My first guess is the battery connector is either damaged or has become slightly unsoldered. I would touch it up with some new solder first. If its not that then its probably circuitry related to whatever switches from battery power to ac power. Maybe a loose connection there, but re-soldering that is flirting with disaster if you ask me. I dunno maybe find one with a broken lens and replace the motherboard if you are feeling up to it, good luck