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Please don't try to use pack voltage to measure amp hour removed. That is a marginal route to a rough guess.

A lead acid cell reads 1.75 volts under load when it is discharged. Actually, since we don't have a constant load even that isn't really accurate (but a real good target to try and stay above when driving.) A resting lead acid battery at about 10% state of charge will read about 2.0 volts per cell, about 1.95 vpc is please-don't-even-make-it-run-your-dome-light dead. The curve goes off a cliff at that point.

A fully charged lead acid battery reads about 2.14 volts per cell. That is the charged and rested voltage, right off charge it may read 2.25 vpc and work its way down over 24 hours. The fully charged voltage is variable, depending on the alloy metals and the specific gravity. A fully charged (and rested) Optima yellow top used to specced at 2.2 volts per cell.
 
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