The Mahle X35 SP1 Range extender battery provides 36V/208Wh of additional power bringing the total capacity to 450W for the X35 drive system. The Range extender includes a holder that mounts to the water bottle bosses on the frame. It charges the main battery while riding, and offers a parallel power supply to the motor when the power draw exceeds 2A. - 36V 208Wh Panasonic - BUS Compatible (Smart), Balanced operation as charger or independent battery - The Mahle charger will recharge both the main and range extender batteries at the same time