See also:

Mercedes-Benz M-Class Owners Manual
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Charging the batter
WARNING   Never charge a battery still installed in the vehicle unless a battery charger unit approved by Mercedes-Benz is being used. Gases may escape during charging and cause explosio ...

Vehicle remote opening
If you have unintentionally locked your vehicle (e.g. the SmartKey is still in the vehicle) and a replacement SmartKey is not available, the vehicle can be opened by a Mercedes-Benz Customer A ...