A battery with low voltage can be charged by which method?

Using external power to charge a battery with low voltage is effective only when the external power source is in the ON position and the affected battery switch is also in the ON position. This configuration allows the aircraft's external power supply to provide the necessary energy to the electrical system, including the batteries. The external power is designed to maintain or recharge the batteries when required, ensuring that they have sufficient voltage for aircraft systems to function properly.

When external power is activated and the battery switch is ON, the electrons flow from the external power source, energizing the systems and charging the batteries. This method is widely utilized on the A320 because it ensures that the aircraft can maintain its electrical integrity and power essential components when the batteries are low.

Other approaches, such as having the battery switch OFF or using the APU, do not adequately allow for battery recharging by external power, which is why those methods would not be efficient or correct in recharging a low-voltage battery.