Patent Publication Number: US-2005127878-A1

Title: Power fault battery protection circuit

Description:
BACKGROUND  
      1. Technical Field  
      This invention relates generally to protection circuits for rechargeable battery packs, and more specifically to protection circuits that disable a rechargeable battery pack due to an excessive amount of power being drawn by the load.  
      2. Background Art  
      Portable electronic devices, like cellular telephones, pagers and two-way radios for example, derive their portability from rechargeable batteries. Such batteries allow these devices to slip the surly bonds of wall mounted power supplies and wirelessly touch the hand of the user wherever he may be.  
      While many people may think that a rechargeable battery is simply a cell and a plastic housing, nothing could be further from the truth. Rechargeable battery packs often include circuit boards, electronic circuitry, mechanical assemblies and electromechanical protection components. The circuits employed in rechargeable battery packs include charging circuits that start, ramp, taper and stop current, fuel gauging circuits, temperature measurement circuits and indicator circuits, just to name a few. Simply put, a battery pack is a complex system of components working in harmony to safely deliver power to a portable electronic device.  
      One of the most fundamental circuits in a battery pack is the protection circuit. Rechargeable battery performance, especially with respect to those having cells constructed of lithium-based materials, may be severely compromised if the cell within the battery pack is over or under charged. For this reason, most all battery packs today include one form of safety circuit or another.  
      Typical safety circuits include voltage and current limits. As such, when the voltage across the cell in a battery pack becomes too high or too low, the safety circuit will open switches within the pack, thereby “turning off” the battery pack. Similarly, if the current flowing either into or out of the cell gets too high, the safety circuit will turn off the battery pack.  
      Despite these voltage and current safety mechanisms, new concerns are arising from “over power” situations. These situations arise when a battery pack is operating within its voltage and current limits, but the total power—the product of voltage and current—becomes too high for a particular electronic device. The concern is that the over power situation may cause components within the electronic device to generate excessive heat.  
      There is thus a need for an improved battery safety circuit that turns off the battery not only due to excessive voltage or current, but for excessive power dissipation as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a block diagram of a safety circuit IC.  
       FIG. 2  illustrates a protection circuit having a safety circuit and overpower circuit in accordance with the invention.  
       FIG. 3  illustrates one embodiment of a power meter in accordance with the invention.  
       FIG. 4  illustrates one embodiment of an analog multiplier in accordance with the invention.  
       FIG. 5  illustrates a protection circuit having a plurality of safety circuits and overpower circuits in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
      This invention provides a overpower protection circuit that may be used in conjunction with an existing battery safety circuit to offer an additional level of protection. The overpower circuit monitors the power being delivered to or from the cells in a battery pack. When the power exceeds a predetermined threshold, the overpower circuit simulates an overcurrent condition with respect to the safety circuit. This overcurrent simulation causes the protection circuit to open one or more serial pass elements, thereby isolating the cells from the external terminals of the battery pack. The overpower circuit then resets itself when the load is removed from the battery pack. The combination of the overpower circuit with the conventional battery safety circuit provides a system that protects not only from excessive voltages and currents, but from excessive power dissipation levels as well.  
      Prior to understanding the overpower circuit, a brief overview of battery safety circuits is warranted. As used herein, a “safety circuit” is any circuit capable of monitoring the voltage across at least one rechargeable cell, in addition to being capable of monitoring the current flowing through the cell or cells. One example of such a circuit is the S8232 series of safety circuits manufactured by Seiko Instruments, Inc. For discussion and exemplary purposes, such a circuit will be discussed herein. It will be clear to those of ordinary skill in the art who have the benefit of this disclosure, however, that the invention is not so limited. Discrete circuits, application specific circuits and safety circuits manufactured by other companies, including Ricoh and Mitsumi for example, may be equally substituted for the Seiko circuit.  
      By way of background, referring now to  FIG. 1 , illustrated therein is a block diagram of an S-8232 safety circuit  100 . The S-8232 safety circuit is designed to be used with two, serial, lithium-based cells. Again, it will be clear to those of ordinary skill in the art with the benefit of this disclosure that the invention is not so limited. The overpower circuit discussed herein may be equally applied to any combination of serial or parallel cells.  
