Living on a boat means that you are in charge of everything you use including your electricity. This is done with your house battery. My house battery is actually six Lifeline 6-volt, 300 amp AGM batteries. I pair the batteries together to make three 12-volt batteries and have a 900 amp hour bank. Due to how important my house battery is, I monitor it everyday.
Well a little over a month ago I noticed when I woke up my house battery was at 11.5 volts. I was at the same amp hours used as normal, but I should have seen the voltage at 12.2 or so. Of course this concerned me and I made a note. Over the next two mornings I had the same thing, but everything that used electricity was working fine. This meant I had power, but something was wrong and it was most likely one of the six batteries had failed. This is not unexpected since each of them is over seven years old.
I had a theory, but had to find out if it was accurate and which batteries was the weak one. I did this by disconnecting each one and testing them each with a voltage meter. They all were giving very close readings, so I called my best friend to talk it over with him. He suggested the solar panels gave the batteries a little charge, which masked the bad one. He suggested turning the solar panels off that night and testing them again in the morning when they have not been charged any! I did just that and viola there the bad sucker was (of course it was the last one I checked!).
I put it to the side along with one other one since two of them make a 12-volt battery and reconnected the remaining batteries. Everything is working great now. In theory I now have a 600 amp hour 12-volt system, but given the age of the batteries (usually you get 3-5 years out of them) I feel I have more like a 300 amp hour house battery. This is low, but workable for me for the rest of the season. In the offseason I will replace the house battery along with other things, so check back and see what I do and if it will help you on your boat.