Re: electrical upgrade question
Just buy another AGM battery for the rear and you're set. And for that small a bank, you don't have to have a negative run from the front battery. Just make sure you have a very good grounding point for the rear battery and make sure your grounds in front are equally good.
If it's a unibody car, ground the rear battery on a strut tower or some other structural panel and if it has a conventional frame, ground to the frame.
As for mixing batteries, the problem is with mixing battery types, not sizes. You said you have an Advance Auto Platinum battery. That's an AGM battery so when you buy the second battery just get another AGM. But even if you mix battery types, the difference in surface charge is only a few tenths of a volt and it's not the catastrophe people make it out to be. All that happens is that the wet cell robs a small amount of surface charge from the AGM. Once they are equal in voltage, they stay that way.
In simple terms, robbing the float charge from an AGM battery is simply drawing a tiny amount of current from it for a short time. But actually using the battery is to draw much larger amounts of current from it so, to arguer that mixing wet and AGM cells shortens the life of the AGM is arguing that using the battery shortens its life. And, well, that's just a fact of life. Using batteries shortens their life. Duh.
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