Just wanted to share an idea I came up with for my Wrangler. I have a winch and fog lights on my Automatic equipped Jeep. I considered an extra battery with an isolator - pretty good chunk of change... then maybe just an extra battery hooked up in parallel, also considerable money, not to mention the desire for a bigger alternator that goes with such. Then I realized that all I really wanted was the ability to get the thing started again if I screwed up and let the battery get too low.
I went and got one of those booster boxes that you can plug into AC or DC to keep hot and bungied it under the hood on the drivers side on the tray under my brake booster. I snipped the lighter plug off the end of the charging cord and wired it to my headlight switch. That way it is only getting charged when my headlights are on. If I haven't used it, it keeps a charge anyway, but this keeps it hot without continual charging.
Now when I need it for myself, no sweat... just hook er up and go. Even better, if someone else needs a jump, I don't have to get my vehicle near theirs - like when they kill it out in the middle of a bog, and I don't have to risk frying my computer or harness with jumper cables.