One Energizer MAX 9 V alkaline battery (522 / 6LR61 size), the rectangular battery with snap terminals on top. It is the standard cell for smoke and gas detectors, guitar pedals and wireless microphones, multimeters, some toys and older alarm sensors. Energizer MAX batteries are single-use alkaline cells with Power Seal technology to hold the charge in storage; the pack states a five-year shelf life. Replace a smoke-alarm battery at least once a year, and test the alarm after fitting the new cell. Do not store loose 9 V batteries with keys or coins, as the terminals can short. Recycle used batteries at a drop-off point.