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.

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2007 Corvette Convertible Torch Red
1996 MBz SL500
2008 Harley Davidson Night Rod Special

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.

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2007 Corvette Convertible Torch Red
1996 MBz SL500
2008 Harley Davidson Night Rod Special

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.

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.

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95 E320 Cabriolet, 159K

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?

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2007 Corvette Convertible Torch Red
1996 MBz SL500
2008 Harley Davidson Night Rod Special

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

If you want to test a modern car for drain, consider Steve Brotherton's words in the thread below.

I hope the dealer solution works out for you. My gut feelings says this was not the problem. I hope I'm wrong.

Don't be surprised if the factory circuits test out OK. I'd bet on an after-market add-on..ie..alarm..ear-busting sound system, etc.

current drain??

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I've found that most of the problems with a discharged, good condition battery, have been caused by lights that should have gone out. You have to inspect the car in total darkness as sometimes they are quite dim.

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'76 240D-Sold
'78 240D-Sold
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet- under restoration
'86 560SL 124K Miles-Sold
'94 320E Wagon, 74K Miles-128K Miles JUNKED
'06 E350 Wagon, 84K Miles
07 SL550, 14K Miles

I had the SAME problem. New battery that would die after a few days of not being driven. The problem, after 2 weeks at the shop was that there was power being leaked through the stereo system. The becker radio was on even though I didnt know it was, and that was draining up the power.

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87' 300E champagne - Euro headlights and corners, 8-hole rims, 140,000miles/220,00km.

Had the identical problem with my '94 SL500, battery dead in 3 days, replaced the battery twice, bought an inline current tester/ammeter, removed every fuse in the car, front fuses, , disconnected the trunk cd player. Removed every interior light bulb, crawled in the trunk at night, removed fuses in the dark looking for a spark, with everything removed and disconnected, i still had a 300 milliamp draw. Took it to an independent electrical shop who indicated it might be a problem with the ignition switch, alleged technical memorandum on this. 5 hours later no diagnosis, would need at least 5 hours more to diagnose. Took it to the dealer, they stated at least 8 hours @$95/hr. Solution: Bought a priority start for $75.00 took 20 minutes to install, memeorized the radio code and I really no longer care where the source of the current leak is. It's a 10 year old car with 22,000 miles. I am not a wealthy man and was not going to spend thousands trying to repair something when $75.00 gives the same result, problem solved.

'91 560 SEL 88,000
'94 SL500 22,000
'86 300SDL 202,000 burnt to a crisp!

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jcciem
'94 SL5000 60,000
'91 350SDL 545,000
'91 350SD 197,000
'00 E55 AMG 77,000

My sentiment exactly,though I try a different approach.I exchange for a new battery yrly in winter at Costco.no charge.
I disconnect battery when on holiday.

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Tom 2010 C250 4-matic Sport (Canada)

Petaling....how do you get costco to exchange your battery every year? When you tell them the battery is bad....they don't check it? I want to know because I like the idea

I have had this battery killing drain in two instances. The first time...it was the radio. No idea why the radio would drain so much...must have been an internal short, though the radio was a decent quality item, and worked just fine.
The other...now this will sound stupid...but once in a while, I would pull the car in the garage....and I would leave the key in the ignition. I know...stupid....but that creates a battery drain having the key inserted in the ignition. Also, the battery was getting a bit old at the time, so it was probably a combination of the key in ignition, and old battery.

I like that priority battery thing...I will keep in mind "just in case"

Did you end up finding out what the problem is?

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1989 300CE Astralsilber/Schwarz
1992 300CE Schwarz/Schwarz

How can the Becker radio be on when you think it is not

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1989 300CE Astralsilber/Schwarz
1992 300CE Schwarz/Schwarz

I had this problem once: turned out the alternator field was remaining energized when the engine was stopped, caused by a bad voltage reg. How I found the problem: I could recharge the battery, and the car would perform perfectly until I stopped it for a period of time. I charged the battery and did not start the car; while looking for the current drain I accidentally leaned ont he alternator and it was warm, even though the car had not been running.
There's always a reason.