Method and system for providing a warning of a remaining energy level of a battery pack in a defibrillator

A method and system for monitoring the capacity of a battery pack is provided. The battery pack is capable of delivering current to a load. The battery pack includes a monitor cell and battery cells. The monitor cell has an initial energy level that is lower by a predetermined difference from the initial energy level of at least one of the other battery cells. A monitoring system including an analog-to-digital converter and a microprocessor is connected to the battery pack and monitors the voltage of the battery pack. When the monitoring system detects a voltage change indicating depletion of the monitor cell, the monitoring system provides a signal indicative that the battery pack is nearing depletion. The proportional rate of discharge of the monitor cell relative to the other cells during normal defibrillator operation is matched to the proportional rate of discharge occurring at other times. Thus, the assumptions as to the remaining energy reserves when the monitor cell is depleted remain accurate whether the battery pack is discharged by normal defibrillator operation or by other phenomenon such as energy drain during non-use. In addition, the system will accurately provide a warning of battery pack depletion regardless of the type of batteries used or the initial energy levels, as long as the batteries are initially similar to one another. Thus, batteries from any manufacturer or production lot may be used for the battery pack without requiring advance knowledge of the energy characteristics of the particular set of batteries.

FIELD OF THE INVENTION
 The present invention relates generally to a method and system for
 monitoring batteries and determining the condition of a battery pack in a
 defibrillator.
 BACKGROUND OF THE INVENTION
 Batteries are often used to power portable electronic devices. After a
 period of use, the batteries in a portable device will become depleted and
 affect the performance of the portable device. When the energy level of
 the batteries falls below a certain threshold, the portable device will
 cease to operate. At this point, the batteries must be recharged or
 replaced. Accordingly, in many applications it would be beneficial to
 monitor the remaining charge in the batteries and alert the user of a
 portable device before the batteries are depleted.
 One environment where monitoring the remaining charge in batteries is
 important is in portable external cardiac defibrillators. Portable cardiac
 defibrillators generate and apply a high energy defibrillation pulse to
 the chest of a patient to cause the patient's heart to stop fibrillating
 and return to a normal rhythm. Sometimes the application of a single
 defibrillation pulse fails to restore the patient's heart to a normal
 rhythm. In such an event, it may be necessary to apply additional
 defibrillation pulses. Portable external defibrillators generally use a
 battery pack containing a number of cells to power the defibrillator. The
 battery pack allows an energy storage capacitor to be charged in order to
 generate a defibrillation pulse. If the battery pack becomes depleted, the
 patient cannot be treated.
 In order to assure that an external cardiac defibrillator is always ready
 for use, it is therefore advantageous to monitor the remaining charge in
 the battery pack. Battery monitoring provides an indication of when the
 battery pack is nearly depleted and needs to be replaced or recharged.
 Various efforts in the prior art have been devoted to monitoring the
 remaining energy in a battery pack. One common approach entails measuring
 an output voltage from the battery pack while the battery pack is
 connected to an electrical load. As the battery pack is discharged, the
 voltage will typically drop. Variations of this approach involve measuring
 other battery parameters, such as the impedance of the batteries in the
 battery pack, to detect changes that indicate that the battery pack is
 nearing the end of its useful life.
 One disadvantage of monitoring the output voltage from the battery pack to
 detect a drop in voltage is that the method is unable to accurately
 monitor the energy level of typical nonrechargeable batteries. FIG. 1
 depicts the output voltage v from a single nonrechargeable lithium battery
 plotted with respect to time. The output voltage from the lithium battery
 remains relatively constant until near the end of its life t when the
 output voltage drops precipitously. Because of the steep drop in voltage,
 it is nearly impossible to give sufficient advance warning when the
 battery pack is nearly depleted. By the time the battery voltage begins to
 drop, insufficient energy often remains in the battery pack to power the
 electronic device for any appreciable period. This is especially true for
 devices which have a high energy utilization rate such as defibrillators.
 Another approach for determining the remaining charge in a battery pack is
 disclosed in U.S. Pat. No. 5,483,165 to Cameron et al. Cameron et al.
 disclose a defibrillator having a main battery consisting of a number of
 identical battery cells connected in series. A sense cell is connected in
 series to the main battery. The sense cell is identical in type and
 manufacture to each of the main battery cells. A current flows through the
 main battery and the sense cell to a load. Because all the cells are
 identical, the sense cell and battery cells in the main battery should in
 normal circumstances be depleted at the same rate. However, Cameron et al.
 disclose a dedicated circuit for drawing additional, incremental current
 from the sense cell. The value of the incremental current equals the value
 of the current delivered to the load scaled by an arbitrary constant. The
 dedicated circuit drawing the incremental current includes a variable
 resistance connected between the sense cell and ground potential. The
 variable resistance is used to vary the value of the incremental current
 drawn from the sense cell.
