September 6th, 2007

belize

Motorcycle won't start -- alternator? battery?

My motorcycle (BMW F650GS) won't start.

-- turn the key, no lights (maybe I left it on?)
-- if I jump it from my car, it'll turn on, and engine will start
-- once I disconnect jumper cables, engine dies

It's that last point which confuses me: once it's going (and I left it run 5+ minutes), why does it need the battery anymore? I don't see why disconnecting the battery immediately kills the engine.

Any suggestions?