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Originally Posted by billrei
This is a posting from Todd Knutson from the MBZ.org list about dead batteries:
Okay, here it goes. The answer is right here in one of a couple
steps. You'll need a voltmeter. Go from the start, and don't skip
steps. You'll know in 5 minutes.
0) No battery installed in car. Completely inspect the wiring to the
alternator. It must be all solid and tightly connected. Check battery
ground cable for corrosion on the terminal and frame connection. Take
the frame connection off if you aren't absolutely sure.
1) Charge battery. Fully charged batteries normally put out about 12.5
volts with nothing attached. Low point is 12.0 volts. Anything less
than 12 volts means a cell failure. Period. I know that a Mercedes
will start with a big battery only putting out 10.5 volts (one
dead cell). If we see 12.0 volts or better, move on to number two.
2) Connect battery to the system, and run a voltmeter with the key
turned off and no accessories or lights on. Check the voltage. If you
see about the same or a bit less, that's okay. You should never, at
all, drop below 12.0 volts. If you do, take the fuses out of the fuse
holder one by one until you find your voltage drop (open circuit fault).
Only the radio or clock circuit may make a small change in voltage.
3) With no accessories on, and lights off, start car. Check voltage.
Let sit for a couple minutes at an idle of at least 1,000 rpm. You must
register at least 13.0 volts, with a probable voltage of 13.4 volts
which is what your voltage regulator is set for. If you have less than
13.0 volts, see if you can get the alternator belt to squeal by blipping
the throttle. If not, you have a bad regulator.
4) With a reading of at least 13.0 (up to 13.4 volts) at 1,000 rpm,
start turning on accessories and keep monitoring the voltage. With all
accessories turned on, including lights, radio, defroster, you must have
at least 12.0 volts showing on your voltmeter. If you have 13.0 volts
showing on your voltmeter, there is nothing wrong. You have probably
tightened some cable enough or improved a ground connection. You'll be
fine.
5) Of course, you'll really see that you have dropped below 12.0 volts.
I'd peg this at 75%. I'm just screwing around here, since I already have
a very strong feeling that 1/2 of your regulator pick up brushes are
screwed. The regulator should have two pickups. If one brush has a poor
contact, like the spring or carbon hangs up in the sleeve, you will only
get a partial charging. This is very, very common. This is a one
minute trial when you have about a 12.2 volt charge with no accessories
and 11.0 with everything turned on. In short order, you've drained the
battery while you are driving.
6) I'm forcing you to read on. If you had total regulator failure,
you'd have zero charging. This would be obvious if you had the car
running and a voltage of 11.5 volts and dropping with no accessories.
Since you seem to have SOME charging, the rebuilt (um...did that come
from me???...I don't think so...I think you got a rebuilt starter) has a
defective set of brushes. Simple. If the regulator has that nice
replaceable pack (two Phillips screws or Allens holding a plate on the
back of the alternator) it takes five minutes and $50.00 to replace. It
takes an hour and $2.00 in brushes from the hardware store, but that
would assume that the problem is not in one of the holder sleeves.
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