 The sense cell in Cameron et al. is used to estimate the remaining charge
 in the main battery. Since the sense cell is discharged at a quicker rate
 than the main battery, the voltage drop characteristic of battery cells at
 the end of their useful life will first occur on the sense cell. When the
 voltage drop on the sense cell is detected by a controller, a signal or
 other warning is generated to indicate that the main battery is also
 nearly depleted.
 While the Cameron et al. approach offers some advantages over other prior
 art techniques for monitoring batteries, it also presents disadvantages.
 The need to draw an additional current from a sense cell adds undue
 complexity and, it will be appreciated, problems attendant with that
 complexity. For example, the Cameron et al. approach requires adding a
 dedicated voltage monitor and a variable resistance circuit to draw the
 incremental current from the sense cell. The additional components and
 battery contacts required to draw the incremental current increase the
 likelihood that a component in the battery monitoring system could fail
 and render the battery monitoring technique ineffective.
 Apart from the increased likelihood of failure, the need to provide a
 voltage monitor and variable resistance poses other problems. The addition
 of the voltage monitor and the variable resistance increases the cost of a
 device incorporating the battery monitoring system. The voltage monitor
 and the variable resistance also add weight and size to the system.
 The use of additional circuitry to draw an incremental current from the
 sense cell also limits the interchangeability of the type of battery cell
 incorporated in a battery pack. For example, if lithium battery cells are
 used in the battery packs, a certain incremental current must be used that
 corresponds to the anticipated voltage curve of the lithium battery cell.
 If there is an improvement in battery technology or if a user desires to
 use a different type of battery cell, such as a rechargeable nickel
 cadmium battery cell, a different amount of incremental current should be
 drawn from the new cell to correspond to the anticipated voltage curve of
 the new cell. An operator of the portable electronic device must therefore
 somehow adjust the amount of incremental current drawn depending upon the
 type of battery pack that is to be used in the device. The need to perform
 an additional adjustment limits the ease with which the user may select a
 type of battery suitable for a particular application.
 In addition to monitoring the remaining charge in the battery pack, it
 would also be advantageous to monitor the health of the individual battery
 cells within the battery pack in order to detect a possible cell failure.
 Individual cell failures decrease the energy storage capacity of the
 battery pack and can also detrimentally affect the performance of the
 battery pack during discharge of the battery pack.
 As can be seen from the discussion above, there exists a need for improved
 methods of monitoring the condition of battery packs. The present
 invention is directed toward addressing this need.
 SUMMARY OF THE INVENTION
 In one embodiment of the present invention, a method and system for
 monitoring the condition of a battery pack of a defibrillator is provided.
 The battery pack includes a plurality of battery cells, at least one of
 which is designated as a monitor cell. In one embodiment, the monitor cell
 has an initial energy level lower than at least one of the plurality of
 other battery cells. The monitoring system includes an analog-to-digital
 converter and a microprocessor connected to the battery pack for
 monitoring the output voltage from the battery pack. When the monitoring
 system detects a voltage change or a rate of voltage change across the
 battery pack that is greater than a threshold value, the monitoring system
 alerts an operator that the battery pack is nearly depleted.
 In accordance with one aspect of the invention, the capacity of the battery
 pack and the monitor cell is predetermined so that the defibrillator may
 continue to operate on the voltage provided by the remaining battery cells
 in the battery pack after depletion of the monitor cells.
 In accordance with another aspect of the invention, the monitor cell is
 rated at less ampere hours than the other cells in the battery pack.
 Alternatively, the monitor cell is discharged to a predetermined level
 prior to being connected to the other cells in the battery pack.
 In accordance with still other aspects of the invention, the monitoring
 system provides an indication of an approximate number of shocks that the
 defibrillator is capable of providing upon detection of the depletion of
 the monitor cell. In embodiments in which multiple monitor cells are used,
 the monitoring system detects the depletion of individual monitoring cells
 and provides an indication of the operating capacity of the defibrillator
 upon depletion of each monitor cell.
