The M12 amplifier can use either 2 "AA" Batteries or AC Power Supply's - (sold separately)
The battery compartments of the M12 amplifiers are located on the left side of the units. you should see a U shaped door. To remove this battery door, simply apply pressure while pushing down. The door will slide off of the amplifier.
Please note: The life of the AA battery will vary depending on how often the base unit is used.