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'92 W140 500 SEL
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7 Posts
Discussion Starter · #1 ·
Alright guys I have a confession to make.
I have been going absolutely mad over the past few months because of my battery problems. I am already on my 4th battery since I bought the car and i just don't know what to do anymore...
The problem is as follows: i buy a battery, everything's fine, car starts nicely and all that, and after about 3-4 weeks the battery just wont charge anymore apparently. I disconnect the battery and leave it be, but once it fails to start the car it seems i cant charge it fully ever again. i have a battery charger at home and it says that the battery is charged but when i try to start it, the key wont even turn to second position. I go buy a new one, and the cycle repeats.

Now, almost everything electrical in the car works, i haven't found any bad fuses and my alternator is also only a few months old. I've been to a local electrician and didn't say anything was wrong.

The main reason why i want to tear off my skin is that i disconnect the freaking battery and it still drains! Is there a scientific name for this phenomenon??

Please help ..
Thanks in advance.
 

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1995 W140 S420, no auto leveling, stock basic sedan 72,000mi. VIN # WDBGA43E8SA229905
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25 Posts
I think it's called residual discharge or parasitic drain. Usually from some accessory that feeds off it slowly when the car is not in use. A voltmeter/amperage test will reveal if that's the case. A small amount of drainage is normal as it feeds the clock, alarm system as an example. If that's the case then you need to find the source. After 3-4 weeks the battery won't charge anymore may be an alternator issue that is intermittent. I had a problem in the past of a brand new alternator that was fine for a week developed an overcharge issue that destroyed the battery and luckily didn't fry any electronics. The voltage regulator was faulty and received a replacement charger under warranty. Have your battery load tested off the car and if that's the case that it cannot hold a charge then something ( over voltage charging ? ) is killing it.
 

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1994 S600 Coupe, 1995 S600 Coupe
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3,201 Posts
Probably a parasitic power drain issue. Get a multimeter with an amp setting of at least 10 amps or so; higher is better. Most top out at 20 amps. With ignition switch off, key removed, disconnect the negative lead from the battery. Make sure the doors are closed and the trunk lights are off or disconnected. Put the multimeter in the amp setting, and connect it between the cable and negative battery post. After a few minutes, the current draw should settle down to 30 milliamps (.03 amps) or less. Each time you disconnect then reconnect the battery, the initial draw will be much higher until the computers go to sleep.

If your draw is higher than that, you have an excess draw. Common suspects are aftermarket electrical components that are failing, early analog odometer instrument clusters have multiple issues that can cause excessive draw, bad alternator diodes, or other failing control units.

Once a battery is drained, it sulphates, which makes it incapable of being recharged properly. There are chargers that have special settings to desulphate batteries, but they are not always successful. If a battery is fully charged, then loses its charge disconnected like yours, it may be non-recoverable.

Before investing in more batteries, test for parasitic draw, or find a diagnostic specialist, and have him run down the issue.

For storage over a week or so, it's best to keep a float type charger on the battery, or disconnect the battery, fully charged. Even 30 milliamps will eventually drain a good battery.

Jon
 

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'92 W140 500 SEL
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7 Posts
Discussion Starter · #4 ·
Thanks you all very much for advice. I will be going to an autoelectrician tomorrow.
 
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