 In accordance with another aspect of the invention, the proportional
 discharge rate of the monitor cell relative to the other cells during
 defibrillation pulse operation is set to be approximately equal to the
 proportional discharge rate that occurs at other times. In other words,
 the estimation of the remaining energy reserve in the battery pack once
 the monitor cell is depleted is based in part on the presumed proportional
 rate at which the monitor cell was discharged relative to the other cells
 of the battery pack. Thus, for example, if the monitor cell is discharged
 at a different proportional rate during a different circumstance (e.g.,
 while sitting around unused for a long time period), then the energy
 assumptions when the monitor cell is depleted may be erroneous. To avoid
 this condition, the system of the present invention matches the
 proportional discharge rate of the monitor cell during defibrillation
 pulse operations to that occurring at other times. Thus, in the present
 system, the depletion point of the monitor cell will still accurately
 indicate a certain remaining energy reserve of the battery pack as a
 whole, regardless of whether the cells of the battery pack have been
 discharged by normal use or by some other means. This is in contrast to a
 system in which the proportional discharge rates are not matched, wherein
 under certain circumstances there may be an inaccurate indication with
 regard to the remaining energy reserve of the battery pack when the
 monitor cell is depleted.
 In another embodiment of the invention, a method for identifying the
 presence of a damaged or depleted battery cell in a rechargeable battery
 pack having a plurality of cells is provided. The battery pack is
 connected to a load to allow the battery pack to deliver current to the
 load. A voltage or rate of change in voltage across the battery pack is
 measured during application of the current to the load. The measured
 voltage or rate of change in voltage is compared with a threshold value to
 provide an indication of the presence of a depleted or damaged battery
 cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 The present invention detects the condition of a battery pack in a
 defibrillator or other device in which the battery pack is located. In
 accordance with one embodiment of the invention, the voltage of the
 battery pack is monitored to provide indications about the remaining
 charge of the battery pack and the condition of individual cells in the
 battery pack. FIG. 2 is a block diagram of one embodiment of the invention
 for monitoring the remaining charge in a battery pack 6 incorporated in a
 portable electronic device, such as a portable defibrillator 4. Battery
 pack 6 comprises a plurality of battery cells C1, C2, C3 and C4 that are
 coupled in series so that the output voltage of the battery pack is equal
 to the sum of the output voltage from each individual battery cell. While
 four cells are depicted in FIG. 2, it will be appreciated that any number
 of battery cells may be incorporated in the battery pack, depending on the
 output voltage required to power the electronic device containing the
 battery pack.
 One of the battery cells C1, C2, C3, or C4 is designated a monitor cell 8.
 FIG. 2 depicts cell C2 as the monitor cell, however, it will be
 appreciated that any of the other cells in the battery pack could be used
 as a monitor cell. A diode D is coupled in parallel with the monitor cell
 so that it is forward biased if the monitor cell is depleted or fails.
 Diode D provides a conductive path so that when the monitor cell is
 depleted, the battery pack continues to produce an output current.
 Preferably, each of the battery cells C1, C2, C3, and C4 have substantially
 the same output voltage and type of construction. The battery cells may be
 any type of cell, including, but not limited to, lithium cells,
 nickel-cadmium cells, alkaline cells and zinc-carbon cells. As discussed
 in additional detail below, in the preferred embodiment the battery cells
 C1, C3, and C4 are fully-charged cells that have approximately the same
 energy storage capacity and initial energy charge level. The monitor cell
 8, however, is configured to have less energy stored in it initially than
 the energy stored in each of the battery cells C1, C3, and C4.
 The negative terminal of battery pack 6 is coupled to ground, and the
 positive terminal of the battery pack is coupled to a load R.sub.L. Load
 R.sub.L represents any load to which current from the battery pack 6 is
 delivered. For example, when the battery pack is incorporated in the
 defibrillator 4, load R.sub.L represents a circuit for charging an energy
 storage capacitor (not shown) to allow a defibrillation pulse to be
 applied to a patient experiencing ventricular fibrillation, or other
 shockable rhythm.
 A monitoring system 22 is provided to monitor the output voltage from the
 battery pack 6 and determine when the battery pack is nearly depleted. The
 monitoring system 22 includes an analog-to-digital converter 10 and a
 microprocessor 12. The positive terminal of the battery pack 6 is
 connected to the analog-to-digital converter 10 by a line 16. The
 analog-to-digital converter 10 measures the voltage across the battery
 pack and converts the voltage into a digital signal. The digital signal is
 provided to the microprocessor 12 over a bus 18 to allow the
 microprocessor to monitor the voltage or rate of change in the voltage
 across the battery pack (hereinafter voltage change shall mean either a
 change in voltage or a change in the rate of change of the voltage).
