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Old 11-28-2006, 02:17 PM
patbob's Avatar
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Join Date: Oct 2006
Location: Portland, OR
Posts: 836
no load = about 12.8V

Originally Posted by unkl300d View Post
A fully charged battery is supposed to float at some said volts. What is the margin of voltage loss just standing without cables attached??
No load, fully charged terminal voltage of a car battery should be 13.2V. You'll only see this for a short while after disconnecting the battery charger. With no load and time, it'll drop to about 12.8V give or take a few tenths of a volt. It should stay there for a very long time until you discharge it some.

12.55V seems a tad low for immediately taking off the charging circuit, but since battery cables were still connected, is probably OK (it sounds like you do still have some load running it down).

You should see ~14V at the battery terminals when the battery is charging. The battery cannot reach full charge until the applied voltage is >13.2V, and you need a little extra on top of that to push some current through it to make the chemical reaction of charging occur. Your readings all sound fine and I wouldn't consider them any problem without any other evidence (in other words, sounds like the alternator/regulator are probably A-OK).

Your declining battery voltage when disconnected seems the likely culprit. I wouldn't fuss over the .01V changes, but wouldn't expect to see 0.1V changes too fast. If it was this way when new, then I'd consider it defective. Test might be to connect a small light across it (e.g. glove compartment light) for a couple of hours. The battery voltage shouldn't dip too much with that small of a load (I'd expect 12V or better). I left my glove compartment open for ~9 hours in my gasser the other day and I didn't even notice the discharge on the battery when I went to start it. Felt like an idiot for doing it, but what can ya do?

All in all, it sounds like the new battery is not (no longer?) holding a charge. As I recall, you said you ran the new battery down pretty deeply. If so, try connecting a 3 Amp trickle charger to it for a few (2-5) days to see if it seems to recover. It is possible one cell is weak from the experience and can't hold much charge anymore. That'll cause the no-load voltage to drop faster and quicker than normal, and will cause that cell to not be able to keep up on the current demand when starting, so you won't get too many tries before there's not enough voltage left to crank over the starter, which is what you seemed to describe. A nice, slow, long trickle charge might get enough current through that cell to get it charged up and able to keep up with the other cells.

Also, check the water level in all six cells to see if they completely cover the plates. Running it down could affect the level, but it shouldn't affect it by enough to expose the plates. If they are exposed, it probably wasn't right when you got it.

BTW, I'd disconnect the battery when not driving until I found and fixed the discharge. As suggested, disconnect battery and put meter in series (or better, a small 12V test light). Pull fuses until meter reads ~0V (test light goes out). Start with known loads -- clock, dome light, etc. -- since you know they will be drawing some. Don't forget, not all circuits have fuses in the box and at least one (the starter itself) has none (but it does have a relay). Check a wiring diagram to make sure you've disconnected the battery from everything.
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Old 11-28-2006, 08:58 PM
unkl300d's Avatar
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Join Date: Jul 1999
Location: San Francisco, Ca
Posts: 2,284
Thanks Diesel Bob,
Your advice is well appreciated. Tomorrow I go see my mechanic for help.

I spoke to Interstate battery cust service and they said that the fully charged battery should float at 12.5 to 12.6 volts for my model MTP-93.

I fully charged the battery twice now on a charger and twice it floated at approx. 12.57 Volts overnight. So I don't suspect the battery anymore. My friend's digital battery tester checks the battery standing, with load, and tests the alt and starter systems. It is a digital device. The tests are positive on all.

P.S. My Fluke digital multimeter seems to measure a bit lower than my friend's
digital battery tester. So, the first drain episode may have been at 11.5V approx. reading. Just below 12 but not enough to even budge the starter.
So I don't think it drained outrageously to affect a cell. My correction.

Today after the battery was surely fully charged, I connected the cables and after 3.5 hours I tested the battery after removing the cables. The digital device disclosed that the battery had gone down 0.14 Volts to 12.43 V.

Interstate said that 12.45 V represented a 75% charge on this battery so I recognize that this deficit is not acceptable in such a short period of time.
Now I can visibly see a small spark after connecting the neg cable (last step).

Supposedly this means an open circuit. It does this even with the clock and radio fuses removed and suspected A/C Amp removed.

I did a second meter in series test removing all fuses. No apparent drops in mAmps.

Did other tests and redundancies. No apparent bugs.

Cables are off now and the battery is on a charge for a bit to have a fully charged battery for tomorrow's trip to the mechanic.

Will post updates.
1979 300D 199 K miles
1995 C280 95 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles
1979 240D 140Kmiles (bought for parents) *SOLD.
1989 300SE 148 K miles *SOLD

Last edited by unkl300d; 11-28-2006 at 09:13 PM.
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Old 11-30-2006, 08:45 PM
unkl300d's Avatar
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Join Date: Jul 1999
Location: San Francisco, Ca
Posts: 2,284
Thumbs up Problem Solved

Thanks to this forum's archives, the respondents to this thread and my mechanic, the electrical drain was found.

I was close to the source but due to my inexperience with ammeters and test lights etc. I failed to take the extra step of testing the ammeter in series with and without the A/C Amplifier.

My mechanic used a test light. He did the same process I did with the fuses and then he investigated the A/C Amp using a test light in series.
I had the amp readily available since I had already inspected it.
My loss of heat was the clue that lead me to do that.

The story ends with the fact that the A/C SERVO was the source of the battery drain.
Unplug the A/C amp and no drain. End of story.
My car has no heat but that is OK. For now. At least my battery is healthy and worry free.

Here is a quote from some member in the archives. Words of wisdom.

"I've been in the auto electric business for over 30 years. The best way to locate a battery draw is by connecting a test light in series between either battery terminal & the battery cable. Don't use a Snap On light, the bulb has too much resistance. After connecting the light momentarily touch the cable end to the batt terminal with test light still connected, then remove. This will allow any components that have capacitors such as radio, a/c amp, clock winding mechanism, etc to recharge. You may see a slight arc when you do's normal. If you use an ammeter you'll be chasing phantoms, as it will show you values you need not wory about. A test light will allow a small amount of current to flow keeping caps charged without glowing. If it's not enough to light the light , it's not enough to drain a battery under normal service intervals. If your test light glows, you have a battery draw. Remove the plug from the alternator & check test light. I've seen many alternators bench test ok for output but still have a draw when not running. From your description you may have an intermittent charging problem. Check all connections from battery to alternator including grounds, especially the ground from the alternator to the bracket. You might want to remove the regulator from the alt to check brush lenght.
Be sure you fully charge your battery before testing alt output, a weak battery will give you low readings, and stress your's made to maintain a good battery, not charge a weak one. Ideally you want around 14.2V , but most MB's seem to run around 13.5V. Below 13.2V is unacceptable.
If all checks ok, start pulling fuses until your light goes out. "

I add this info and my story for the benefit of the archives and future researchers of problems.

1979 300D 199 K miles
1995 C280 95 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles
1979 240D 140Kmiles (bought for parents) *SOLD.
1989 300SE 148 K miles *SOLD
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