Finally there is an environmentally friendly alternative to the Invisible Fence Power Cap. The Impi EcoCap represents a break-through in battery chemical composition enabling the creation of long life, high capacity battery to operate in all Invisible Fence brand collar receivers including the R21, R22, R51 and Mircolite.The typical Invisible Fence brand collar will use 4 Power Caps per year. If purchased individually at a cost of $19 for a single Invisible Fence brand battery that"s $80 per year, per dog. The Impi EcoCap charging station comes with one EcoCap and a charging base capable of charging 2 EcoCaps. Depending on how frequently the dog tests the fence the EcoCap will operate for up to 20 days on a 4 hour charge and has been fully tested to cycle upwards of 500 times. At this rate the EcoCap will power an Invisible Fence collar receiver for up to 20 years!