 In the preferred embodiment, the monitor cell has approximately the same
 energy storage capacity as the other battery cells. In actual practice,
 one way to help ensure that the batteries have similar characteristics is
 to use batteries from the same lot from the manufacturer. In other words,
 batteries that have been manufactured by different machines or in
 different production runs may have significant variations in their energy
 characteristics. Thus, one simple way to improve the odds at the start
 that the batteries in a given battery pack will have similar
 characteristics is to simply use batteries from the same lot. It should be
 noted that this technique can not be used in a system utilizing different
 sized batteries that are necessarily manufactured in different production
 runs.
 A particular advantage of the preferred embodiment is that it does not
 require advance knowledge of the energy characteristics of the particular
 set of batteries being used in the battery pack. In other words, as long
 as the batteries are initially similar to one another, the system will
 accurately provide a warning of battery pack depletion regardless of which
 manufacturer or production lot the set of batteries came from.
 Prior to assembly of the battery pack 6, the amount of energy in the
 monitor cell 8 is set so that it is less than the initial energy contained
 in each of the battery cells C1, C3, and C4. Two techniques may be used to
 ensure that the monitor cell contains less energy than the other cells.
 The monitor cell may be selected so that it is rated at less ampere-hours
 than the other battery cells when in a fully charged state. Alternatively,
 the monitor cell C8 may be discharged by drawing a known current from the
 monitor cell C8 for a predetermined period of time. For example, the
 discharge can be accomplished by connecting a resistor to the monitor cell
 C8 to draw a current from the monitor cell for a predetermined time.
 After the amount of energy in the monitor cell is set, the battery pack is
 assembled and placed in the defibrillator. As current from the battery
 pack 6 is delivered to the load R.sub.L, the battery cells C1, C3, C4 and
 the monitor cell 8 are discharged at an approximately equal rate. Because
 the battery cells C1, C2, C3, and C4 are connected in series and the
 initial energy level of the monitor cell 8 is lower, the monitor cell 8 is
 depleted before the battery cells C1, C3, and C4. When the monitor cell 8
 is depleted, the output voltage from the battery pack will drop as
 depicted in FIG. 3.
 FIG. 3 is a graph of the output voltage from the battery pack 6. The
 abscissa represents time, while the ordinate represents voltage. The
 initial input voltage V.sub.i from the battery pack is equal to the sum of
 the output voltages from all the cells in the battery pack. For most of
 the battery pack life, the output voltage remains at or near the initial
 voltage level V.sub.i. At a time t.sub.2, the monitor cell 8 becomes
 nearly depleted and the output voltage from the monitor cell drops
 quickly. The output voltage from the battery pack therefore drops by a
 corresponding amount, from the initial voltage level V.sub.i to an
 intermediate voltage level V.sub.w at time t.sub.3. The intermediate
 voltage level V.sub.w is equal to the sum of the output voltages from the
 remaining cells C1, C3, and C4, minus the forward drop of diode D.
 It will be appreciated that the monitor cell 8 may be a high impedance
 circuit when depleted. Diode D bypasses the monitor cell and acts as a
 current bypass around the monitor cell 8 so that the battery pack will
 continue to provide current to the defibrillator.
 The battery pack continues to provide the intermediate output voltage
 V.sub.w for a period of time following time t.sub.3. In the preferred
 embodiment, the output of the battery cells and thus the intermediate
 output voltage V.sub.w is predetermined to be adequate to continue to
 power the defibrillator with little or no loss of device performance. The
 continued delivery of current by the battery pack 6 will ultimately cause
 battery cells C1, C3, and C4 to become depleted. At time t.sub.4, the
 battery cells C1, C3, and C4 are nearly depleted and the output voltage
 from the battery pack drops quickly. Following time t.sub.4, the output
 voltage from the battery pack may be insufficient to power the
 defibrillator.
 The detection of the voltage change from the initial voltage V.sub.i to the
 intermediate voltage V.sub.w is used to warn an operator of the portable
 equipment of the remaining charge in the battery pack 6. When the
 microprocessor 12 detects the voltage change caused by the depletion of
 the monitor cell, a warning is provided to the operator on a display 14.
 The warning may be an audible or visual alarm. The warning indicates to
 the operator that the battery pack 6 will soon be completely discharged
 and that the battery pack should be replaced or recharged before the
 device fails.
 As indicated above, the voltage of the battery pack remains approximately
 constant until the battery pack is nearly depleted. Therefore, the voltage
 change caused by the depletion of the monitor cell 8 is easily detected by
 the microprocessor. The microprocessor is preprogrammed to detect when a
 voltage change greater than a predetermined threshold is detected. Thus,
 the microprocessor detects when the monitor cell is depleted and then
 provides a warning to the operator to indicate that the monitor cell is
 depleted, and thus the battery pack 6 is nearing depletion.
