Systems and methods used to reserve a constant battery capacity

An implantable pulse generator for generating electrical pulses for stimulating tissue of a patient, the implantable pulse generator comprising:

• pulse generating circuitry for generating electrical pulses;
• communication circuitry for communicating with an external controller
• a rechargeable battery for powering the implantable pulse generator, the rechargeable battery subject to capacity degradation when a voltage of the rechargeable battery falls below a first voltage level;
• charging circuitry for recharging the rechargeable battery by converting RF power into charging current;
• circuitry for monitoring one or more states of the rechargeable battery;
• control circuitry for controlling the implantable pulse generator, the control circuitry controlling the pulse generating circuitry according to one or more parameters;
• wherein the control circuitry, using the circuitry for monitoring, calculates a reserve threshold value indicative of an amount of time remaining until the voltage of the rechargeable battery will reach the first voltage level, the control circuitry calculating the threshold value, in part, by compensating for loss of capacity of the rechargeable battery to maintain a substantially constant reserve over a life of the implantable pulse generator;
• wherein the control circuitry communicates, using the communication circuitry, a signal to the external controller indicating the rechargeable battery should be recharged when the calculated reserve threshold value falls below a predetermined amount.