The PetSafe Passport Rechargeable Battery and Passport Key both work with the Passport Pet Access Smart System. The rechargeable battery can be used for full operation of the Passport or as a battery back-up option in case of a power loss. Worn on your pet's collar, the Passport Key uses radio-frequency identification (RFID) technology by using a unique code to signal the Passport to operate as programmed for your pet. The Passport Key Features: Does not require a battery to operate. Up to 20 Passport Key accessories can be programmed to the Passport The Passport rechargeable Battery Features: Use for full operation of the Passport or as a battery back-up option in case of power loss, indoor use only Passport Key Includes: 1 Passport Key, 2 Split key rings Passport Rechargeable Battery Includes: 1 Rechargeable lithium-ion battery (additional batteries sold separately), charging station and power adapter and operating guide Specifications for Rechargeable Battery: Lithium-ion Battery: 11.1 VDC / 2.2 Ah / 24.42 WhPower Adapter Input: 100 to 240 VAC / 50 Hz to 60 Hz / 0.8 A Max, Charger Output: 12.VDC / 1.1 ACharge Time: Less than 4 hours Dimensions for Passport Key: 4" W x 0.625" D x 5" H; 0.04 lbs. Dimensions for Passport Rechargeable Battery: 5.62" W x 2.72" D x 11.02" H; 1.485 lbs. Warning for Passport Rechargeable Battery: To reduce risk of injury, user must read operating guide. Only use designated charger to charge battery. Do not charge if cracked or damaged. Do not disassemble or throw into fire. Recycle battery properly. Warning for Passport Key: Keep small parts away from young children and pets who might mistakenly or intentionally swallow them. Small parts can be a choking hazard.