Page "Diode" Paragraph 124

Diodes will prevent currents in unintended directions.

To supply power to an electrical circuit during a power failure, the circuit can draw current from a battery.

An uninterruptible power supply may use diodes in this way to ensure that current is only drawn from the battery when necessary.

Likewise, small boats typically have two circuits each with their own battery / batteries: one used for engine starting ; one used for domestics.

Normally, both are charged from a single alternator, and a heavy-duty split-charge diode is used to prevent the higher-charge battery ( typically the engine battery ) from discharging through the lower-charge battery when the alternator is not running.