      The safety circuit  100  may be as simple as a single integrated circuit (IC) that provides a means for monitoring of cell voltage and current, and thereby controls the charging and discharging of the cells within a battery pack. Discrete equivalents of the IC may also be substituted. The safety circuit  100  includes an overcharge detector  101  that monitors the voltages across the corresponding cells. The overcharge detector  101  compares these voltages to a predetermined maximum cell voltage. When the cell voltage exceeds this threshold, the overcharge detector  101 , via some internal logic circuitry  103 , causes a push-pull output stage  114  to actuate the charge pin  107 . When the charge pin  107  is coupled to a disconnect means, like a transistor acting as a switch in its non-linear region, actuation will prevent any further charging of the cells.  
      Similarly, the safety circuit includes an overdischarge detector  102  that ensures that the voltage across the cells does not fall below a predetermined threshold. If it does, the overdischarge detector  102  causes an output stage  113  to actuate the discharge pin  106 . When the discharge pin  106  is coupled to a disconnect means, like a serial transistor, actuation prevents any further discharge.  
      Cell current is monitored by way of an overcurrent detection pin  108  coupled to an overcurrent detection circuit  104 . The overcurrent detection pin  108  senses the voltage between the Vss pin  112  and the overcurrent detection pin  108 . When this voltage exceeds a predetermined threshold, as will be explained in more detail later, the overcurrent circuit  104  causes the discharging pin  106  to actuate, thereby stopping the flow of current in the discharge direction. In some situations, with some safety circuits, the charging pin  107  may also actuate.  
      When the load is removed, as evidenced by an impedance greater than 200 MΩ appearing between the Sens pin  110  and the overcurrent pin  108 , the safety circuit  100  resets, thereby deactuating the discharge pin  106 . This action will be more evident with the discussion of  FIG. 2  below.  
      Other components of the safety circuit  100  include a Vcc pin  109 , a center tap pin  111 , and a Vss pin  112 , that monitor the voltage at the cathode, between, and at the anode of serial cells, respectively. Additionally, a delay circuit  105  provides some hysteresis and transient immunity.  
      Referring now to  FIG. 2 , illustrated therein is one preferred embodiment of a battery protection circuit in accordance with the invention. The safety circuit  100  from  FIG. 1  is coupled to a pair of rechargeable cells  201 ,  202 . The charge pin  107  and the discharge pin  106  are coupled to disconnect means  203 ,  204 , respectively, which are in turn coupled serially with the cells  201 ,  202 . The disconnect means  203 ,  204  in this exemplary embodiment are field effect transistors (FETs), although other devices, including switches, relays, circuit breakers and controllable fuses may be substituted, depending upon the application.  
      The overcurrent pin  108  is coupled to the low side  205  of-the circuit, such that the overcurrent pin  108  may work in conjunction with the Vss pin  112  to sense the voltage across the FETs  203 ,  204 . When this voltage becomes too high, the safety circuit  100  knows that the current being drawn from the cells  201 ,  202  is correspondingly too high. When this occurs, the discharge pin  106  causes the FETs  203  to open, thereby preventing current from flowing to the external terminals  206 ,  207 . The safety circuit  100  resets, and thus closes the FET  203 , when an impedance greater than 200 MΩ is sensed between the Sens pin  110  and the overcurrent pin  108 . This occurs when a load (not shown) is removed from the terminals  206 ,  207 , thereby creating an open circuit between the terminals  206 ,  207 .  
      The overpower circuit  208  includes a power meter  209  that acts as a means of monitoring power being delivered to or from the cells  201 ,  202 . The power meter  209 , explained in more detail with the discussion of  FIG. 3 , is any circuit that is capable of determining whether the product of the voltage across the at least one rechargeable cell and the current flowing through the at least one rechargeable cell exceeds the predetermined threshold. It may include a circuit that generates a signal that is proportional to the product of the voltage across the cells and the current flowing through the cells. It may also be a circuit that simply generates a binary, up or down signal that indicates whether the power is above or below the threshold. The power meter  209  may be an accurate, linear power meter, or may be a simpler circuit that approximates power, for example by way of piecewise linear or other approximation means.  
