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billd View Drop Down
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Direct Link To This Post Topic: Battery FACTS
    Posted: Aug/13/2013 at 7:17am
This will follow along the lines of the alternator/regulator type/model/connection posts I've made.
This post is about automotive batteries. It will be factual, no wives' tales, no urban legends, and no mistaken notions plagiarized from other sites.
All information here-in is from documents released by:
GM, Ford, Chrysler, Sun (now Snap-On), Allen-tronix, Motorola Automotive Products Division (they made automotive radios, charging systems, tachometers, electronic ignition systems and other products for cars), and from my college papers and tests and text books, etc..

First we need to establish what constitutes a fully charged battery. I've seen no small number of posts stating "fully charged battery will be 12.4 volts".
That is ...... Incorrect.
At 12.4 volts, the battery is only 75% charged. That is correct, a FULLY charged battery at rest with the surface charge removed will be 12.6 volts.
This is because a 12 volt battery is made up of 6 cells, each producing ~ 2.1 volts. These are tied together in series, meaning the voltage of each individual cell is multiplied by the number of cells to get the total voltage of each.
There is a very big difference when a car battery drops even a small amount of voltage. When an automotive battery voltage drops from 12.6 to 12.0, for example, its 'power' drops from 100% to 25%. At 12.4 volts, a car battery is 75% charged while at 12.2 volts its 50% charged. That is correct, at 12.2 volts, consider the battery HALF DISCHARGED.
For simplicity's sake, you can consider your car battery charged at 12.4 volts or higher and discharged at 12.39 volts or less, but 12.6 is 100% charged, 100% rated capacity.

Here is a chart taking this even further - comparing battery voltage with specific gravity and state of charge as well as the freezing point of the battery's electrolyte, the latter being of interest to our friends in MN and IA next winter  (Credit to General Motors) ->

(as with the regulator and other posts, this will be added to as time permits)
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