold level, the thermal protector  115  will be caused to trip. When this happens, the electronic switching devices  105  and  110  are caused to turn OFF (assume a non-conducting condition), effectively interrupting the current path from the power cell  90  through the load  85 . In particular, under normal operation, the electronic switching devices  105  and  110  are controlled by the outputs of the integrated circuit control chip  100  to be ON, the electronic switching device  125  is OFF, and effectively no current flows into the collector node (where the junction of the cathodes of the two diodes  130  and  135  connect) because the collector-to-emitter connection behaves like an open circuit. This means that the two diodes  130  and  135  are essentially out of the picture, because the diodes  130  and  135  are not conducting and behave like open circuits. The electronic switching device  125  is OFF because the thermal protector  115  in ON (typically at a relatively low impedance). As described above, thermal protector  115  robs the base drive current flowing through the resistor  120 , effectively bypassing the base of electronic switching device  125 . When the thermal protector  115  trips due to a high temperature, the impedance of the thermal protector  115  changes from a low impedance to a high impedance. Current now flows through the resistor  120  to the base of electronic switching device  125 . As a result, the electronic switching device  125  turns ON, driving the collector voltage down, close to the emitter voltage. The result is that the voltage at the collector node, to which the cathodes of the diodes  130  and  135  are connected, is also driven low. As a result, the diodes  130  and  135  turn ON and conduct current. This then robs the drive of the integrated circuit control chip  100  to the electronic switching devices  105  and  110 , thereby turning them both OFF. Thus, the implementation described herein effectively causes the control electronic switching devices  105  and  110  of the PCM  95  to act as temperature-dependent switches (on top of their other duties in the PCM  95  as required by the integrated circuit control chip  100 ). In this sense, the temperature cutoff protection provided by the thermal protector  115  is independent, meaning that it is not under the control of any controller or processor, and in particular not under the control of the integrated circuit control chip  100 . 
     In a variation of the embodiment depicted in  FIG. 6 , a thermal protector  115  can be utilized that has a very high impedance during normal temperature conditions, and a low impedance during overtemperature conditions. With such a thermal protector  115 , the electronic switching device  125  can be eliminated and the thermal protector  115  can be directly coupled between the lower power rail and the cathodes of the diodes  130  and  135 . In normal operating conditions, the impedance of the thermal protector  115  is high, and virtually no current is able to flow through the thermal protector  115 . As a result, the diodes  130  and  135  are OFF. In overtemperature conditions, however, the impedance of the thermal protector  115  is low, effectively pulling down the cathode voltage of the diodes  130  and  135 , turning them ON and turning OFF the electronic switching devices  105  and  110 . In this variation, the thermal protector  115  can serve both as a temperature-sensitive element and as a switching element to turn ON diodes  130  and  135 . 
       FIG. 7  is a schematic illustration of an equivalent conceptual circuit  140  for the rechargeable battery pack  80 . The equivalent circuit  140  includes a resistor  145  that represents the resistance of the cell  90 , resistors  150  and  155  that represent the resistance of the electronic switching devices  105  and  110 , and a resistor  160  that represents the resistance of connectors and other conductors in the series path to the load  85  (which is represented by the resistor  165 ). As can be seen in  FIG. 7 , the circuit path that includes the cell  90  and the electronic switching devices  105  and  110 , which provide power to the load  85 , does not include any resistance that is attributable to the thermal protector  115 , because any resistance that is attributable to the thermal protector  115  is negligible. As described above, nearly all current passing through the 90 cell passes through the load  85 , while a much smaller and comparatively insignificant amount of current flows through the thermal protector  115  and other circuit elements. 
     The circuit described above may realize one or more advantages. Because the thermal protector  115  is not provided in the circuit path that includes the cell  90  and the load  85  and because the circuit path that includes the resistor  120  and the thermal protector  115  has a comparably higher impedance, very low currents will flow through the thermal protector  115 . This disclosure is not limited to any particular amount of current flowing through thermal protector  115 , but in general, the amount of current flowing through thermal protector  115  is negligible, such that thermal protector  115  has a negligible effect upon the ESR. Because very low currents will flow through the thermal protector  115 , a smaller, typically less expensive component can be utilized as the thermal protector  115 . 
     In addition, in the case where the thermal protector  115  is a positive temperature coefficient (PTC) thermistor, the effect of residual high post-trip resistance (that is often the case with PTC thermistor) will not add to the ESR of the rechargeable battery pack  80 . More specifically, as is known, once tripped, the resistance of a PTC thermistor never quite relaxes back to its original level, but rather settles at a higher level (this is the high post-trip resistance cited above). Thus, when a PTC thermistor is used in the circuit path that includes a cell in a rechargeable battery, as is the case in the rechargeable battery pack  5  of  FIG. 1 , the fact that post-trip resistance is increased adds to the ESR of the battery pack  5  in  FIG. 1 . An increased post-trip resistance in the configuration of  FIG. 6 , however, would not necessarily result in a corresponding increase in ESR. 
     Thus, the rechargeable battery pack  80  provides a reduced ESR, and therefore increased battery time. Increased battery time results in increased device usage time between chargings. Further, the configuration depicted in  FIG. 6  protects against overvoltage, overcurrent and overtemperture conditions, thereby effectively preserving the safety of the rechargeable battery pack  80 . 
     While various embodiments have been described and illustrated above, it should be understood that these are exemplary and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope hereof. The described devices and techniques are not limited to the particular circuit elements shown. In particular and without limitation, various transistors and switching elements in  FIG. 6  need not employ the particular circuit elements shown, but may include different transistors or switching elements. In addition, the particular circuit elements may be, but need not be, arranged as depicted in the Figures. For example, if integrated circuit control chip  100  includes a housing, components of the thermal protection circuitry, such as the thermal protector  115  or the electronic switching device  125 , may be employed inside the housing and need not be physically separate from the integrated circuit control chip  100 . 
     Further, various embodiments may include elements not depicted in any of the figures. For example, circuit elements depicted as directly coupled may be coupled via one or more intermediate elements, such as a resistor or diode. In particular and without limitation, the electronic switching devices  105  and  110  in  FIG. 6  could be coupled via resistors to the integrated circuit control chip  100 . Accordingly, these and other embodiments are within the scope of the following claims. 
     The above-described embodiments are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope, which is defined solely by the claims appended hereto.