They're standard silver fish bstteries. You can get them from loads of Chinese suppliers. BMSBATTERY is pretty reliable. They give you the lock-rod and the base-plate too for guaranteed compatibility. You can even upgrade to a 12 ah or 15 ah one http://www.bmsbattery.com/36v/475-36v-15ah-lithium-ion-electric-bicycle-battery-pack.html They advertise it as "Included 10s polymer cells". Is there a good chance they're the Phylion prismatic stuff?