 In the preferred embodiment, the battery pack 6 is used within an external
 defibrillator. In such embodiment, the battery pack is carefully sized so
 that upon depletion of the monitor cell the battery pack still contains
 enough energy to operate the defibrillator during a predetermined range of
 shocks. Thus, the battery pack is sized to insure that the operator is
 provided an indication of the battery pack nearing depletion prior to the
 defibrillator's being unable to defibrillate a patient. In the preferred
 embodiment, the battery pack's energy level is sized to allow the
 defibrillator to operator for approximately a predetermined number of
 defibrillation shocks. Thus, the microprocessor may also provide an
 indication to the operator of approximately how many defibrillation shocks
 the defibrillator can provide prior to depletion of the battery pack. The
 indication of the number of shocks left within the battery pack is, of
 course, approximate because of variations in battery cell construction,
 varying discharge rates of the batteries.
 The initial energy level of the monitor cell 8 is calibrated so that the
 approximate amount of charge remaining in battery cells C1, C3, and C4 is
 known when the monitor cell is depleted. The calibration is, of course,
 approximate because of variations in the battery cell construction,
 varying discharge rates because of climatic conditions to which the
 battery pack is exposed, and other factors.
 In alternative embodiments of the invention, multiple monitor cells can be
 provided within the battery pack. Each monitor cell can have a
 predetermined initial energy level that is different from the other
 monitor cells. As the battery pack is discharged, each of the monitor
 cells having a different initial energy level will be depleted at a
 different time. The monitor circuit detects the depletion of each
 individual monitor cell and provides an indication of the remaining energy
 within the battery pack upon depletion of each individual monitor cell.
 Therefore, the microprocessor can be programmed to provide an indication
 of the remaining power within the battery pack based upon the measure of
 voltage after depletion of each individual monitor cell.
 Based upon the measured voltage in the battery pack, the microprocessor can
 calculate roughly the energy storage capacity left in the battery pack and
 thus how many times the defibrillator can be operated prior to battery
 failure. The indication of remaining energy within the battery pack or how
 long the defibrillator can be operated for prior to depletion of the
 battery pack will be approximate due to the varying parameters associated
 with the charge and discharge of each of the cells within the battery
 pack, system inefficiencies within the defibrillator, and the energy
 provided in each shock, etc.
 The present method and system for monitoring the remaining charge in a
 battery pack provides a simple, reliable technique to anticipate battery
 pack failure. The present invention has application in a variety of
 possible environments that require reliable battery power including
 external cardiac defibrillators. Conventional external cardiac
 defibrillators include many of the components required to implement the
 present invention in the defibrillator. In many cases, only a new battery
 pack including the diode D and the addition of software will need to be
 added to practice the invention. Some systems may also require the
 addition of a display or indicator for conveying the information regarding
 the status of the battery pack.
 Another aspect of the present method and system involves the proportional
 rate at which the monitor cell is designed to be discharged with respect
 to the other cells during normal use. It has been discovered that in some
 systems, if the proportional discharge rate of the monitor cell with
 respect to the other cells is not the same during normal use as it is
 during other times, then inaccurate energy reserve assumptions may result
 at the time when the monitor cell is depleted. The following examples are
 useful for illustrating this concept, both with respect to a system
 according to the present invention where the proportional discharge rates
 are matched, and with respect to a prior art system where the proportional
 discharge rates are not matched. These examples include a number of
 simplifying assumptions and are given for purposes of illustration only.
 In the first example, a system formed in accordance with the present
 invention has the following characteristics:
 (a) The first example system provides twenty 360-joule shocks, for a total
 of 7,200 joules;
 (b) The monitor cell is set to be depleted and the alarm given when the
 system is at one-half initial capacity with enough energy remaining in the
 system to provide ten more 360-joule shocks;
 (c) Assuming that all four cells will be drained at an equal rate during
 each 360-joule defibrillation pulse operation, each cell will be drained
 by 360-joules.div.4=90 joules per shock, such that the monitor cell should
 be set at 900 joules so that it is drained after ten shocks;
 (d) With the monitor cell set at 900 joules, the remaining three cells
 should be set at (7,200-900).div.3=2,100 joules apiece to start (since
 there needs to be 7,200 total joules in the system at the start in order
 to provide twenty 360-joule shocks).
 Thus, after ten shocks, the monitor cell will be completely depleted at
 which time the alarm signal will go off, and the remaining three. battery
 cells will have (2,100-(10.times.90)=1,200) joules apiece remaining, which
 is enough to provide ten more 360-joule shocks.
