Abstract:
In a method and apparatus for determining the depleted capacity of a CF x  type battery used in an implantable medical device, average values of battery voltage and battery current drawn from the battery are measured during a measurement time, the length of which exceeds a battery voltage recovery time after a load change, and wherein the actual depleted capacity of the battery is determined by predetermined relations between combinations of the average values of voltage and current and depleted battery capacity.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and an apparatus for determining depleted capacity of a battery of CF x  type used in an implantable medical device. 
   2. Description of the Prior Art 
   So called CF x  (carbon monoFluoride) batteries offer the possibility to use fast microprocessors in implantable medical devices like pacemakers, since this type of battery has the capability of delivering current pulses in the milliampere range required by most suitable microprocessors. Further, there is a growing interest in multi-chamber pacing and also in high rate pacing for arrhytmia suppression and termination which also increases the need of the battery to deliver higher battery current. Future products will require high speed and long range telemetry, which also requires higher battery current. 
   However, the determination of the state of discharge or remaining capacity of this kind of battery currently causes considerable difficulties, since there is no single electrical quantity which is well correlated to remaining usable battery capacity. 
   The battery voltage exhibits very long time constants after load changes and as a consequence there is no useful relation between the instantaneous battery voltage and the state of discharge or remaining battery capacity unless the battery load is constant. Measuring the battery impedance is not useful either for this purpose, since it does not provide useful data during the whole discharge period but only in the latter part of the battery lifetime. Thus, known conventional methods of determining the remaining capacity of batteries used in implantable medical devices cannot be used for CF x  type of batteries. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a new technique for determining the status of CF x  type batteries, when used in implantable medical devices, especially implantable heart stimulators. 
   The above object is achieved in accordance with the principles of the present invention in a method and apparatus for determining the depleted capacity of a CF x  type battery used in an implantable medical device, wherein average values of battery voltage and battery current drawn from the battery are measured during a measurement time, the length of which exceeds a battery voltage recovery time after a load change, and wherein the actual depleted capacity of the battery is determined by predetermined relations between combinations of the average values of voltage and current and depleted battery capacity. 
   Thus if the battery voltage and the current drawn for the battery are averaged over a sufficiently long periods these average values can be used for determining the remaining capacity of the battery. The voltage and current are averaged over a measurement time exceeding the length of a battery voltage recovery time after a load change, the measurement time exceeding the recovery time preferably by a predetermined factor between 5 and 10. The recovery time can be defined as the time needed for the battery voltage to reach a certain percentage, e.g. 90%, of its steady state level. 
   In another embodiment of the method according to the invention, the average values of voltage and current are entered into a predetermined look-up table providing depleted battery capacity for different average voltage and current combinations. In practice such a look-up table is available from e.g. the battery manufacturer Wilson Greatbatch based on constant current data. Experiments have, however, shown that if the battery voltage and current are averaged over a sufficiently long measurement time combinations of the average voltage and average current values can be used for obtaining reliable values of depleted battery capacity from such a table. 
   In other embodiments of the method according to the invention depleted battery capacity is also determined by time integrating the total current drawn from the battery. This technique for determining depleted battery capacity is per se previously known when applied to other types of batteries for implantable medical devices, see e.g. U.S. Pat. No. 5,769,873. According to the invention an alarm is preferably triggered if the difference between depleted battery capacities, determined from measured average values of battery voltage and current and determined by time integration of the current drawn from the battery, respectively, exceeds a predetermined threshold value. The triggering of the alarm then indicates that the depleted battery capacity has to be further considered or investigated. 
   In a further embodiment of the apparatus according to the invention the averaging unit is adapted to determine the average values by sampling and integrating battery voltage and current during the measurement time. As discussed above the measurement time is in practice comparatively long, e.g. 24 h, and the sampling frequency is chosen high enough to get good accuracy of the average values, e.g. a sampling frequency of 256 Hz. With the use of an optional filter in front of an analog to digital converter, the sampling frequency can be reduced, e.g. to the range 0.1 to 1 Hz. 
   In further embodiments of an apparatus according to the invention an impedance measurement unit is provided to measure the internal battery impedance when depleted battery capacity reaches a predetermined threshold value and a second determining unit is provided to thereafter determine depleted battery capacity from the measured internal impedance. Internal impedance measurements give reliable values of the depleted battery capacity only in the fatter part of the battery lifetime. A first triggering unit is therefore preferably provided to trigger the impedance measurement unit when depleted battery capacity reaches the predetermined threshold value, determined from measured average values of battery voltage and current as described above. 
   In another embodiment of the apparatus according to the invention a second triggering unit triggers an alarm if the difference between depleted battery capacities, determined from measured average values of battery voltage and current, and determined by time integration of the current drawn from the battery or determined from the measured internal impedance, respectively, exceeds a predetermined threshold value. Thus, if there are discrepancies in the depleted battery capacities determined by the different methods, this is indicated to the patient and/or the physician so that further investigations can be made. In this way improved security with reference to the battery status is obtained. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plot of battery voltage and discharge capacity versus time obtained from measurements on a CF x  type battery. 
       FIG. 2  is a block diagram of an embodiment of the apparatus according to the invention. 
