This is what Harold Barre has to say about using an engine alternator to charge an RV house battery on page 112 of Managing 12 Volts:

… If your house batteries are discharged to at least 50 percent of charge, the standard alternator charges your battery with about 15-20 amps until the batteries approach full charger. … You want to run the engine until the alternator output drops to about 7 or 8 amps; at this point it does not produce enough amperage to make running the engine worth while.

Keeping that in mind as well as the fact that my batteries are at about 98% right now, I am quite satisfied that as soon as I turned on the engine to Miranda tonight, voltage in the house leaped from 12.55V to 14.04V and I had 6.5A coming in. 🙂

It definitely wouldn’t be worth running the engine for two hours a day to get 12A in, but on a grey day it would be worth running the engine that long to get the 14.04V to print, charge the computers, or do whatever AC I can and want to do through my inverter.

We’ve got a grey spell ahead, so I just may let my batteries get a little lower than comfortable to see if I do get more amps when my batteries are further discharged.

It will also be interesting to see how much charge my batteries get while I am driving.

For the curious, I’ve got about 12.27V showing and I have on two LED lights, the fridge (on propane, which still needs a little DC since a modern one *g*), the whole house inverter, the UPS in the study, and three external drives. Total amps going out: 3.85. Hours till the battery is fully discharged: 35, meaning I could run like this for 21 hours.

Last but not least, I get a slightly less (and I do mean slightly) jaw dropping voltage drop when I charge the computer through the factory-installed outlet in the living room, so I will be using that one until I can hard wire in a new one in the study.

I really was ready to throw in the electrical towel, but the profound satisfaction I feel tonight makes the frustration worthwhile.