The battery charger is used only without an Uninterruptible Power Supply (UPS). The charger contains:
A DC-power relay to switch the batteries into the power circuit if a main power failure is detected
The circuit breaker protects the AC input power to the cabinet and serves as the main AC input disconnect switch.
If a problem develops, the circuit breaker automatically trips (opens) and removes the AC power input.
The batteries trickle-charge from the battery charger.
When AC power restores after an outage, the battery charger converts a 120 VAC input to a DC voltage that recharges the batteries (usually within 24 hours).