Indicator of remaining energy in storage cell of implantable medical device

A manganese dioxide (MnO2) or silver vanadium oxide (SVO) or other battery of an implantable medical device having a relatively flat quiescent battery voltage during a beginning portion of the battery's useful life, makes it difficult to use quiescent battery voltage as an indicator of remaining battery energy during this portion of the battery life. A substantially constant load current pulse is drawn from the battery and a pair of loaded battery terminal voltage measurements is taken during this pulse. A difference between the voltage measurements is computed. This difference can be expressed as a rate of change, a slope, or a polarization angle, and can be used with stored data from similar batteries to determine remaining energy of the battery. A quiescent battery voltage can also be used in combination with this technique, and/or for distinguishing between different remaining energies corresponding to the same difference, slope, or polarization angle.

TECHNICAL FIELD

This document relates generally to energy storage cells and particularly, but not by way of limitation, to an indicator of remaining capacity of an energy storage cell, such as in an implantable pacer/defibrillator device.

BACKGROUND

Implantable medical devices include, among other things, cardiac rhythm management (CRM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) devices, as well as combination devices that provide more than one of these therapy modalities to a subject. Such devices are typically powered by self-contained energy sources, such as batteries. It is useful to know how much energy capacity remains in a battery carried within an implanted medical device, such as to ascertain when the implanted device should be explanted from the subject and replaced by a device with a fresh battery. Determining how much energy is left in a battery is particularly difficult when a measured battery characteristic (e.g., the quiescent voltage at the battery terminals) does not change appreciably during a large portion of the battery life. Yet such a characteristic is particularly desirable for use in a cardiac rhythm management device because it provides a predictable battery characteristic during that large portion of the battery life.

SUMMARY

In certain examples, this document describes a method. The method comprises drawing a substantially constant first current pulse from an energy storage cell during a first time period between a starting time and an ending time, measuring a first change of a terminal voltage across the cell during the first time period, and comparing the measured first change to first stored data to determine the energy remaining in the cell.

In certain variations, the drawing the first current pulse from the cell comprises drawing the first current pulse from a manganese dioxide battery. In certain variations, the drawing the first current pulse from the cell comprises drawing the first current pulse from a silver vanadium oxide battery. In certain variations, the drawing the first current pulse comprises drawing a substantially constant current of approximately between 2 amperes and 4 amperes. In certain variations, the drawing the first current pulse comprises drawing a substantially constant current of approximately 3 amperes. In certain variations, the first time period is approximately between 3 seconds and 30 seconds. In certain variations, the first time period is approximately 6 seconds. In certain variations, the measuring the first change comprises measuring a polarization angle. In certain variations, the measuring the first change comprises measuring a first terminal voltage across the cell just after the starting time, measuring a second terminal voltage across the cell just before the ending time, and dividing a difference between the first and second terminal voltages by a time difference between the measurements. In certain variations, the first stored data includes two different stored capacity values corresponding to a single change in terminal voltage across the cell during the first time period, and the method further comprises measuring a quiescent voltage of the cell, and comparing the measured quiescent voltage to a predetermined threshold to distinguish between the two different stored capacity values that correspond to the single change in terminal voltage across the cell. In certain variations, the method further comprises measuring a quiescent voltage of the cell, and comparing the measured quiescent voltage to second stored data to determine the energy remaining in the cell. In certain variations, the method further comprises using the measured first change to determine the energy remaining in the cell during an earlier portion of a life of the cell, and using the measured quiescent voltage to determine the energy remaining in the cell during a later portion of the life of the cell.

In certain examples, this document describes a method. The method comprises drawing a substantially constant first current pulse from an energy cell during a first time period, measuring a first change in a terminal voltage across the cell during the first time period, drawing a substantially constant second current pulse from the cell during a different second time period, measuring a second change in the terminal voltage across the cell during the second time period, and comparing the measured second change to first stored data to determine an energy remaining in the cell, including comparing the first and second changes to distinguish between two different stored capacity values that correspond to a single change in the terminal voltage across the cell.

