See also:

Important notes
In order to prevent the vehicle's battery from discharging, the interior lighting functions are automatically deactivated after some time except for when the SmartKey is in position 2 in the i ...

Problems with the SmartKey
Problem Possible causes/consequences and Solutions You cannot lock or unlock the vehicle using the SmartKey. The SmartKey battery is discharged or nearly discharged. ...

Charging the battery
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 explosions th ...