Finally there is an environmentally friendly alternative to the Invisible Fence Power Cap. The WiseWire Phoenix represents a break-through in battery chemical composition enabling the creation of a long life, high capacity battery to operate all Invisible Fence brand collar receivers including the R21, R22, R51 and Mircolite. The Wisewire Phoenix kit includes one rechargeable battery and a charging base capable of charging 2 batteries simultaneously. Depending on how frequently your dog challenges the fence, the Phoenix battery will operate up to 14 days on a 4 hour charge and has been fully tested to cycle upwards of 500 times. At this rate the Phoenix will power an Invisible Fence collar receiver for up to 20 years! The WiseWire Phoenix detects the condition of the battery and charges it with the optimum current, based upon temperature and voltage. Charging temperature range is 32-113 degrees F. The trickle charge feature prolongs battery life and keeps a full charge when left in the charger. The Green LED remains solid during trickle charge. Charging base LED indicator lights: LED is Red when charging base is ready. LED is flashing Red when battery is damaged and will not charge. LED is flashing Green when battery is charging. LED is solid Green when battery is fully charged. LED is flashing Yellow when it is time to recharge battery currently in use. Lasts up to 20 days on a 4 hour charge Recharges up to 500 times for a 20 year life expectancy LED indicator displays when battery is completely charged Smart charging system senses battery charge and only charges up to capacity before switching to a trickle charge.