In certain variations, the drawing the second current pulse includes drawing the second current pulse of a like magnitude and duration as the first current pulse. In certain variations, the drawing the first current pulse from the cell comprises drawing the first current pulse from a manganese dioxide battery. In certain variations, the drawing the first current pulse from the cell comprises drawing the first current pulse from a silver vanadium oxide battery. In certain variations, the measuring first and second changes comprises measuring a polarization angle. In certain variations, the method comprises measuring a quiescent voltage of the cell, and comparing the measured quiescent voltage to stored quiescent voltage data to determine the energy remaining in the cell. In certain variations, the method comprises using the measured change to determine the energy remaining in the cell during an earlier portion of a life of the cell, and using the measured quiescent voltage to determine the energy remaining in the cell during a later portion of the life of the cell.

In certain examples, this document describes a system. The system comprises: an energy storage cell; a current source/sink circuit, coupled to the cell, to draw a substantially constant first current pulse; a voltage measurement circuit, coupled to the cell, to measure first and second voltages during the first current pulse; a difference circuit, coupled to the voltage measurement circuit, to compute a difference between the first and second voltages; and a processor circuit, coupled to the difference circuit, the processor circuit including a memory circuit to store first data relating cell capacity to the difference between the first and second voltages, the memory circuit also including a cell capacity indicator storage location to provide an indication of cell capacity, the processor configured to use the difference between the first and second voltages obtained from the difference circuit and the stored first data indicative of cell capacity to provide the indication of cell capacity.

In certain variations, the energy storage cell comprises a manganese dioxide battery cell. In certain variations, the energy storage cell comprises a silver vanadium oxide cell. In certain variations, the voltage measurement circuit is also configured to measure a quiescent voltage. In certain variations, the processor is configured to compare the measured quiescent voltage to a predetermined threshold to distinguish between two different stored cell capacity values that correspond to a single difference in terminal voltage across the cell. In certain variations, the memory circuit is also configured to store second data relating cell capacity to the quiescent voltage, and in which the processor is configured to compare the measured quiescent voltage to the second data to determine the energy remaining in the cell. In certain variations, the processor is configured to determine the energy remaining in the cell using the difference, during an earlier portion of a life of the cell, and using the measured quiescent voltage, during the later portion of a life of the cell. In certain variations, the processor is configured to compare first and second differences to distinguish between two different stored first data values that correspond to a single stored difference. In certain variations, the processor is located within an implantable medical device. In certain variations, the processor is located within an external remote interface device.

DETAILED DESCRIPTION

FIG. 1is a schematic diagram illustrating generally, by way of example, but not by way of limitation, one example of a system100. In this example, the system100includes an implantable device102and an external remote interface circuit104. In the illustrated example, the implantable device102represents a cardiac rhythm management (CRM) device, and the external remote interface circuit104represents a remote programmer device. The device102includes an energy storage cell, such as a battery106. The battery106provides energy to load circuits108. For an implantable CRM device, such load circuits108typically include, among other things, analog circuits, a digital microprocessor circuit, a memory circuit, pacing therapy circuits, and defibrillation therapy circuits. The load circuits108typically draw a relatively stable quiescent current from the battery106. One exception, however, is a charging circuit for occasionally charging one or more defibrillation energy storage capacitors to a high voltage. This stored high voltage is used for subsequently delivering a defibrillation shock to a subject. During such occasional operation, the high voltage charging circuit typically adds a substantial additional load current beyond the background quiescent current drawn by the load circuits108.

Because the battery106typically has a finite energy storage capacity, there is a need to obtain an indication of how much stored energy remains in the battery106. This battery status information is useful to a physician or other caregiver, such as for determining when the battery106is depleted enough to require replacing the battery106(or, more typically, replacing the entire implantable device102). This battery status information is also useful for other components of the implantable device102. For example, near the end of the useful life of the battery106, it may be desirable to automatically turn off one or more “nonessential” circuits to conserve energy. This preserves and prolongs the ability of other more “critical” circuits to provide therapy to the subject.

In certain circumstances, the terminal voltage (across the battery terminals110A–B) during quiescent current draw does not vary appreciably over a significant portion of the useful life of the battery106. This is true, for example, during a significant portion of the beginning of the useful life of the battery106, where the battery106includes a manganese dioxide (MnO2) battery chemistry.

