Mercedes-Benz GLC Owner's Manual page 445

Battery (vehicle)
442
Allow the battery to warm up gently first, if
necessary. Otherwise, the service life can be
shortened and the starting characteristics
impaired, especially at low temperatures.
PLUG-IN HYBRID vehicles: if the battery
charge is sufficient, the high-voltage battery
can also supply the 12 V battery with energy.
This only happens if the battery charge of the
12 V battery requires this, e.g. after using
electrical consumers for an extended period
with the engine switched off. As the on-board
voltage is continuously monitored this can
also be performed when the engine is
switched off. The battery charge of the 12 V
battery and the on-board voltage are thereby
kept stable for longer.