Abstract: A hybrid battery which provides back-up energy for a main power source to a load. The hybrid battery comprises a rechargeable nickel cadmium battery, a primary lithium battery and a power transistion circuit for controlling the supply selection to the load based on the highest voltage available. The hybrid battery further comprises a charge controller circuit for recharging the nickel cadmium battery from the main power source. Both the power transition circuit and the charge controller circuit isolate each of the energy sources from one another preventing draining whenever one or more of the power sources are off.