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In a method and apparatus for determining the depleted capacity of a CFx 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 tim

In a method and apparatus for determining the depleted capacity of a CFx 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.

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The invention claimed is: 1. A method for determining depleted capacity of a CFx type battery in an implantable medical device, comprising the steps of: measuring an average value of battery voltage and an average value of battery current drawn from a CFx type battery during a measurement time havi

The invention claimed is: 1. A method for determining depleted capacity of a CFx type battery in an implantable medical device, comprising the steps of: measuring an average value of battery voltage and an average value of battery current drawn from a CFx type battery during a measurement time having a length exceeding a battery voltage recovery time of said CFx type battery after a load change; and determining actual depleted capacity of said CFx type battery from predetermined relationships between combinations of said average values of voltage and current and values of depleted capacity. 2. A method as claimed in claim 1 comprising measuring said average voltage and average current during a measurement time which exceeds said battery recovery time by a predetermined factor. 3. A method as claimed in claim 2 wherein said factor is between 5 and 10. 4. A method as claimed in claim 1 comprising measuring said average voltage and said average current from a CFx type battery having a battery recovery time in a range between 3 and 50 hours. 5. A method as claimed in claim 1 comprising measuring said average voltage and said average current for a measurement time exceeding 24 hours. 6. A method as claimed in claim 1 wherein the step of determining said actual depleted capacity comprises storing predetermined combinations of average voltage and average current, with respect to depleted battery capacity, in a look-up table, and consulting said look-up table to determine a depleted battery capacity therein corresponding to said combinations of said measured average voltage and said measured average current. 7. A method as claimed in claim 1 further comprising integrating current drawn from said CFx type battery over time, thereby obtaining an integrated current value, and also determining said actual depleted battery capacity from a relationship between depleted battery capacity and said integrated current value. 8. A method as claimed in claim 7 comprising triggering an alarm if a difference between the actual depleted battery capacity determined from said combination of average battery voltage and average battery current, and the actual depleted battery capacity determined from said integrated current value, exceeds a predetermined threshold value. 9. An apparatus for determining depleted capacity of a CF x type battery in an implantable medical device, comprising: an averaging unit adapted for connection to a CFx type battery for measuring an average value of voltage drawn from said CFx type battery and an average value of current drawn from said CFx type battery during a measurement time having a length exceeding a battery voltage recovery time of said CFx type battery after a load change; and a determining unit connected to said averaging unit for determining actual depleted battery capacity of said CFx type battery from predetermined relationships between combinations of said average values of voltage and current and depleted battery capacity. 10. An apparatus as claimed in claim 9 wherein said determining unit includes a look-up table wherein predetermined relationships between combinations of average battery voltage and average battery current and depleted battery capacity are stored, and wherein said determining unit consults said look-up table to determine a depleted battery capacity corresponding to said combinations of the measured average voltage and the measured average current. 11. An apparatus as claimed in claim 9 wherein said averaging unit determines said average value of battery voltage by sampling an integrating said battery voltage during said measurement time, and determines said average value of battery current by sampling and integrating said battery current during said measurement time. 12. An apparatus as claimed in claim 9 wherein said determining unit includes an integrating unit which integrates a total current drawn from said CFx type battery during said measurement time, to determine a total current depleted from said battery, and a second determining unit which also determine said actual depleted battery capacity from a relationship to said total current. 13. An apparatus as claimed in claim 12 comprising a triggering unit which triggers an alarm if a difference between said actual depleted battery capacity determined from said combinations of average values of voltage and current, and said actual depleted battery capacity determined from said total current, exceeds a predetermined threshold value. 14. An apparatus as claimed in claim 9 further comprising an impedance measurement unit which measures internal battery impedance of said CFx type battery when said actual depleted battery capacity reaches a predetermined threshold value determined from said measured average values of battery voltage and battery current, and further comprising a second determining unit which thereafter determines said actual depleted battery capacity from a relationship between depleted battery capacity and said measured internal impedance. 15. An apparatus as claimed in claim 14 further comprising a triggering unit which triggers an alarm if a difference between said actual depleted battery capacity determined from said combination of average values of battery voltage and battery current, and said actual depleted battery capacity measured integral impedance exceeds a predetermined threshold value.