Find your fuse panel. Take a picture of it with your cell telephone. With the car off, with a volt meter, remove the fuses one at a time. Measure every slot to find out what is hot and what is not. Any Hot circuits should be examined closely to find out what is ON when the car is OFF.
Places to look ... light in the trunk??? is it on or off with the lid closed? Under the hood? Same. Do you have a radio that comes on with the key off??? GPS, you leave plugged in? Cell phone or other gadgetry you may have plugged in to the aux power supply? Any other lights? Parking lights for example? Security system? CB radio?
After you have checked these items, and if you find nothing, you may want to have your alternator examined. If you have a shorted diode in the alternator, the power may be draining back to ground through this shorted diode. You could perform this test yourself by unplugging or disconnecting the alternator during the periods when you are not driving the car. If this is the trouble, on day three, when you reconnect the alternator, two things you will note: 1) there will be a pretty big spark & 2) you will still have charge in the battery (evidenced by the spark)(there will be some spark no matter what ... but a big spark will mean a big drain). I have seen shorted diode problems fixed by inserting a toggle switch in the alternator power circuit. The alternator will still charge the battery when the switch is ON with the car running. When you stop the car, turn the switch off, power will not drain through the alternator to ground. Probably not the solution for your Mercedes. The real solution is to replace the alternator.
BTW - do you have a security system in your car? A thief proof radio? Check your owner's manual for precautions you should take before disconnecting your battery with regard to the anti theft features of your vehicle.
You say your battery is new and dead after three days. If fully charged on day one, and dead on day three, that means your battery has expended between 500 (economy battery) and 1000 (premimum battery) "amp hours" of power (500 amps / 72 hours = about 7 amps per hour going somewhere. (An 'amp hour' is one amp for one hour) Ohm's Law applies.
I hope you find your trouble and I hope I have given you a few leads. Thanks for allowing me to assist you with this problem this morning ... good luck.
Thanks for using FixYa.com today.
Your battery are okay but you may have an alternator/generator problem, it's not charging back electricity to the battery. That's the common interpretation of that battery warning light on the dash...
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