According to one aspect of the present invention, a battery charging apparatus, which is attachable/removable to/from an ink jet printer, and which electrically charges a secondary battery to supply electric power to the ink jet printer. The battery charging apparatus has a connector to supply the electric power from the battery to the ink jet printer, a power source voltage detection unit to determine an electric charging condition for electrically charging the battery, a serial communication unit to receive a battery charging control signal from the ink jet printer, and a microcomputer to control electric charging of the battery in correspondence with the received charging control signal and the electric charging condition.