 This first example shows a simple proportional discharge rate of a
 one-to-one ratio of a monitor cell with respect to the other cells. In
 other words, in this first example, the discharge rate of 90 joules per
 shock operation for the monitor cell is the same as the 90 joules for each
 of the other cells, thus resulting in a simple one-to-one ratio. In
 accordance with the invention, this is designed to be the same as the
 proportional discharge rate under other conditions, such as when the
 battery pack is not being used, as will be described below.
 The proportional discharge rate during non-use can be described as follows.
 A general assumption is made that all four batteries have been depleted by
 approximately the same number of joules due to non-use. One model used for
 nonuse energy drain is a shunt resistor across the terminals of each of
 the batteries which draws approximately the same current from each of the
 batteries over time. Actual battery drain is more complex than this,
 although the characteristic represented by this simplified model is part
 of the process, and provides a simple illustration of how the present
 inventive system compensates for this type of energy drain Under this
 assumption, the monitor cell will drain at approximately the same rate as
 the other battery cells. For purposes of illustration, assume that the
 monitor cell and the three additional cells of the first example have been
 depleted by 900 joules apiece during a non-use period. After such a
 period, the monitor cell will have 0 joules remaining and the three
 remaining cells will have 1,200 joules apiece. When the system of the
 first example is activated, the alarm will go off immediately, thus
 properly indicating that there is only enough energy remaining in the
 system for ten 360-joule shocks.
 As described above, the model for energy drain during non-use has assumed a
 one-to-one ratio for the proportional discharge rate of the monitor cell
 with respect to the other cells. The system of the first example was thus
 designed to have a similar proportional discharge rate for the monitor
 cell during normal defibrillation pulse operation. Thus, the system
 continues to provide an accurate warning signal with regard to remaining
 energy reserves, even when the batteries have been partially drained by
 non-use. In contrast, a system in which the proportional discharge rates
 are not matched, may provide inaccurate indications as to energy reserve
 levels, as illustrated in the second example below.
 The second example uses a system similar to the one described in U.S. Pat.
 No. 5,483,165 to Cameron et al., that was discussed previously in the
 "Background of the Invention" section. That system also uses a "monitor
 cell" that is designed to run out of energy before the other battery
 cells. As described above, that system during defibrillation pulse
 operation draws more energy from the monitor cell than the other cells so
 that the monitor cell will run out of energy faster. The system is also
 designed so that a warning signal will be given when the monitor cell is
 depleted and there is presumed to be a certain amount of energy remaining
 in the system. The second example is structured with the following
 characteristics:
 (a) The example system provides twenty 360-joule shocks, for a total of
 7,200 joules;
 (b) Giving the system three regular battery cells, and one monitor cell,
 the four battery cells will therefore store (7,200.div.4=1,800) joules
 apiece at the start;
 (c) To have enough energy for one half the starting capacity, or 10 shocks
 remaining in the system when the alarm goes off, the monitor cell must be
 drained when there are 3,600 joules left in the system or after ten
 shocks. Therefore, the sense/monitor battery cell must be drained at a
 rate of (1,800.div.10=180) joules per shock;
 (d) If the sense/monitor cell is drained at 180 joules per shock, the
 remaining 180 joules for each 360-joule shock must come from the other
 three battery cells, which must be drained at a rate of 180.div.3=60
 joules per shock.
 Thus, after ten shocks in this second example system, the sense battery
 cell will be completely drained of energy, the warning signal will go off,
 and there will be (1,800-(10.times.60)=1,200) joules remaining in each of
 the last three battery cells, which is half the original capacity and is
 enough energy to provide ten more 360-joule shocks.
 The following description illustrates how this second example system can
 have erroneous energy assumptions when some of the batteries have been
 partially depleted by non-use. This discussion also assumes that the
 monitor cell drains at the same proportional rate as the others from
 non-use. Assuming for the above example that from non-use the three
 battery cells and the sense cell have drained by 900 joules each, each of
 the battery cells and the monitor cell will have 900 joules apiece
 remaining on them. Now, when the monitor cell is depleted by 180 joules
 per shock, it will be depleted after five shocks, although now, the other
 three batteries will have been depleted by (five shocks.times.60 joules
 per shock=300 joules), so each of the other three battery cells will only
 have 600 joules each (for a total of 1,800 joules) remaining when the
 alarm goes off This amount of energy is no longer enough to provide the
 expected ten 360 joule shocks that are supposed to be remaining after the
 alarm goes off, since there will only be 1,800 joules remaining in the
 total system at this time. Thus, the defibrillator operator would discover
 that the defibrillator had gone dead after five shocks after the alarm
 went off, even though there were supposed to be ten shocks left in the
 system after the alarm.
