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How do I find the real AH and C ratings of a flooded lead acid?

Charge it fully. Connect it to a constant current load of N amps. Record the start time. Monitor the voltage until it falls below a terminal voltage. (for a 6 cell Lead acid battery, Vt usually is 9 or 10 volts.) Then record the end time. The End time minus the start time in hours is the elapsed time The AH capacity of the battery for that N amp load is going to be N * Elapsed time The capacity will vary, getting lower as the N increases. Manufacturers data sheets often show a family of curves for different N. Or, knowing the approximate C in advance, they use a 1/10 C discharge rate.

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