old battery and new battery over heating in my LM2100 this just started I tried to find schematics but not available
Given that this "just started", and that it happens to both your batteries, in inclined to say the device is at fault. Are you noticing decreased cutting power? Check that everything spins freely (batteries removed!). If you feel extra resistance, there's a the problem. If it spins OK, could be some kind of internal short.