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Re: Battery storage / Voltage

To: BobMGT@aol.com
Subject: Re: Battery storage / Voltage
From: DANMAS@aol.com
Date: Wed, 25 Feb 1998 20:24:32 EST
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

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
                    http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition, slated for a V8 soon!
'68 MGBGT---organ donor for the '74

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