Battery dies after auto being idle for 2-3 days.

[spinedoc]

1996 SL500. I have had a very weird problem the last couple of months. If I leave my auto sitting without starting it for 2-3 days it will not start. If I leave it for a week or 2 then the battery will be completely dead.

I would assume the alternator is working because once I jump it and run it on the highway the car starts fine as long as I keep it started once a day. I can only assume something is drawing power, but am not sure what. The only aftermarket thing I have is my head unit and that has been installed for almost 2 years and I have never had any battery issues.

I even bought a new battery from the dealership and installed it, same thing. Any help would be appreciated.

[spinedoc]

I did check the voltage at the battery and with the car on it is 14.2 with all the accessories on (also 14.2 with radio and heater on). From what I read this is normal range for alt voltage. The battery is fully charged and brand new from the dealer.

[bart]

I had a similar problem with my 82 380SEL Mercedes......My problem and yours probably arent related however I noticed my car would not start after about three days of sitting idle. My car also had a viper anti theft system connected to the electrical system.......Turned out that this anti theft system was the culprit.....I disconnected all leads going to the viper box and problem was solved. I dunno if it was simply connected wrong or what, but anyway this fixed my problem.

[Kestas]

One way to diagnose this problem is to disconnect the battery, lay the cable next to it, and complete the circuit with an ammeter. The draw should be no more than 30-50 milliamps. I'll bet you get something around 500 ma. Disconnect the fuses one-by-one until you see a drop in current draw. That fuse will point you to the circuit that's at fault. Make sure the doors are closed and no lights are on in the car, such as in the glovebox or trunk.

[spinedoc]

Took it to the dealer today and they said the battery was kaput, I bought it from them about a month ago so they replaced it for free. They also found a broken/clogged pressure valve in the right rear that was causing moisture in the battery area, they said this might be causing my drain. The alt checked out OK.

Replacing the valve parts and labor was $395, I hope this solves the problem, but does not seem like a definite cause. Any thoughts?

[Jim H]

Batteries can have internal faults that discharge the battery and imitate an external load.

If your battery is discharging, and you can't find the sneak load, charge it up and disconnect it. If it then goes flat in a few days, it's no good, since there was no wiring connected to drain it.

Best Regards,
Jim

[lino]

Originally Posted by spinedoc

Took it to the dealer today and they said the battery was kaput, I bought it from them about a month ago so they replaced it for free. They also found a broken/clogged pressure valve in the right rear that was causing moisture in the battery area, they said this might be causing my drain. The alt checked out OK.

Replacing the valve parts and labor was $395, I hope this solves the problem, but does not seem like a definite cause. Any thoughts?

Did you end up finding out what the problem is?