Why does my w211 battery drains overnight

Hi @saniumarisa

What is the year model of the vehicle.

You have to find out which circuit in the car is draining the power from the battery.

One way to do this is to disconnect the -ve battery cable from the battery and connect a DMM (digital multimeter) (examples only) set to measure Amps i.e. an ammeter, between the -ve battery terminal and the battery cable to measure the current flow with the engine turned off.

Then if it is >50-80mA (average quiescent current discharge for most vehicles) you will need to remove each fuse from the engine fuse block and the instrument panel fuse block, one at a time and then check if the current falls back to the average value. Be sure that you replace each fuse before removing the next one when checking.

Once you know which fuse it is you will know which circuit it feeds power to which will help to find out what's wrong.