Patent ID: 7251527

Claim:
A system comprising: a lithium battery having an internal resistance; a charge storage capacitor electrically connected to the lithium battery; a first device electrically connected to the lithium battery and adapted to be powered by the battery; and at least one second device electrically connected to said charge storage capacitor, wherein the at least one second device is adapted to perform a charge time measurement, the at least one second device being adapted to: access a predetermined set of charge time measurement set-up parameters and a predetermined elective replacement time (ERT) charge time limit determined from a battery resistance ERT target that corresponds to a battery charge depletion target wherein the predetermined set of charge time measurement set-up parameters include: an initial reference voltage (VI) to begin the time measurement for determining the rate of charge storage in the capacitor, VI being larger than a discharge voltage for the capacitor (Vd); and a physician-programmed final voltage (VF) to end a time measurement for determining the rate of charge storage in the capacitor; determine a rate of charge storage in the capacitor when the capacitor is charged from VI to VF; compare the determined rate of charge storage in the capacitor to the predetermined ERT charge time limit for the predetermined set of charge time measurement set-up parameters; and declare an ERT based on a number of comparisons between the determined rate of charge storage in the capacitor and the ERT charge time limit.