In an exemplary embodiment, a battery conditioning system monitors battery conditioning and includes a memory for storing data based thereon; for example, data may be stored representative of available battery capacity as measured during a deep discharge cycle. With a microprocessor monitoring battery operation of a portable unit, a measure of remaining battery capacity can be calculated and displayed. Where the microprocessor is permanently secured to the battery so as to receive operating power therefrom during storage and handling, the performance of a given battery in actual use can be accurately judged since the battery system can itself maintain a count of accumulated hours of use and other relevant parameters. Where an embodiment includes a main battery and a backup battery, the voltage of each may be individually measured, and each may be conditioned. Battery capacity of each may be measured. In a unique low voltage sensing circuit applicable to power sources generally, an automatic voltage regulator may itself sense a low power condition without requiring additional precision components. As an example only, such detected low power condition may be utilized to effect switch over to a backup battery or other auxiliary power supply.