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Old 04-12-2010, 07:06 PM   #1 (permalink)
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How much current should the battery draw when parked?

I'm trying to diagnose a chronic dead battery. The battery goes dead when the car is not driven for a week or more. Both battery and alternator have been eliminated as the cause of the issue.

How much current should I see coming out of the positive terminal of the battery when the ignition is in position 0? With all lights turned off, only the clock and maybe the radio should be using a tiny amount of power, right? I'm just looking for a ballpark figure that I can compare against.
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Old 04-12-2010, 07:43 PM   #2 (permalink)
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Should be somewhat less than 500 milliAmps. Becker radios of this vintage are noted for a failure mode that causes this symptom.

Remember: Testing for Amps with a meter set for Ohms will terminate your meter.

Scott
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Old 04-12-2010, 07:55 PM   #3 (permalink)
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Should be somewhat less than 500 milliAmps. Becker radios of this vintage are noted for a failure mode that causes this symptom.

Remember: Testing for Amps with a meter set for Ohms will terminate your meter.

Scott
While we were in SC for winter, I noticed that Sears had nice clamp on meters that read both AC & DC amps. About $60, I think. Should have bought one!

PS: My W210 is doing same thing. If I don't run it once a week, battery will barely turn engine over.
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Old 04-12-2010, 08:11 PM   #4 (permalink)
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Last time I looked into it (and it's been a while) the clamp-on type weren't able to read very low currents. They may have gotten better in the years since, but I haven't checked. If you run across any info, please post it.

Thanks,
Scott
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Old 04-13-2010, 06:35 AM   #5 (permalink)
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Last time I looked into it (and it's been a while) the clamp-on type weren't able to read very low currents. They may have gotten better in the years since, but I haven't checked. If you run across any info, please post it.

Thanks,
Scott
Scott,

The one I was looking at, was the $59.95 one in this link:

http://www.sears.com/shc/s/search_10...=21&y=8&vName=

It says it has an accuracy of +/- 2.5% in the 0-300A range.

In this link, down about post 8, someone mentions using it and checking accuracy at 200Ma.

Now I really wish I had bought one. A friend has one, but he is the other side of town.
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Old 04-13-2010, 07:43 AM   #6 (permalink)
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Originally Posted by RadioTek View Post
Should be somewhat less than 500 milliAmps. Becker radios of this vintage are noted for a failure mode that causes this symptom.

Remember: Testing for Amps with a meter set for Ohms will terminate your meter.

Scott
I never heard that about the vintage Becker radios. Until I figure this out, I think I'll get one of those battery terminal switches from Harbor Freight to disconnect the positive post when not in use. Thanks, all!
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Old 04-13-2010, 08:36 AM   #7 (permalink)
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I never heard that about the vintage Becker radios. Until I figure this out, I think I'll get one of those battery terminal switches from Harbor Freight to disconnect the positive post when not in use. Thanks, all!
I have thought of doing that and even bought the switch. But of course I would lose my settings (and there are a lot on a W210). So alternative will be to connect a trickle charger when the car is left for longer periods. (we don't use E320 much in summer - I drive the SL.
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Old 04-13-2010, 07:13 PM   #8 (permalink)
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It is probably going to be something that is not obvious and hard to see. You might check the trunk light light switch and make sure it is actually going off when closed. It takes amps to drag a mercedes battery down not milli amps. If the trunk light is operational remove the bulb for a few days and see if that changes anything. I consider myself pretty good at electrical stuff, not an expert but pretty good. I ran into the same problem a few years ago on my 380. Battery checked good. Could not find a significant draw. My best friend is a professional mechanic (30+ yrs but not mercedes) and he could not find it either. Just because it made him mad he ordered a new battery (even though it checked good) and the problem dissapeared. I don't understand why but I never argue with success. Might be worth a try if you don't find anything drawing power and the battery is more than a few years old.
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Old 04-13-2010, 07:35 PM   #9 (permalink)
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The normal procedure for chasing one of these is to put the ammeter in series with the battery (after pulling the bulb to the courtesy lights and trunk light, if needed) then to start pulling fuses one by one until the draw drops. Since there have been a fair number of tales about the stereo unit doing this, I would start there. Eliminate the easy stuff first.
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Old 04-13-2010, 08:08 PM   #10 (permalink)
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Originally Posted by RadioTek View Post
Last time I looked into it (and it's been a while) the clamp-on type weren't able to read very low currents. They may have gotten better in the years since, but I haven't checked. If you run across any info, please post it.

Thanks,
Scott
I thought the clamp meters could read AC or pulse circuits???
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