       FIG. 3  illustrates an example of a look-up table suitable for use for determining depleted battery capacity in accordance with the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a plot of battery voltage and discharged capacity versus time obtained from measurements on a CFx type battery. The battery was subjected to different load patterns in a 17 weeks test sequence. More precisely  FIG. 1  shows the results obtained for four different load patterns simulating various types of pacemaker loads. 
   Pattern 1 includes 3 hours of 10 μA load followed by 9 hours of 5 μA load, repeated 14 times, which gives a total time of 7 days. This pattern simulates a typical low current sequence with 3 hours of load threshold (Autocapture) single chamber pacing, followed by 9 hours of inhibition. 
   Pattern 2 includes a fixed load of 6.25 μA during 7 days, representing the average load of pattern 1. 
   Pattern 3 includes 3 hours of 100 μA load followed by 9 hours of 5 μA load, repeated 14 times, which gives a total time of 7 days. This simulates 3 hours of high threshold, multiple chambers pacing, followed by 9 hours of inhibition. 
   Pattern 4 includes a fixed load of 28.75 μA during 7 days, representing the average load of the pattern 3. 
   Between each week of loads simulating typical pacemaker loads according to patterns 1–4 above one week follows with a heavy load of approximately 900 μA in order to discharge the battery within a reasonably short time. In  FIG. 1  such cycles are shown repeated 8 times. In  FIG. 1  total depleted battery capacity is also showed as a function of time. 
   In  FIG. 1  can be seen that the dynamic impedance is high in the beginning of the battery lifetime and then successively decreases. It can also be seen that the recovery time increases with the depletion of the battery. Thus at the time of about 60 days in  FIG. 1  steady state is reached after a period of heavy load within a few days, whereas at time 100 days steady state is hardly reached within one week. 
   In  FIG. 2  an embodiment is shown of the apparatus according to the invention implemented in a pacemaker. 
   A shunt resistor Rs typically of 100 Ohm is connected to the battery  2  of CF x  type to be tested. This resistor Rs converts the current from the battery  2  to a voltage. The current drain from the battery  2  consists of the internal housekeeping current and the current used for therapeutic treatment, i.e. pacing pulses. 
   A voltage controlled oscillator (VCO)  6  converts the voltage across the resistor Rs to a pulse train with a frequency, which is proportional to the voltage. 
   The counter  8  counts the pulse train pulses from the VCO  6 . The count is read by the microprocessor  10  every 24 hours. Thereafter the counter  8  is reset by the microprocessor  10  and starts counting for another 24 hours period. 
   A stabilizing capacitor Cs typically of 47 μF is used for stabilizing the supply voltage during varying battery current loads. 
   An analog to digital converter (ADC)  12 , preceded by a RC-filter  14 , converts the battery voltage to a digital word. The microprocessor  10  controls the counter  8 , reads the ADC  12  and calculates remaining capacity of the battery  2  as will be further explained in the following. 
   The device circuitry  4  represents the complete normal circuitry of the pacemaker. 
   The average battery voltage is determined over a period of 24 hours. The battery voltage is sampled by the ADC  12 . The 24 hours average voltage is calculated by the microprocessor  10  by calculating the sum of all sampled digital values during 24 hours and then dividing this sum by the number of samples. The voltage is sampled with such a high frequency that good accuracy of the true average value is achieved, e.g. a sampling frequency of 1 Hz. 
   The average battery current is also calculated over a period of 24 hours. The current from the battery  2  is measured by measuring the voltage across the resistor Rs. The measured voltage is supplied to the VCO  6 , which is generating a pulse train with a frequency proportional to the measured voltage, and consequently proportional to the current. This digital signal with a varying frequency is supplied to the counter  8 . The counter value is read every 24 hours. The counter  8  is then immediately reset to be ready for counting during the following 24 hours period. 
   In the embodiment shown in  FIG. 2  a VCO  6  is used for current measurements and an ADC  12  is used for voltage measurements. As alternatives either VCOs or ADCs can be used for both current and voltage measurements. As another alternative the microprocessor can be replaced by hard-wired logic. 
   The 24 hours average voltage and current values are entered into a lookup table as shown in  FIG. 3  to obtain remaining battery capacity. 
   The table in  FIG. 3  is an example based on constant load current data available from the battery manufacturer Wilson Greatbatch. However, experiments have shown that corresponding tables are valid for variable loads when using voltage and current average values determined as described above. 
   The table in  FIG. 3  is used as follows. The average current for the last 24 hours is determined to e.g. 30 μA. The corresponding average voltage has been determined to e.g. 2.940 V. The 30 μA row is then followed in the table until the measured average voltage of 2.940 V is reached. This column in the table is followed to the top of the table where the depleted capacity can be read to 0.6 Ah. Interpolation is used to determine a value for the depleted capacity when one or both of the average voltage and the average load current values are in between the values in the table. 
   The look-up table is preferably stored in the memory of the microprocessor  10  and the described procedure is executed in automated fashion. The invention can then be used as a new advantageous RRT (Recommended Replacement Time) indicator for CFx type batteries. 
   Although modifications and changes may be suggested by those skilled in the art, it is the invention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.