The battery is at the bottom of the bike, in front of the rear wheel. Remove two bolts, and the bottom swings open. Disconnect the battery cables first, and you will have a lot more room if you roll the rear tire up on a 2x4 so the box swings all the way open without hitting the ground.
When you put a new battery in, use super glue to hold the nuts on the back of the terminals, and use a magnetic screwdriver to start the bolts once the battery is back in. Use lock washers or star washers under the bolts to keep them from vibrating loose.
