A portable emergency vehicle battery charger has a portable case, a battery and a circuit board. The battery and circuit board are mounted in the portable case. The case mainly has a power out socket connected to the battery through the circuit board, a power input socket connected to the battery and a power supply switch connected between the circuit board and the battery. The power output socket is a cigar-lighter socket. When a vehicle battery does not have sufficient charge to start an engine, the driver uses a jumper cable connected between a cigar-lighter in the vehicle and the power output socket to charge the battery. Then the driver turns the power supply switch on to provide a constant current to the vehicle battery.