Re: Interior lights start to dim
I'm pretty familiar with the 120 volt AC to 12 volt DC converters from the RV industry. It sounds like the one you have isn't keeping up with the demand. When the batteries are fully charged, most converters are designed to sense that and switch to trickle charge (.5 amp or so). I've seen many instances where the converters don't sense it, never switch out of fast charge mode (around 10 - 20 amp, depending on the model), and end up overcharging and boiling out the batteries. While I've never heard of it, it is possible that the converter is stuck in trickle charge mode. This condition could also be due to your battery condtions/ages. Sometimes, a dead cell in one battery will be a "black hole" for the charge to enter. This black hole will also suck voltage from other batteries in the system. Thus, some charge is going to the black hole, and some to the black hole. The black hole, recharging the good battery, running the lights/other accessories is just too much for any good converter to keep up with.
When the boat is plugged in to shore power, your converter is definitely charging (should be putting out 13.5 - 14.5 volts, depending on the model). This is shown by your meter reading 14 volts. Is that 14 volts while the batteries are fresh, or long after they are dimming down? After dimming down, the converter will still put out 14+ volts, but that voltage will be much lower when the lights/dying batteries demand the bulk of the amperage (thus decreasing the voltage). We'll save the further technical data (voltage/resistance/amperage relationship) for another discussion.