[brucepick]

Hooo-boy, that's a lot of questions. I'm a man of many words, for better or worse, so here goes...

What you saw re. battery fading in a long coast - I saw the same thing last winter. Mine didn't drop quite as far as yours did, but would drop through the 11.x range and down into 10.x if I didn't re-engage the alternator by bump starting the engine.

I came up with a cheap + quick solution I liked for last winter, and then built a better one this past summer that I like even better. With the new setup I've successfully tested a driving range of 3 hours plus, WITH headlights running. About 6 hrs. driving without headlights, if I use each of my two batteries in succession, one after the other. I have switches that can do that.

My cheap + quick remedy for last winter was a second starter battery, not a deep cycle, about 2x the size of the Civic starter battery. It came out of a Jeep, we just had it in the garage so I put it to use. In other words, it was not a monster size, but bigger than the Civic starter battery. I hooked it in parallel with the under hood battery and could easily coast 1-2 miles with lights on. Starting was fine too, engine would crank over very easily with the two batteries in parallel. Both would get charged when the alternator ran, of course.

Generally they say do NOT put two batteries in parallel, and now that I've set up a pretty evolved deep cycle system, I have two and they're separated, not in parallel. I DEFINITELY wouldn't put two different type batteries in parallel, such as deep cycle and starting type. I also wouldn't parallel two nice new carefully selected batteries, it would be bad for both of them. One will always be "stronger", aka higher voltage, and will discharge into the other battery. You lose a lot of charge that way, due to the various inefficiencies of batteries charging each other. (RVs and boats use multiple deep cycle batteries, those are the folks who learned the lesson to not parallel them. Better to use one, then use another, etc. - or have them run separate devices.)

In my current system -
For the starter, and to power the headlights:
I use a WalMart marine deep cycle Group 29, installed in the trunk, left side, ~60 lb. It's rated 122 AH, but that's at only 1A draw. I figure it might be about 100AH at 10 or 20A draw. The headlights are powered via relays now, so that made it possible to power them from the battery of my choosing.

I estimate the car draws about 10-12A, and the headlights also draw 10-12A, so 20-24A when running with headlights. I've run the whole car with lights using just the WalMart deep cycle but I wouldn't want to do that regularly, driving about 2.5-3 hours daily - and in winter that's all with headlights on.

The rest of the car (not headlights + starter) runs off a 40AH lithium LiFePO4, 12.8V pack. It's rated to handle a 100A load, and is internally circuit-protected with 100A as a limit. In actual use, it can run car + headlights but only for about 30-40 minutes. The nominal 12.8V pack drops to 11.5 or so in such use, and I don't want to take it lower than that. When I first had the deep cycle setup, I had the old starter battery up front, just powering the starter. And the lithium ran everything else including headlights. If I needed the lights more than 40 minutes or so, I'd have to stop and switch the alternator back into the circuit and run everything off the starting battery, just like it was originally built. I didn't want to charge the 12.8V lithium pack off the alternator, it's not built for that job and I didn't want the alt to mess up the pricey lithium pack.

So with the 40AH lithium being the only deep cycle in the car, I felt a clear need for more battery capacity. Thus the aprox $90, 100AH or 122AH WalMart lead acid battery.