      The power signal  210  is then coupled to a comparator  211  that has a signal  212  (like a reference voltage) that is proportional to the predetermined threshold of power. When the power signal  210  exceeds the predetermined threshold  212 , the comparator  211  actuates. The predetermined threshold may be set to any level required by the application. One exemplary threshold for a two-serial-cell configuration, that is intended to comply with a corresponding temperature threshold limit set forth by the Atmospheric Explosive (ATEX) directive set forth by the European Union, is nine watts. In any event, when the signal proportional to power is below the predetermined threshold, the output  216  of the comparator  211  is in a first state. The output  216  of the comparator  211  switches to a second state when the signal proportional to power exceeds the predetermined threshold.  
      When the power sourced from the cells  201 ,  202  exceeds the predetermined threshold, the overpower circuit  208  simulates an overcurrent condition within the safety circuit  100 , causing the FET  203  to open or enter a high impedance state, thereby preventing current from flowing from the cells  201 ,  202 . The overcurrent condition is simulated by sourcing current into the overcurrent pin  108  (as a result of increased voltage at the overcurrent pin  108 ), and thus into the overcurrent detection circuit within the safety circuit  100 .  
      Such an overpower condition would arise as follows: The power meter  209  would be continually monitoring the power sourced from the cells  201 ,  202 . The load connected to the terminals  206 ,  206  would begin drawing power in excess of the predetermined threshold. The power meter  209  determines that this is the case, causing the power signal  210  to rise above the power threshold signal  212 . This, in turn, actuates the comparator  211 .  
      A switch  219 , shown here as a FET, is responsive to the comparator and closes upon actuation of the comparator  211 . This pulls the overcurrent pin  108  to the cell voltages, thereby causing current to flow into the overcurrent pin  108  through a current limiting resistor  213 . To the safety circuit  100 , this appears to be an overcurrent situation. The safety circuit  100  then opens the discharge FET  203 , thereby preventing any current from flowing out of the cells  201 ,  202 . As such, the cells  201 ,  202  are disconnected from the terminals  206 ,  207  as a result of power dissipation exceeding the predetermined threshold. An optional delay circuit  218 , for example a R-C filter, may be coupled between the comparator  211  and the switch  219  where a delay prior to opening the FET  203  is desired. Such a delay may be desirable when a host device needs time to complete an operation prior to power down.  
      In parallel, an optional second disconnect means  214 , shown here as a FET, may be coupled to the comparator  211  so as to be responsive to the comparator  211 . The second disconnect means  214 , coupled serially between the terminals  206 ,  207  and the cells  201 ,  202 , operates as a secondary circuit breaker and opens when the comparator  211  is actuated. As such, if the discharge FET  203  fails, the second disconnect means will still disconnect the cells  201 ,  202  from the terminals  206 ,  207  when an overpower condition occurs.  
      Note that this second disconnect means  214  is optional, as it is advantageous in some designs. For example, in circuits where redundancy is needed for reliability, a designer may decide to employ two separate safety circuits, using the second safety circuit to control a second discharge FET, which would thus serve as the optional second disconnect means. In such a case, one example of which is illustrated in  FIG. 5 , either the overpower circuit  208 , or a redundant overpower circuit  501 , would be connected to the overcurrent pin  502  of a second safety circuit  503 , with the charge and discharge FETs  504 ,  505  of the second safety circuit  503  being coupled serially with the terminals  206 ,  207 , the cells  201 ,  202 , and the first charge and discharge FETs  203 ,  204 . Other designs may need neither the second safety circuit nor the second disconnect means.  
