LakeSuperiorGeezer, they were not in a state of discharge for even 20 minutes. I have been living on the boat for a year and half. This morning when I woke up and checked the voltage they were reading 12.5 with no load. Upon turning on a small flourescent light however they drop to 11.6. This has been going on for some time. The battery hydrometer that I am borrowing I am afraid is junk. I will search the net for the temperature tables and see if someone has a descent one. A friend said that he has a charger that he can loan me to equalize the batteries.
Neither voltage nor specific gravity (as measured by a hydrometer) are indications of the capacity
of the battery. Rather, they are indications of state-of-charge (SOC) of the battery. These are not the same thing.
Sounds very much like your batteries are at the end of their useful life. Those with a high SOC will show near normal resting voltage but won't be able to deliver significant amperage for very long. Think car batteries: you can have an older car battery which is fully charged, has a "normal" resting voltage, but can't turn over your engine. It's capacity
is shot (see below).
Before concluding that this is the case with your batteries, either borrow a real battery tester which measures capacity (like the Midtronics series...these cost $600 and more, so most boaters don't have them), or do a controlled load test over time.
What kills battery capacity?
. sulfation (formation of PbSO4 crystals on the plates, which become embedded and limit available plate area)
. contamination of the electrolyte
. corrosion and erosion of the plates
. stratification (different concentrations of electrolyte at different levels)
. physical damage to plates or connecting rods
. buildup of sluffed-off material in the bottom, causing intermittent shorts, or worse