The system100provides, among other things, devices and methods for determining the battery status, that is, the remaining energy in the battery106. This includes a pulsed constant current source/sink circuit112, which is connected across the battery terminals110A–B. A voltage detector circuit is also connected across the battery terminals110A–B, such as for measuring a battery terminal voltage one or more times during a constant current pulse drawn from the battery106by the current source/sink circuit112. The voltage detector circuit114includes at least one output coupled, at node/bus116, to at least one input of an analog-to-digital (A/D) converter circuit118. The voltage detector circuit114outputs a voltage measurement of the battery terminal voltage. The A/D converter circuit118receives and digitizes this voltage measurement. At least one output of the A/D converter circuit118is coupled, at node/bus120to at least one input of a microprocessor, controller, or other processor circuit122. The processor122includes stored executable instructions124, such as for performing various operations and issuing control signals to other circuits of the implantable device102.

The processor122includes a difference circuit126, which includes at least one input that is coupled, at the node/bus120, to the at least one output of the A/D converter circuit118to receive the digitized battery terminal voltage measurements. The difference circuit126calculates a difference between first and second voltage measurements taken (separated by a known time interval, Δt) during the constant current pulse drawn from the battery106by the current source/sink circuit112. In one example, this difference is expressed as a “polarization angle,” as discussed below. In another example, this difference is expressed as a “slope” or “rate of change,” as discussed below. The processor122includes an onboard or separate memory128. The memory128includes stored data130. The stored data130is representative of remaining battery energy as a function of the difference between the first and second voltage measurements taken during the constant current pulse drawn from the battery106(or, alternatively, as a function of the “polarization angle,” the “slope,” or “rate of change”). As discussed below, the processor122executes instructions that use the difference to look up the remaining battery energy. The memory128includes at least one storage location132for storing an indicator of the remaining battery energy. In one example, the implantable device102further includes a telemetry or other communication transceiver circuit134. The transceiver134includes at least one input that is coupled to at least one output of the processor122, such as at a communications node/bus136. The transceiver134transmits information indicative of the remaining energy indicator in the storage location132to the external remote interface circuit104. Among other things, this informs the physician or other caregiver of how much useful life remains in the battery106before replacement of the battery106(or the device102) is needed.

FIG. 1illustrates an example in which the battery status determination is made within the implantable device102, with the result communicated to the external remote interface circuit104. In an alternate example, however, the processor includes instructions to control the obtaining of the first and second voltage measurements, and these first and second voltage measurements (or, alternatively, the difference between these first and second voltage measurements) are communicated by the transceiver134to the external remote interface circuit104. In this example, the external remote interface circuit104includes instructions for performing the necessary computations for determining battery status.

FIG. 2is a flow chart illustrating generally, by way of example, but not by way of limitation, one example of a method of determining remaining battery capacity, such as for a MnO2battery for which the battery terminal voltage does not vary appreciably during quiescent current conditions—particularly during the beginning portion of the useful life of the battery106. In the example ofFIG. 2, at200, the constant current source/sink112is turned on to draw a substantially constant current having an amplitude (in addition to the quiescent/background current drawn by the load circuits108) of approximately between 2 amperes and 4 amperes, such as about 3 amperes. In one example, this constant current pulse is of a fixed predetermined duration that is approximately between 3 seconds and 30 seconds, such as about 6 seconds. At202, first and second voltage measurements (separated by the known time interval, Δt) are obtained, such as by the voltage detector circuit114, during the constant current pulse. In one example, the first voltage measurement is obtained just after the constant current pulse commences (e.g., after any initial turn-on transients stabilize), and the second voltage measurement is obtained just before the constant current pulse ceases. At204, an indication of the difference between the first and second voltage measurements is computed, such as by the difference circuit126. This indication of the difference may, but need not, be expressed as a slope or rate of change of the battery terminal voltage during the constant current pulse, or as a “polarization angle,” as discussed below. At206, the indication of the difference is used to compute the remaining energy of the battery106. At208, an indication of the remaining energy of the battery106is stored in the memory location132. At210, information indicative of the remaining energy of the battery106is communicated from the implantable device102, such as from the transceiver134to the external remote interface circuit104, to be displayed to a physician or other caregiver.

