A battery charger module to be mounted in a vehicle for charging a vehicle battery is disclosed. The battery charger module may include a capacitor, and a direct current (DC) voltage sensor configured to sense a DC voltage across the capacitor. The battery charger module may further include a controller configured to compare the sensed voltage to a voltage threshold, and to determine that an alternating current (AC) power supply is unstable if the sensed voltage fails to exceed the voltage threshold. A method is also disclosed for determining a state of an AC electrical power supply at a battery charger module for charging a vehicle battery. The method may include sensing a DC voltage across a capacitor, comparing the sensed voltage to a voltage threshold, and determining that the AC power supply is unstable when the sensed voltage fails to exceed the voltage threshold.