Originally Posted by Faster
Unfortunately voltage alone doesn't tell you much - a rough indication at best, esp with no load.
With your shore charger you only "put in" 8 amps, and while motoring only 15 to 20 I'd guess.... So how much charge you got to depends on how run down the battery actually was vs its amphours capacity.
Right. The unloaded voltage of a lead-acid battery tells absolutely nothing on the state of charge (well, nearly nothing, you can tell that the battery is fully discharged).
You'd need a battery monitor, which continuously senses current flowing in and out of the battery to have a more or less exact measurement on the level of charge.
Measuring voltage with load can give an estimation of charge, but this would usually be only enough to differentiate a depleted battery from a partially or fully charged. It won't tell you whether it's 75% or 90%.