Hello!
My batterry drained few times, with or without my 'approval'
I would like to discuss the scenarios and if anybody has an idea how to prevent the bad drain.
To make a long story short, twice my battery drained without me wanting. So, I've researched the forums and over the net and got pointed to my leaving the car locked and the keyfob left withing the distance of lock/unlock the car.
That is, the ECU was talking to my keyfob so much that drained the battery. Hard to swallow reason so I've recharged the battery and moved the keyfob further so ECU won't reach it. No more drain.
When put back within the ECU reach, bingo...drained again.
Question: Do I miss any car setup of sorts so I don't need always to put the keyfob faraway from the car?
Thanks.

 

Yes your car will have ECU activity when the fob in the car. And yes its enough to drain the battery to the point that the ECU will never go to sleep so it will disconnect the battery due to low voltage. Its supposed to save enough energy to start the car but this rarely works. Dont keep the FOB near the car and your problems solved.

 

Thanks Robert.
One note on this subject: If I double tap the lock on keyfob and receive double chirp/flashlights from the car, then it seems ECU isn't looking for keyfob, so isn't draining battery.
Surprised to see such a marvel of engineering company doesn't have a software patch like for this issue

 

Yes that is odd but at least you found a temporary fix. I asked GROk and here is the answer to your question.

Quick Workarounds (No Tools Needed)
The easiest and most popular fix among W204 owners is to temporarily disable the passive/keyless reception on the fob when you're home/parking for extended periods:

Double-press the lock button quickly right after locking the car (or even when you're just storing it nearby).
Many owners report this puts the fob into a "sleep" or deactivated state where it stops responding to the car's proximity pings until you press any button again (like unlock). You'll usually see the hazard lights flash twice or hear a confirmation beep/honk to indicate it's deactivated.
This is the same double-press you already use for the horn honk on lock—it often doubles as the deactivator for KEYLESS-GO functions in many Mercedes keys (including older W204 ones).
Test it: After double-pressing lock, leave the fob close to the car for a day or two and see if the battery holds up better. Reactivate by pressing unlock or lock once.

If double-press doesn't fully stop the drain on your specific fob/year (some variations exist by market or software), try this sequence reported for similar models:
Hold the lock button down, then quickly press unlock twice while still holding lock. Release. This fully disables passive features on some keys (LED may flash to confirm).

These button tricks are free, reversible, and widely recommended on forums like MBWorld and Reddit for preventing exactly this proximity drain without killing convenience entirely.
Other Options

Store the fob farther away — You've already found this works perfectly (e.g., in a different room, metal box, or Faraday pouch/bag to block signals completely). This is the simplest long-term habit if the double-press doesn't cut it 100%.
Faraday pouch/bag — Cheap on Amazon (search "RFID Faraday pouch" or "key fob signal blocker"). Drop the fob in when at home—it blocks all signals, zero drain, and also protects against relay theft.
No permanent menu/setting disable exists in the W204 instrument cluster for KEYLESS-GO passive listening (unlike some newer models). Advanced coding with Mercedes STAR/XENTRY diagnostic tool (dealer or indie shop) might tweak antenna power or wake-up sensitivity, but it's not standard, can be hit-or-miss, and usually isn't worth it for this.
Rule out fob fault: If drain happens even when far away, the fob might have an internal issue (rare, but replace battery first with a quality Panasonic CR2025, not cheap generics).