Methods and apparatus for monitoring battery charge depletion

1. A system comprising:

an implantable battery;

an implantable medical device powered by the battery, the device having first and second current drain states; and

a first resistance connected to the battery through a first current path when the device is in the first current drain state, wherein current is prevented from flowing through the first current path when the device is in the second current drain state;

a second resistance connected to the battery through a second current path when the device is in the second current drain state, wherein current is prevented from flowing through the second current path when the device is in the first current drain state;

a first memory register configured to store a cumulative first current drain state count indicative of total charge drawn from the battery when the device is in the first current drain state; and

a second memory register configured to store a cumulative second current drain state count indicative of a total charge drawn from the battery when the device is in the second current drain state.

2. The system of claim 1 , wherein:

the system comprises a voltage-controlled oscillator (VCO) having an input, an output and a VCO signal at the output having a frequency that is proportional to a signal applied to the input; and

the first and second resistances are electrically coupled to the input;

a first voltage is generated at the input responsive to a first current passing through the first resistance along the first current path when the device is in the first current drain state; and

a second voltage is generated at the input responsive to a second current passing through the second resistance along the second current path when the device is in the second current drain state.

3. The system of claim 2 , comprising a counter at the output of the VCO, wherein:

the counter is configured to count the VCO signals;

generate a first current drain count while the device is in the first current drain state; and

generate a second current drain count while the device is in the second current drain state.

4. The system of claim 3 , wherein:

the first memory register updates the cumulative first current drain state count responsive to the first current drain count generated by the counter; and

the second memory register updates the cumulative second current drain state count responsive to the second current drain count generated by the counter.

5. The system of claim 1 , further comprising:

a first switch in series with the first resistance and the first current path, the first switch having a closed state, in which the first current travels through the first resistance and the first current path, and an open state, in which current is prevented from traveling through the first resistance and the first current path; and

a second switch in series with the second resistance and the second current path, the second switch having a closed state, in which the second current travels through the second resistance and the second current path, and an open state, in which current is prevented from traveling through the second resistance and the second current path.

6. The system of claim 5 , further comprising switching circuitry coupled to the first and second switches, wherein the switching circuitry is configured to place the first switch in the closed state and the second switch in the open state when the device is in the first current drain state, and the switching circuitry is configured to place the first switch in the open state and the second switch in the closed state when the device is in the second current drain state.

7. The system of claim 1 , further comprising a telemetry transceiver adapted to transmit the cumulative first current drain state count and the cumulative second current drain state count.

8. A battery charge depletion monitor for monitoring an amount of charge delivered by a battery of an implantable medical device operable in at least first and second current drain states, the monitor comprising:

a voltage-controlled oscillator (VCO) having an input, an output and a VCO signal at the output having a frequency that is proportional to a signal applied to the input;

a counter coupled to the output and configured to count the VCO signals;

a first resistance coupled to the input;

a first switch coupled to the battery and the first resistance, the first switch having a closed state, in which current can travel from the battery through the first resistance, and an open state, in which current is prevented from traveling through the first resistance;

a second resistance coupled to the input; and

a second switch coupled to the battery and the second resistance, the second switch having a closed state, in which current can travel from the battery through the second resistance, and an open state, in which current is prevented from traveling through the second resistance.

9. The monitor of claim 8 , further comprising switching circuitry coupled to the first and second switches, wherein the switching circuitry is configured to place the first switch in the closed state and the second switch in the open state when the device is operated in the first current drain state, and the switching circuitry is configured to place the first switch in the open state and the second switch in the closed state when the device is in the second current drain state.

10. The monitor of claim 8 , further comprising:

a first memory register storing a cumulative first current drain state count indicative of total charge drawn from the battery when the device is in the first current drain state; and

a second memory register storing a cumulative second current drain state count indicative of a total charge drawn from the battery when the device is in the second current drain state.

11. A method of monitoring an amount of charge drawn from a battery of a device operable in at least first and second current drain states comprising:

operating the device in the first current drain state;

during operating the device in the first current drain state:

connecting a first resistance in series with the battery;

conducting a first current through the first resistance responsive to connecting a first resistance; and

generating a first current drain state count responsive to the first current;

operating the device in the second current drain state; and

during operating the device in the second current drain state:

connecting a second resistance in series with the battery;

conducting a second current through the second resistance responsive to connecting a second resistance; and

generating a second current drain state count responsive to the second current.

12. The method of claim 11 , further comprising:

summing the first current drain state count with a cumulative first current drain state count; and

summing the second current drain state count with a cumulative second current drain state count.

13. The method of claim 12 , further comprising calculating a total charge drawn from the battery as a function of the cumulative first current drain state count and the cumulative second current drain state count.

14. The method of claim 12 , further comprising wirelessly transmitting the cumulative first current drain state count and the second cumulative current drain state count.

15. The method of claim 12 , wherein summing the first current drain state count and summing the second current drain state count respectively occur responsive to transitioning the device from operating in the first current drain state to operating in the second current drain state, and from operating in the second current drain state to operating in the first current drain state.

16. The method of claim 11 , wherein generating a first current drain state count responsive to the first current comprises:

applying a voltage generated at the first resistance responsive to the first current to an input of a voltage-controlled oscillator (VCO); and

counting pulses of a VCO signal generated at an output of the VCO.

17. The method of claim 16 , wherein generating a second current drain state count responsive to the second current comprises:

applying a voltage generated at the second resistance responsive to second first current to an input of a voltage-controlled oscillator (VCO); and

counting pulses of a VCO signal generated at an output of the VCO.

18. The method of claim 11 , further comprising calculating a total charge drawn from the battery as a function of the first and second current drain state counts and the first and second resistances.

19. The method of claim 11 , further comprising treating a medical condition of a patient while operating the device in at least one of the first and second current drain states.

20. The method of claim 11 , wherein:

preventing current from traveling through the second resistance during operating the device in the first current drain state; and

preventing current from traveling through the first resistance during operating the device in the second current drain state.