FIG. 3is a current vs. time graph illustrating generally one example of a substantially constant current pulse drawn from the battery106, such as by the current source/sink circuit112. The current pulse is turned on at time t1, and reaches its full amplitude Ioby time t2. This current amplitude Iois in addition to any quiescent/background current being drawn from the battery106, such as by the load circuits108. The amplitude remains stable at Ioat time t3, just before the current pulse is turned off at t4. The amplitude returns to zero at time t5.

FIG. 4is a corresponding voltage vs. time graph illustrating generally one example of a battery terminal voltage signal during the substantially constant current pulse ofFIG. 3. Initially, the battery terminal voltage is at a background or quiescent voltage VQ. At time t1, when the current pulse is turned on, the battery terminal voltage begins to drop until it reaches the first loaded voltage V1Bat time t2. During the substantially constant current pulse, the battery terminal voltage continues to drop slightly until it reaches the second loaded voltage V2Bat the time t4. At that time, the substantially constant current pulse is turned off, and the battery terminal voltage returns to the quiescent voltage VQat time t5.FIG. 4illustrates a conceptual example for which the battery106is near the beginning of its useful life.

FIG. 5is a voltage vs. time graph, similar toFIG. 4, but illustrating a conceptual example for which the battery106is near the middle of its useful life. ComparingFIGS. 4–5, the battery terminal voltage drops, between times t2and t4, more quickly when the battery106is near the beginning of its useful life (seeFIG. 4) than when the battery106is near the middle of its useful life (seeFIG. 5). This rate of change, or “slope,” therefore, provides a useful indicator of the energy remaining in the battery106. The slope can alternatively be expressed as a polarization angle θ, as illustrated inFIGS. 4–5. The polarization angle θ=tan−1(Δt/Δv), where Δt is a time difference and Δv is a corresponding voltage difference.FIGS. 4–5illustrate θ1(for the beginning of the useful life of the battery106) as being less than θ2(for the middle of the useful life of the battery106.

FIG. 6is a conceptualized voltage vs. time graph of quiescent battery terminal voltage vs. depth of discharge of a MnO2battery106. As seen inFIG. 6, the quiescent battery terminal voltage does not vary appreciably during a beginning portion of the MnO2battery life, when the battery is relatively full of stored energy. This makes quiescent battery terminal voltage difficult to use for determining the battery's stored energy status during the beginning portion of the MnO2battery's useful life.

FIG. 7is a conceptualized graph of a polarization angle (i.e., 90−θ), representing the slope of the battery terminal voltage during the constant current pulse, such as between times t2and t3or between times t2and t4. As seen inFIG. 7, the polarization angle changes significantly during the beginning portion of the MnO2battery life, when the battery is relatively full of stored energy. This makes such slope or polarization angle a useful indicator for determining the battery's stored energy status, particularly during the beginning portion of the MnO2battery's useful life. By measuring the battery terminal voltage at two times during the constant current pulse, taking the difference between these measurements, and dividing by the time difference, At, between these two battery terminal voltage measurements, the angle θ is obtained. The polarization angle quantity (90−θ), where θ is expressed in degrees, is compared to a lookup table or equation fit (from data previously obtained from similar batteries) of remaining battery energy vs. (90−θ). This permits the remaining battery energy to be determined.

However, as seen inFIG. 7, the “bathtub” shaped curve of polarization angle vs. depth of discharge maps one value of the polarization angle to two different values of remaining battery energy. The present inventors have recognized several techniques for overcoming this potentially confounding factor. First, the polarization angle can be used to determine remaining battery energy during a beginning portion of the battery's useful life, then switching to use the quiescent battery terminal voltage to determine remaining battery energy during a later portion of the battery life, as discussed below with respect toFIG. 8. Second, the curve ofFIG. 7can be divided up into a beginning of life segment and an end of life segment, and the polarization angle or remaining energy can be used to switch between the two segments, such as discussed below with respect toFIG. 9. Third, the curve ofFIG. 7can be divided up into the beginning of life and end of life segments, and a quiescent battery terminal voltage can be used to switch between the two segments, such as discussed below with respect toFIG. 10.

