Rechargeable Wii Energy Pak for Wii Balance Board for $7.02.  Here is a screenshot for your reference. Shipping is free on orders over $25.  If needed, use this Filler Tool to pad the order to qualify for free shipping.  This rechargeable Lithium-Ion powered Energy Pack fits directly into the Balance Board's battery compartment, powering the Balance Board for up to 30 hours. Most places have it for about $19.