Battery/alternator problem ????

[mercbnz300sd]

I have been having a problem with my car for about 3 months now, and I have no idea what is wrong with it. My battery died in June, so I took the battery to Interstate Battery (I still have a warranty), and they tested it and it was fine. It happened in July also. Same thing, the battery tested good. Then I took my car to a new mechanic (old mechanic was too expensive) and had the alternator replaced. The car was fine for a couple of weeks. A couple of days ago my car wouldn't start again. A friend of mine gave me a jump, and I took the battery back to Interstate Battery. They gave me a loaner while they tested mine. The battery (the loaner, mine was still at Interstate Battery) died by that night, and everytime I tried to start the car, it got weaker and weaker. I don't know what it could be, and I don't believe the Interstate Battery guys or the new mechanic. I have a 1983 300SD Turbo Diesel. Can anyone help? I am tired of wasting money, and getting things replaced that may not need to be replaced.

[blindside.AMG]

Hm, sounds like you have a parasitic draw on your battery. Meaning, something is drawing a small amount of current from your battery when the car is off. It happens a lot in the newer vehicles since they have so many control modules. I'm not sure what could be staying on in your '83 since I don't have much experience in older Mercedes. Do you have an aftermarket stereo? Those sometimes stay on, maybe an interior/exterior light you haven't noticed isn't shutting off.

I'm surprised the guys at Interstate haven't done a parasitic draw test on your car yet, or your mechanic for that matter. If you have any more information I'd be glad to help if I can.

[podunk]

try this at night, or in a dark garage, with the doors closed check out the interior lights and make sure none are on, the vanity light on the passenger side is a common culpret of this problem also sometimes it can be that the light in the trunk is staying on.