The Clarity battery life is officialy is 2-3 hours.
Airplan Flight Equipment (who I bought mine from) are doing it with a free external power pack at the moment.
Airbox Freedom Clarity : Airbox - Airplan Flight Equipment
It holds 2x 'AA' rechargeable batteries (supplied) and comes with adaptors to charge USB devices, phones and ipod. Extends battery life on the Clarity unit to approx 8 hours. The power pack itself is charged from a USB port.