DANMAS@aol.com wrote:
>
> In a message dated 2/25/98 12:00:03 AM Eastern Standard Time, Bob MGT writes:
>
> > My car runs 15V at the battery terminals with the engine running and the
> > lights off. With the lights on it runs 14.5V. These were measured with an
> > accurate meter. I assumed that this was too high, as I've always heard
> > that 14V was the proper level. Is 15V ok? I don't want to cook my battery.
>
> Bob,
>
> I don't know for sure, but i wouldn't think 15 volts would be a real problem.
> Most every source I've seen says the alternator output should be 14.6 volts.
> For the sake of argument, let's assume the 14.6 V is chargine at a rate of 10
> amps. Using ohm's law, the says the equivelent battery resistance is 14.6/10
> = 1.46 ohms. With 15 volts, the charging current is 15/1.46 = 10.37 A, less
> than a 3 % increase. As far as I know, the most significant factor in damage
> from over charging is heat. An extra 3 % charging current shouldn't be
> critical.
>
> Keep in mind, though, that the resistance we're speaking of is not a true
> resistance. It is a function of the chemical nature of the battery, and is
> basically the result of "back EMF" from the battery -- EMF being
> "electromotive force", what we normally call voltage. The amount of back EMF
> depends on a lot of things, such as temperature, state of charge, etc. My
> knowledge of such things is very limited.
>
> Dan Masters,
> Alcoa, TN
>
Kepp in mind that all modern regulators are temperature compensated.
Even our generator / regulators are temp regulated. The colder it gets,
the higher the voltage.
I don't know too much about these batteries, but as for the sealed gel
cell type lead acid batteries, the maximum voltage under any condition
is 15.00 volts. Above this voltage the gasses created by the charging
chemical reaction cannot be re-combined. Pressure can build up and the
vents release.
While this is not a problem in an automobile battery, the higher the
charging voltage, the more water is released to the air. This means that
you must be more diligent in the maintenance of the battery water.
Chuck
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