If the batteries are bulging after short usage, it sound to me like the voltage regulator has gone kaput. You can check this with a
DC voltmeter, use jumper cables off of a car to start the generator but with the car engine off so you don't read the car alternator. The voltage with the generator running should be about 13 to 14 volts, read across the jumper cables. If the reading is higher, the voltage regulator is probably shot. If the reading is lower, then the alternator on the generator engine has probably quit. Do the batteries boil themselves dry when they self- destruct?