Our old house battery bank consisted of three Group 31 AGM batteries. This gave us 315 amp hours (but only 157.5 hours are usable). The house bank is only 6 months old and yet we noticed that the voltage would drop to 10.9V when the fridge would kick on. As soon as the fridge would turn off, the voltage would return to 13.5V.
The fridge will cause a drop in voltage when the compressor turns on, but the voltage sag should not be that extreme. This motivated me to renew the electrical connections to the batteries. The cables were in good shape, but the terminals left much to be desired.
The corroded terminals were creating too much resistance and not allowing the electricity to flow out of the batteries and into the system. I also found an old Sammy feather down in the battery compartment that I took out for good measure.
When the voltage would drop on the electrical system, I put volt meter leads directly onto the poles and the batteries were not dropping down to the 10.9V of the rest of the boat, they actually remained around 12.4V! This confirmed my suspicion that the battery terminals were shot and needed to be replaced.
I replaced them with shinny new terminals and cleaned up the ends of the battery cables. I also added two more Group 31 AGM batteries to the house bank for good measure. This brings our electrical storage up to 525 amp hours (262.5 usable amp hours)!
When adding new batteries, the concern of mixing battery ages always comes up. Being how the old batteries were only 6 months old and relatively new by those standards, I was less concerned with the mixing. To prevent any potential problems though, we grouped the three old batteries on Bank 2, and the two new batteries on Bank 1.
Now the five house batteries are organized into their two banks while keeping the new and old batteries separate. With the new terminals (and extra amp hours) the voltage sag has been much less prominent when the fridge kicks in, keeping our electrical needs met in comfort.
You might be wondering why I never changed those old terminals when I replaced the house batteries, and the reason was laziness. They still went over the terminal poles and the insides were cleaned with a wire brush. When the battery cables were connected, the lights turned on in the boat and I was satisfied with having electricity flowing through our boats wires. It wasn't until the fridge issue caused the poor connections to become a more pressing concern.
If we didn't have a fridge, the old terminals would have remained in place for many more years because they looked like crap but still worked enough to power the running and interior lights.
