Battery Voltages vs Capacity
I have a classic 1 x 2 battery set up in my Catalina. The starter battery charges up to 13+ volts and holds there rather well. The house batteries charge up to a slightly lower voltage. (BTW - these are big batteries - I forget the group but they are approximately 16" long by 12" wide - they are also less than 3 months old.)
Here is my problem. Tonight we went over from our dock to the fuel dock for fuel. The only draw on the house batteries was from our Raymarine ST50 gear - maybe 2 to 3 amps according to the ammeter. We ran the boat about .3 hours - based upon the engine hour meter. When we returned to our dock the voltage reading with all circuts off on the house bank was about 12.2 or 12.3 volts. This seems very low to me. I checked to see if something was drawing on the batteries but the ammeter said no - 0 AMPS.
My question - what voltages are considered to be 75%, 50%, and 25% charged presuming that there is no load on the battery and it has had a few minutes to recover from the previous load.