What action should be avoided when the battery voltage is low?

Connecting external power is the action that should be avoided when the battery voltage is low because it can pose risks to both the aircraft and the electrical systems. When the battery is low, utilizing external power may lead to overloading the electrical systems, potentially causing damage to equipment or creating an unstable power supply. External power also can overwhelm a depleted battery, which might lead to complications such as reversed polarity or electrical surges.

It's critical to ensure that the aircraft's electrical systems are functioning properly before connecting external power. In situations where the battery is low, operating emergency power, hydraulic systems, or aircraft lights may still be feasible, albeit with caution, as these actions can be managed without introducing the complexities that external power connection entails.