The industry standard for a fully depleted flooded battery is 10.5 volts. This does not vary with load.
The 20-hour rate is the generally used rate for deep-cycle batteries. As noted above, it is measured by applying a resistive load which is 1/20th the rated AH capacity to a fully charged battery. At the end of this period, a healthy battery should show 10.5 volts.
If you reach 10.5 volts in less than 20 hours, the battery's capacity has been reduced. Batteries begin to lose capacity when they leave the factory. If you reach 10.5 volts in, say, 5 hours, your batteries have reached the end of their useful life.
Statements like, "I got seven years out of my batteries" are virtually meaningless because they say nothing about the real residual capacity of the batteries...only that they're not performing as well as the owner thinks they should. Your car battery is "good" -- even with 90% reduced capacity -- right up to that morning when it won't start your car.
It really isn't rocket science! Resting voltages (i.e., no charge or discharge overnite) for flooded batteries will indicate the state-of-charge (SOC) of the battery as follows:
12.6 is 100% charged
12.4 is 75% charged
12.2 is 50% charged
12.0 is 25% charged
AGMs and gels will show slightly higher voltages at each SOC.
If you know your boat well, an accurate digital voltmeter will give you a good enough indication of SOC (e.g., on my boat during the day with the frig running and the VHF
and instruments on, I'll see about 0.1V less than the above).
When you see 12.2 volts or less, it's time to charge!
Put as much in as you comfortably can, because ALL batteries -- flooded, AGM
, and gel -- will sulfate if not fully charged frequently at voltages above about 14.2.
And, be sure to keep in mind that SOC does not indicate capacity of the battery!
After years of use and abuse, a fully charged battery may only have a tiny portion of its original capacity left.