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Battery Drain on 2004 Mercedes E320

I have a Mercedes E-320 2004. I have a new battery and alternator works fine. After I drive my car for a while- the battery will drain rapidly overnight. I had a mechanic check for a draw and he found none. Now here is the weird thing. I have installed a cut off switch to the negative side of battery. If after I drive my car and switch off the battery and quickly reconnect it-the draw stops and does not drain battery ever. If after driving again and I don’t do this it will rapidly discharge again. Any ideas please?

Does this Merc still have its factory radio or aftermarket replacement? Does it have an aftermarket security system or remote starter? All 3 tend to go bad and create parasitic drains.

None of the above! I been struggling with this for a couple weeks now. Seems like something does not shut down after I drive it and thats why I put in the cut off switch. When I isolate car from battery and then connect it again it will not drain it anymore.

You could start pulling fuses each night to find the one related to the issue.

I have tried doing that but when I do it-it shows no drain. It will stay charged indefinitely when I cut the power from battery and reconnect it. Very frustrating and any thoughts or help is graciously accepted.

How many fuses do you pull at one time? I’d pull half of them at the same time. That way you can narrow it down to half of them in one night. Then pull half of the half that the offending circuit is on. Continue until you find the parasitic circuit.

I’d start looking around for door, hood or trunk switches that might not be switching properly. If the car “thinks” you have a door open, it will “wake up” the electronics that might be needed.

There is also the possibility that your mechanic isn’t very good at diagnosing electrical problems…

It is a very fast draw when it is doing it but like I said it does does not show up when I disconnect and reconnect battery. And it does not show on multimeter. It can go from 12.7V to 12.3 in just an hour or two.

There are fuses and relays between the battery and all of the electronics. One fuse box is in the trunk and the other is on the side of the dashboard assembly, visible when the driver’s door is open. Did you pull any of those fuses/relays to find the parasitic circuit?

Yes I have!

All of them? Did you pull them one at a time, in groups, or all at once?

The drain is only after I drive the car for a couple miles. When I come come and disconnect the battery and reconnect it. It does not drain anymore. Its like a relay or something sticking. And when I disconnect and reconnect it it does not drain-almost like it is resetting something.
So when I disconnect the negative cable to check for a draw and tie in my multimeter it won’t register a draw at all. Makes it really difficult to find the draw then.

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Take a jump pack, put black on the negative battery terminal, put the red on positive battery terminal. With jump pack turned on, remove positive battery terminal from battery post with jump pack still attached to battery cable. This will keep current unintertupted. Then hook your multimeter for amps between positive battery post and battery cable. Remove jump pack and wait fifteen minutes. Then start pulling fuses one by one checking amperage until culprit is found.

You or your shop is gonna have to determine if there’s an unexpectedly large drain current (over 50 mA) or not. Until you know that, it’s gonna be hard to solve. BTW, when you say there’s “no drain current”, that’s unlikely, there’s also some. What exactly does your current meter say is the phantom drain current?

For example, it’s possible there is no drain unusual current measured b/c there isn’t any unusual drain current; i.e. whatever the car won’t do the next morning you think it should isn’t caused by a drain current, but something else. What is it the car is doing or not doing the next morning that makes you think something is wrong? Another possibility, the battery is self-discharging, either b/c it is defective, or there’s a current path on the outside case. Try cleaning all 6 sides of the case with baking soda and a brush.

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