 It will be appreciated that in the present inventive system, by matching
 the proportional discharge rate of the monitor cell during normal
 defibrillation pulse operation to that occurring at other times, that the
 remaining energy reserve of the battery pack may be more accurately
 predicted. As described earlier, to compensate for energy drain during
 non-use, for which the assumed proportional discharge rate of the monitor
 cell is approximately a one-to-one ratio, the system of the present
 invention is designed to discharge the monitor cell during normal
 defibrillation pulse operation at approximately a proportional rate of
 one-to-one with respect to the other cells of the battery pack. As
 described earlier, this is in contrast to a system such as that described
 in the Cameron et al. patent, wherein the proportional discharge rate of
 the monitor cell is determined by a calculation of when a desired alarm is
 to be given, rather than with consideration to matching the proportional
 discharge rate to that which occurs for the monitor cell at other times.
 In this manner, the system of the present invention increases the accuracy
 of the predicted energy reserves so that the warning signal can be given
 at the proper time.
 In a system where the proportional discharge rate of the monitor cell
 during non-use was not initially a simple one-to-one ratio, means for
 drawing more or less current from the monitor cell could be provided. It
 should be noted that this adjustment could be made to the monitor cell
 during either the use or non-use periods. For example, during the non-use
 period, an adjustment in the above described system could be made by
 selecting the diode D so that the proportional discharge rate of the
 monitor cell 8 during non-use is made to be the same as that during use.
 Alternatively, a way to adjust the proportional discharge rate of the
 monitor cell during the use period would be similar to that shown in U.S.
 Pat. No. 5,483,165 to Cameron et al. When such a system was used in
 combination with the monitor cell method of the present invention,
 adjustments would need to be made to the energy level of the monitor cell,
 or to the energy level assumptions, to compensate accordingly.
 In another embodiment of the present invention, a degraded or damaged
 battery cell within a battery pack can also be detected by monitoring the
 voltage change of the battery pack during discharge of the battery pack.
 It will be appreciated that battery cells may sometimes become damaged,
 causing irregularities in their ability to retain, accept, or deliver
 charge. Such damage to battery cells may result from a variety of
 circumstances. For example, a degradation in battery cell performance
 occurs when the seals of a cell fail and let it dry out. As another
 example, contamination of a battery cell can occur when the battery cell
 is recharged. Such contamination can undesirably generate leakage currents
 within the battery cell, and thereby deplete the energy stored therein.
 When damage to a battery cell occurs, the damaged battery cell is
 preferably detected and removed from further use.
 The present invention detects a damaged battery cell in a battery pack by
 monitoring the voltage change of the battery pack during discharging. FIG.
 4 is a block diagram of a system for monitoring the voltage on a battery
 pack 42. As will be noted, the system of FIG. 4 is similar to the system
 of FIG. 2 without the use of the diode D. The battery pack 42 comprises a
 plurality of battery cells C7, C8, C9, and C10 that are connected in
 series. The voltage of the battery pack 42 is equal to the sum of the
 voltages of each battery cell. While four cells are depicted in FIG. 4, it
 will be appreciated that any number of battery cells may be incorporated
 in accordance with the present invention.
 Each of the battery cells C7, C8, C9, and C10 initially have a positive
 voltage and are connected so that the negative terminal of one battery
 cell is connected to the positive terminal of an adjacent battery cell.
 The battery cells C7, C8, C9, and C10 are rechargeable cells that are not
 necessarily fully charged. The positive terminal of the battery pack 42 is
 connected to ground potential through a resistor R. Resistor R represents
 any load to which current is delivered. For example, if the battery pack
 42 is used to charge an energy storage capacitor in a defibrillator,
 resistor R represents a circuit for charging the energy storage capacitor
 and delivering a defibrillation pulse to a patient experiencing
 ventricular fibrillation.
 A preferred monitoring stem 52 is provided to detect the voltage change of
 the battery pack 42 either during or after some current has been delivered
 to resistor R. The monitoring system 52 includes a microprocessor 48 and
 an analog-to-digital converter 46. The positive terminal of the battery
 pack 42 is connected to the analog-to-digital converter 46. The
 analog-to-digital converter 46 measures the voltages across the battery
 pack 42 and converts the voltage into a digital signal. The digital signal
 is provided to the microprocessor 48 over a bus 56. The microprocessor 48
 calculates the voltage change of the battery pack 42 due to the delivery
 of current to resistor R. More specifically, as described in more detail
 below, the microprocessor 48 detects damage to a battery cell by
 identifying when the voltage change exceeds a predetermined threshold.
