A method and an apparatus are disclosed for extending the service life of a battery source by fully utilizing the energy in batteries and/or providing higher energy reserve using multiple batteries. An universal battery bank consists of an ensemble of possibly heterogenous batteries all connected in series to provide a usable operational energy source. For a universal battery bank consists on N batteries, the minimum operational voltage for the individual batteries (Vmin) can be lowered to Vmin/N on average, thereby extending the time interval between battery replacements and draining the individual batteries more efficiently. To exploit this power supply that may vary over a wide voltage range, electronics are designed to monitor the individual batteries, guard against excessive voltage and current, and provide constant current sources and/or constant voltage sources for various appliances.