Wow! Lots to think about -- some great answers. But I am still confused about one thing that Roy said: "What will happen is the alternator will only keep your truck start battery charged up and all your remote battery will ever see will be the 13.2 float voltage coming out of the alternator."
When I started my truck, and the spare battery was plugged into my 7 pin via smkettner's amazing pigtail device, I measured the voltage at the spare battery, and it was reading 13.9. Does that mean that my alternator somehow recognized that it needed to bump up the voltage due to the extra draw of the spare battery?
Or is it possible that this is just a temporary phenomenon -- maybe after I run the truck for a while, the truck battery will be fully charged and the alternator will dial back on the voltage??
If that's true, and the voltage slips from 13.9 down to 13.2 or whatever, then it will take a long time for the spare battery to get from a 50% charge (12.1 V) to 90%.
Also, how would I determine the amperage of the output from the 7 pin adapter? Is there a standard number? I would guess that since the 7 pin usually feeds into the converter in the trailer, there must be a "sweet spot" for the converter -- too high and a fuse would blow, too low and there would not be enough power.
Roy mentioned that the apparent reading on a newly-charged battery can be misleading -- I can tell you that he is exactly right. Just after taking a battery off a short time on a charger, it'll often read 12.7. Come back an hour later, and it is down to 12.3 or whatever.
And the link to the "homework" -- the 12 Volt side of life -- is useful but very daunting. Over the last several years, I have read through that website several times. Only a little bit of it sinks in each time. There is really too much info for the novice reader to absorb.
