The battery adapter system includes a housing having a body portion and a reversible cap adapted to screw on and off of the body portion. The body portion has an interior sized to accommodate the whole of a relatively short and wide 3-volt lithium battery or to partially accommodate the narrower, taller 1.5-volt AA battery. The reversible cap has an open end and an interior, and has outer threads that allow the cap to screw to the body portion in either of two orientations. When using the smaller and wider lithium battery, the cap is screwed onto the body portion in a first orientation that forms a first sealed housing interior that does not include the cap interior. When using the taller and thinner AA battery, the cap is screwed onto the body portion in a second orientation wherein the cap open end is first placed over the portion of the AA battery that protrudes from the body portion. This forms a second sealed housing interior that includes the cap interior. The housing is designed to provide an electrical connection between the battery housed therein and a voltage regulating circuit, which is adapted to provide a 3 volt DC output for an input voltage anywhere between 0.6 volt and 3 volts. The output voltage is suitable for powering an electrical and/or optical device such as a light-emitting device.