Which position must the EMER EXT LT switch be in for the DC DCHED ESS Bus to charge the internal batteries?

The EMER EXT LT switch must be in the AUTO or OFF position for the DC DCHED ESS Bus to charge the internal batteries. In these settings, the emergency external lights are either automatically managed or turned off, which facilitates the charging process of the internal batteries when needed.

In the AUTO position, the system is designed to manage the power distribution and ensure that essential functions can still operate while allowing the bus to charge the batteries. Similarly, in the OFF position, the emergency lights do not draw power, thereby enabling the internal batteries to receive the necessary charge without interference from the emergency lighting system.

The ARM position is generally involved in preparing systems for activation rather than charging batteries. The MANUAL position typically indicates that systems are set for manual override, which may not allow the desired charging function. The ON position would activate the emergency lights, thereby preventing the bus from charging the internal batteries since power is being diverted to operate the lights.