FIG. 8is a flow chart illustrating generally one example of a technique that uses both polarization angle (or the slope, or similar indication using the difference between the two battery terminal voltage measurements obtained during the constant current pulse) and quiescent battery terminal voltage (obtained at a time other than during the constant current pulse) for determining the remaining energy in the MnO2battery. At800, upon implantation of the device102, it is the beginning of the battery's useful life. At802, the polarization angle (or the slope, or similar indication using the difference between the two battery terminal voltage measurements obtained during the constant current pulse) is used to determine the remaining energy, such as discussed above with respect toFIG. 2. At804, the resulting remaining battery energy is compared to a threshold (e.g., corresponding to the minima of the curve ofFIG. 7). If the remaining energy exceeds the threshold, then process flow returns to802—remaining battery energy continues to be computed using polarization angle. However, if at804the remaining energy is less than the threshold, then subsequently, battery terminal voltage is measured under quiescent current conditions and used to determine remaining energy, at806. This includes comparing the measured quiescent battery terminal voltage to a lookup table of remaining energy vs. quiescent battery terminal voltage. As seen in the curve ofFIG. 6, during this portion of the battery life, quiescent battery terminal voltage changes more appreciably as a function of remaining battery energy. Therefore, quiescent battery terminal voltage provides a more useful indicator of remaining battery indicator during this latter portion of the battery's useful life than during the earlier portion of the battery's useful life, when the curve is relatively flat.

FIG. 9is a flow chart illustrating generally a technique that splits the curve ofFIG. 7into a beginning of life segment that precedes the minima, and an end of life portion that succeeds the minima, and that uses remaining energy to switch between the two segments. At900, when the device102is initially implanted, the MnO2battery is at the beginning of its useful life. At902, the polarization angle (or the slope, or similar indication using the difference between the two battery terminal voltage measurements obtained during the constant current pulse) is used to determine the remaining energy, such as discussed above with respect toFIG. 2, using the beginning of life segment of the curve ofFIG. 7. At904, the resulting remaining battery energy is compared to a threshold (e.g., corresponding to the minima of the curve ofFIG. 7). If the remaining energy exceeds the threshold, then process flow returns to902—remaining battery energy continues to be computed using polarization angle and the beginning of life segment of the curve ofFIG. 7. However, if at904the remaining energy is less than the threshold, then subsequently, remaining battery energy is subsequently computed at906using polarization angle and the end of life segment of the curve ofFIG. 7. At904, as an alternative to comparing remaining energy to a threshold, the corresponding polarization angle can be compared to a corresponding threshold (e.g., corresponding to the minima of the bathtub curve ofFIG. 7).

FIG. 10is a flow chart illustrating generally another technique that splits the curve ofFIG. 7into a beginning of life segment that precedes the minima, and an end of life portion that succeeds the minima, and that uses a determination of quiescent battery voltage to switch between the two segments. At1000, when the device102is initially implanted, the MnO2 battery is at the beginning of its useful life. At1002, the polarization angle (or the slope, or similar indication using the difference between the two battery terminal voltage measurements obtained during the constant current pulse) is obtained for determining the remaining energy, such as discussed above with respect toFIG. 2. At1004, a quiescent battery terminal voltage measurement is obtained. At1006, the quiescent battery terminal voltage is compared to a threshold value (e.g., corresponding to the minima of the curve ofFIG. 7). If the threshold quiescent battery terminal voltage measurement is greater than or equal to the threshold value, then the first (beginning of life) segment of the curve ofFIG. 7is used to determine remaining battery energy at1008. Otherwise, at1010, the second (end of life) segment of the curve ofFIG. 7is used to determine the remaining battery energy.

Although the above examples have been described for an example using an MnO2battery chemistry, the present devices and methods will also be useful for determining battery status for batteries of other chemistries such as, for example, a silver vanadium oxide (SVO) battery chemistry. For example, for a SVO battery chemistry, the above-described techniques using polarization angle to determine remaining battery life may require that the SVO battery not be discharged too quickly, thereby diminishing the polarization angle effect from which remaining battery life is determined. Also, although the above-described techniques are particularly useful for batteries where the quiescent battery terminal voltage does not vary appreciably over the usable life of the body, such techniques are not limited to use with such batteries, but can be used with other batteries as well.