 When used to monitor a charged battery pack, prior to any delivery of
 current, the voltage of the battery pack 42 has a maximum value which, as
 stated above, is the sum of the voltages of each battery cell. As
 described above, as the battery pack 42 delivers current to the resistor
 R, the voltage V.sub.1 of the battery pack 42 will very gradually decrease
 until the battery pack nears depletion and will then drop sharply. In the
 event that a battery cell within the battery pack 42 becomes damaged, the
 voltage of the battery pack 42 will quickly decrease as the battery cell
 fails and will then level out at a new voltage V2, and again very
 gradually decrease until the battery pack nears depletion at which time it
 will drop quickly. The rapid voltage change and then leveling out of the
 voltage is characteristic of a depleted cell or a damaged cell in the
 battery pack 42.
 FIG. 5 illustrates the reversal of a battery cell as a function of voltage
 of the battery pack 42 plotted against time during discharge of the
 battery pack. At a time t.sub.10, the voltage of the battery pack 42 is
 decreasing gradually while discharging. At a later time t.sub.11, the
 voltage of the battery pack 42 decreases at a relatively fast rate as
 compared to the rate at time t.sub.10. This sudden change in the rate of
 voltage drop is characteristic of battery cell depletion and reversal. The
 reversal of the battery cell lasts until time t.sub.12, when the voltage
 of the battery pack 42 returns to a gradual decrease in voltage, as was
 true at time t.sub.10 prior to reversal. At time t.sub.13, the voltage of
 the battery pack 42 continues to slowly decrease at a rate approximately
 equal to the rate of voltage change at time t.sub.10.
 The rapid voltage change between times t.sub.11 and t.sub.12 is detected by
 the microprocessor 48. Preferably, the microprocessor 48 is programmed to
 store a predetermined threshold value prior to reversal. It will be
 appreciated that the threshold for a particular battery pack prior to
 reversal depends on the type of battery cells, the initial charge level,
 the load to be placed on the battery pack, and other factors. During
 operation, the microprocessor 48 periodically compares the voltage change
 to the threshold value. If the magnitude of measured voltage change is
 larger than the threshold value, the microprocessor 48 detects a reversal,
 indicating the presence of a depleted or damaged battery cell.
 It will be appreciated by those skilled in this art and others that high
 rates of discharge of the battery pack 42 through a battery cell during
 reversal or continued charging of a battery pack having a bad cell may
 damage the battery pack, further damage the bad cell, or prevent the
 battery pack from charging or discharging properly. If a bad battery cell
 is charged or discharged improperly, damage to the battery cell may be
 increased to a point which permanently and irreparably compromises the
 utility of the battery cell or battery pack so that it cannot be recharged
 or discharged. Accordingly, when the microprocessor 48 detects a reversal
 caused by battery cell depletion or damage, a warning is preferably
 provided to a user of the battery pack 42. The warning is provided
 visually through the display 50 connected to the microprocessor 48 through
 line 58 and/or audibly through a speaker (not shown). The user may then
 remove the battery pack 42 and replace the damaged battery cell.
 Generally, battery packs contain cells of approximately the same energy
 storage capacity. Therefore, early depletion of a single battery cell
 generally means that the battery cell is damaged and has failed to
 properly take a charge or is failing to discharge correctly. However, in
 some causes an individual cell may not be damaged but may not have been
 originally fully charged. Therefore, after detection of a depleted or
 damaged battery cell, the battery pack may be recharged in an attempt to
 bring the depleted or damaged cell back to a similar energy storage
 capacity as the remaining cells in the battery pack. However, if the
 battery cell is damaged, it will fail to obtain an energy storage capacity
 that is comparable with the remaining cells in the battery pack.
 Therefore, during cycling of the battery pack, the damaged cell will again
 cause a premature voltage change that can be detected and thus provide an
 indication of the presence of a damaged cell within the battery pack.
 The embodiment of the present invention for detecting a damaged battery
 cell within a battery pack has application in a wide variety of possible
 environments including external cardiac defibrillators or any other
 devices including rechargeable battery packs or batteries.
 While the preferred embodiment of the invention has been illustrated and
 described, it will be appreciated that various changes can be made therein
 without departing from the spirit and scope of the invention. While
 developed for use in an external cardiac defibrillator, the method and
 system in accordance with the present invention may also be used in
 technologies unrelated to defibrillators.