Thermostat battery recharging during HVAC function active and inactive states

A thermostat for controlling one or more HVAC functions in an HVAC system the thermostat comprising:

• a rechargeable battery;
• charging circuitry adapted and arranged to recharge the battery;
• power harvesting circuitry adapted and arranged to harvest power from a circuit of an HVAC function during times when the HVAC function is active and during times when the HVAC function is inactive, and to supply power to the charging circuit for recharging the battery, wherein during the times when the HVAC function is active, the power harvesting circuitry repeatedly charges and discharges a capacitive element to store electrical energy for use by the charging circuitry by repeatedly opening and closing a solid-state switching circuit for activating the HVAC function, such that during times when the solid-state switching circuit is closed, the HVAC function continues uninterrupted while simultaneously charging the capacitive element;
• and
• control circuitry adapted and arranged to control the one or more HVAC functions using power from the rechargeable battery.