"No need to buy the expensive charger! I have successfully used both a 9 volt and a 12 volt power supply to recharge the battery. My research indicated that the battery pack contains an MP26123 monolithic DC-DC stepdown switching charger, so the exact voltage is not critical. Nine and twelve volt power bricks are very easy to find, and are also inexpensive. I wouldn't recommend going below 9 or above 12 volts."