      A leakage current path in parallel with the second disconnect means  214  is provided by a resistor  215 . The resistor  215  provides a latching mechanism that causes the safety circuit  100  to remain in the simulated overcurrent condition. Recall that the safety circuit  100  resets when the impedance between the sense pin  110  and the overcurrent pin  108  exceeds 200 MΩ. This would be the case when the second disconnect means  214  opens. As such, a leakage path provided by the resistor  215  ensures that the safety circuit  100  stays latched in the simulated overcurrrent condition until the load is removed from the terminals  206 ,  207 . Resistance values for resistor  215  range from 100 kΩ to 500 kΩ, preferably about 200 kΩ. Note that when a second safety circuit is used to control the second disconnect means  214 , the leakage current path may not be necessary, as the second safety circuit provides an internal leakage path.  
      As stated, when the overpower, and thus the simulated overcurrent, condition is initiated, the cells  201 ,  202  are disconnected from the terminals  206 ,  207  by way of the FET  203 . In such a state, it is not desirable to have electrical components within the battery pack discharge the cells  201 ,  202 . As such, the invention provides a means of disabling the overpower circuit  208 . Disablement is accomplished by a switch coupled between the high side terminal  206  of the circuit and the overpower circuit  208 . This switch  217 , shown for exemplary purposes as a FET, is coupled to the discharge pin  106  of the safety circuit  100 .  
      When the discharge pin  106  is actuated, the switch  217  turns off, thereby blocking current from flowing to the overpower circuit. Thus, in an overpower condition, the overpower circuit  208  first simulates an overcurrent condition in the safety circuit  100 , thereby causing the discharge pin  106  to actuate. This, in turn, causes the switch  217  to open, thereby deactivating the overpower circuit  208 .  
      Such a scenario is perfectly acceptable, in that the overpower circuit  208  is no longer needed to monitor power being delivered from the cells  201 ,  202 , as there is no power being delivered from the cells  201 ,  202  since the charge and discharge FET  203  is open. Upon removal of the load, however, the safety circuit  100  resets, thereby causing closure of the FET  203 , thereby closing the switch  217 , thereby reactivating the overpower circuit  208 . The safety circuit will then revert back to normal operation.  
      Referring now to  FIG. 3  illustrated therein is one example of a power meter  209  in accordance with the invention. The power meter  209  includes a means for measuring or sensing the voltage across the cells, as well as a means for measuring or sensing the current flowing through the cells. Both the means of measuring voltage and current may comprise analog amplifiers  301 ,  302  coupled to the cells. In the case of voltage, the amplifier  301  may have inputs coupled across the cells to measure the voltage. The gain of the amplifier  301  would be scaled such that the product output is at a level that is acceptable by the comparator.  
      In the case of current, the amplifier  302  input may be coupled to a means of sensing current, like a current sense resistor for example. As with the voltage amplifier  301 , the gain of the current amplifier  302  would be scaled such that the product output is at a level that is acceptable by the comparator.  
      The outputs of the amplifiers  301 ,  302  are then fed into an analog multiplier  303 . The analog multiplier produces a product output  304  that is proportional to the product of voltage and current. This output  304  is then fed to the comparator  211 . One example of an analog multiplier is shown in  FIG. 4 , and is also taught in U.S. Pat. No. 3,562,553, entitled “Multiplier Circuit, issued to Roth, which is incorporated herein by reference for all purposes.  
      Note that the power meter of  FIG. 2  and the multiplier circuit of  FIG. 3  are but one exemplary embodiment of a power meter in accordance with the invention. It will be clear to those of ordinary skill in the art who have the benefit of this disclosure that the invention is not so limited. Numerous other power measurement circuits, including those employing logarithmic amplifiers, microprocessors with analog to digital converters, hall effect multipliers, and other analog and digital circuits may be equally substituted. The only requirement is that the power measurement circuit be capable of producing a signal proportional to the amount of power being sourced from, or delivered to, the cells.  
      While the preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims. For example, while one preferred embodiment of the invention is a rechargeable battery pack comprising the battery protection circuit taught in  FIG. 2 , the invention is not so limited. It may be applied to any power source, including power supplies, fuel cells, solar cells and the like. Additionally, it may be incorporated into the host device as well as within the battery pack.