To provide a battery charger that can determine whether or not an internal combustion engine has successfully started up when it starts up by means of a timer and a reset circuit.

A battery charger that rectifies the output of a generator 12 connected to an engine 11 and uses the rectified output to drive an internal combustion engine and other devices than the internal combustion engine and charge a battery has a timer 31 charged by the generator 12 and the battery, a generator voltage detection circuit 32 for detecting the output voltage of the generator 12, and a timer reset circuit 33 that resets the charging operation of the timer when the output voltage of the AC generator voltage detection circuit 32 is equal to or less than a predetermined voltage.