They are not interchangeable as the 627 watt hour power tube is 70mm longer. I guess, in theory, one could drop a 500 watt unit in an enclosure made for the 625 but the battery needs to be held securely in place at the bottom where the electrical connecting tabs insert and make contact with the inner terminals of the battery. It it rattled and bounced during the ride, there would be intermittent disconnects.