Re: voltage low on car battery

On Thu, 05 Feb 2009 02:01:50 -0500, E Z Peaces <cash@xxxxxxxxxxxxxxx>
wrote:

mm wrote:
On Wed, 4 Feb 2009 04:54:17 -0800 (PST), ransley
<Mark_Ransley@xxxxxxxxx> wrote:

On Feb 4, 12:43 am, mm <NOPSAMmm2...@xxxxxxxxxxx> wrote:
On Tue, 03 Feb 2009 19:52:20 -0500, E Z Peaces <c...@xxxxxxxxxxxxxxx>
wrote:

The battery looks old. It will hold at 12.3 volts for many days, so it
looks as if all cells hold their charge.

It depends what you mean by "their charge". 12.3 is too low. When
the connections between the cells were available for voltage
measurement, one could measure the voltage on each cell. Are 5 of
them at 2.1 and the sixth way too low? Maybe. Are all 6 cells at
2.05? Frankly, I doubt it. You probably have a bad cell. A
hydrometer would find it, and I think it is worth the money to have
one and to learn something. When I buy things I keep them forever and
my hydrometer is probably 40 years old now, so the cost of the item
isn't that important. (Unless you are short of storage space.)

A bad cell might have shorted plates. It's more complicated than the
explanatory drawings. There are maybe 6 plus and 6 minus (or more)
plates in each cell and they alternate. This maximizes the power
available for a given size battery. But it also means cells can short,
a plus plate can touch a minus plate, and I don't think there is
anything you can do about it, because the access hole is so small.


If five cells charged normally and one was .3V low, that cell would be
discharged, and I would not expect the charged battery to run the lights
and start the engine normally.


It's not important what you would expect.
An OPEN or high resistance cell will make it so the lights don't light
and the starter doesn't crank properly.

A fully shorted cell is like that cell just isn't there at all - it's
been "jumpered out" - but a fully shorted cell would mean the voltage
(open circuit, no charge being applied) would be 2.1 +/- volts low -
so insted of 12.6 you would have 10.5

10.5 volts from a fully charged 5 cell battery WILL crank and start a
car. When cold, cranking voltage van drop to 9.6 volts and still start
a car.

Because a cell can be PARTLY shorted, it can reduce voltage by less
than a full cell - it may still crank the engine if it is not too
cold, and lights will still work - but your capacity will be quite
reduced.

Like I said before - the ONLY way to know if today's low maintenance ,
sealed, or semi-sealed batteries are any good is to test them on a
"midtronics" type battery tester.

They can test a battery with less than 30% charge remaining with an
accuracy of somewhere around 95%.
Caveat - the midtronics will NEVER give a false "bad" test - but has
been known to say a battery with an intermittent open intercel
connector was good - ONCE.
Put a good load on it, then test it again, and it fails. I've had that
experience and it is not good when a customer is told the battery is
just fine, and then the car can't start to leave the lot!!!!!

A load test, as recommended by some others, is ONLY accurate if the
battery is fully charged - and there is some question whether this is
the case on this particular battery/vehicle.
If one cell had serious leakage, I would expect the battery voltage to
drop to about 10.5 within a week. It stayed at 12.3.




BTW, same chart points out that even when a car battery is 75 percent
discharged, it will still have have 95 percent of its voltage, 12.05
volts**. Twelve sounds like it's almost 12.6 and people might imagine
that a battery with 12 or 12.3 volts is nearly charged.

**WE went over the arithmetic on this when I took high school
chemistry, and it's pretty simple and clear once you see it done. It's
not black magic that the voltage continues to a much higher percentage
of normal than the perecentage of charge is.

I'd like to know more about the arithmetic. It seems to me that if the
voltage of a certain chemical reaction were 2.1, a 6-cell battery
measured with a 10 megohm meter should read 12.6 whether 1% charged or
100% charged.


So my brother gave me the car when he went to Viet Nam, and I took it
to Chicago, and then to Sears, and said I wanted to buy a battery, and
the service guy said, Do you want our free two thousand point
multi-check, and I said, "Not really. I just need a battery." and he
said "It's free. You ought to take it", so not wanting to argue with
the guy, I let him test the car, and he found a bad connection between
the positive battery cable and the starter motor solenoid stud. He
took off the cable, scraped it with a knife on both sides and put it
back. And the car worked fine thereafter. The dealer, out of
business less than 4 years after I got the car, I think spent too much
time replacing big boxes at the end of the wires and too little time
looking at the wires that connected the boxes. I was very impressed by
Sears.

It did happen that everytime I left the lights on for more than an
hour, again the car woudn't start, even with a jump. I used to crawl
under the car, remove the nut, clean the cable end with a knife and
put it back togehter. By the second or third time this was too much
work, too dirty and time consuming, so I just reached down, grabbed
the cablen, and twisted it around the solenoid stud. 10 degrees or
less was enough to make the car start.

I thought about replacing the battery cable--there must have been
something different about it, but I learned not to leave the lights
on.

Neighbors kept calling me when their battery ran down. It seemed to be
the regulator built into their Delco alternator, and I couldn't find the
problem. One day they phoned when their car wouldn't start at the post
office. I happened to touch the positive battery cable and it started.
They'd had somebody replace their starter and the cable wasn't
tightened down at the starter.

Everything had always looked good idling in their driveway, but I guess
sometimes on the road the loose connection would confuse the regulator.
My neighbors called me a fool for not fixing their car sooner. They
considered me their fool because I helped them for free.

.

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