I know that some earlier models of Yamaha synthesizers and keyboards had an onboard battery, but I don't think the newer models do-- although I don't know when Yamaha stopped using them. The manuals of newer models still have some sentences in the first few pages which say something like "This keyboard may have a battery in it," but I think that statement is left over from years ago when some models did have onboard batteries-- it was presumably just never edited out of the preliminary "warnings and warranties and disclosures" legalese that's given at the beginning of the manuals.
In short, if the service manual doesn't show the location of an onboard battery or list it in the parts listing, then there is none.
As far as "some weird things are happening," sometimes the electronic circuits inside a keyboard can start to get out of whack, producing signals that are no longer within the expected tolerance range, or they might get burned out completely. I'm no electrician or electronics engineer, so I might not be describing things correctly, but basically-- shorts do happen, circuits can start to run hot and act wonky, etc.
