Here's my guess at this point.
The factory install was two house batteries on Island Ranger - each at 105 amp hour. If we only want to use a max of 50%, we have 105 amps to work with. I was guessing we use something in the 50 AH per day - running the fridge all day and night. 'course it is not very hot here. So that gives me 1-2 days. I have now added two additional house batteries, so have 210 AH to play with or 2-4 days. That seems to match what happened (on the low side). On many of our days we only went 10 miles (2 hours), so there was never time to recharge anyway.
The 80 amp alternator expects to run at ~2800-3000 engine RPM. When we run it at 1700, we don't get 80 Amps. I guessed more like 40 amp, but sure would like to know.
When runnning: radar, autopilot, C120, fathometer, vhf, and fridge are all drawing power. Intermittently running the pressure water, Wallas, radio, computer also takes power. I plan to measure soon, but it might be a significant share of the 40 amps (I think my notebook computer draws 8+ amps all by itself.
So, the alternator keeps up with the draw, but I don't think there is anything left to recharge the batteries.
The solar panels could be confusing the alternator circuit as well - as soon as the sun hits 'em, they are producing at a pretty high voltage. Suppose the alternator sees this high voltage, and decides the batteries are charged and regulates back to a trickle? I don't think this is happening 'cause I think we had this problem (not recharging at 1700 rpm) during our Alaska trip when we didn't have solar panels. But I wasn't sure, and had no way to check.
We don't plug in as a rule - I don't want my boat to be the anode for another boat in the marina (I do have the galvanic isolator but still eat through zincs pretty quickly). Plugged in, you only get the 5 amps/hr from the charger so it takes a while to recharge - but at least the fridge can run from AC and not draw on the battery.
I now have 756.7 hours on the Yanmar 110. Still doing fine. Mostly just changing the oil and filter & cleaning the raw water intake filter. I have been monitoring the oil with a test kit, so far so good.
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