Volkswagen Golf GTI 2016, Owner'S Manual

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When the engine is started, the power supply to the 12 Volt
sockets is temporarily interrupted.
The onboard electrical system management cannot always keep
the battery from being drained. For example, the battery will drain if
the engine is not running, but the ignition is switched on or the
parking lights are left on for a long time when parked.
What drains the vehicle battery?
Long periods when the engine is not running, especially when the
ignition is on.
Using electrical systems or devices when the engine is switched
off.
Leaving the vehicle unlocked for several days when not in use.
The selector lever is left for a long period of time in any position
other than Park (P) when the ignition is switched off → Automatic
transmission: Selector lever .