Running 2nd battery with isolator in vehicle…

Hi, I’ve got a 2015 Hyundai Elantra and I currently have 2 ham radios and some other small accessories that I run in my vehicle. Nothing that pulls more than 20ish amps at any given time, and nothing that should be pulling more than 10-15 amps for more than 30-60 seconds at a time. I‘ve managed to acquire a 4 year old car battery that I’m gonna test to see if it’s any good. If it is, I’d like to mount it in my trunk and wire all my radios and accessories to it and have it connected to the main battery via a battery isolator so that I can use any of this stuff without the engine running and not worry about killing the starting battery. As far as wire gauge to run from the starting battery to the 2nd battery, what is the guideline on that? I’ve watched a lot of YouTube videos and everybody seems to be running 0 gauge wire or bigger and that seems a bit excessive for what I’m doing. Does the battery need that much current flow to charge, or are you supposed to base the wire size on what you’re powering? Cause most everything I’ve seen is also for car audio pulling a ton of amps.

Also, if this battery isn’t any good, I was contemplating getting a lower Ah LiFePO4 battery but wasn’t sure how that would work charging off of the alternator? What special steps might I need to take for this?
It's generally recommended to use a wire gauge that is appropriate for the amount of current that will be flowing through it. For your application, where you're only expecting to draw up to 20 amps at a time and possibly as little as 10-15 amps for short periods of time, a wire gauge that can safely handle these currents would be sufficient. You could consider using a wire gauge in the 8-10 range, such as 8 gauge or 10 gauge wire. These wire gauges should be able to safely handle the currents you're expecting to draw and should be sufficient for your application.

As for charging a lower Ah LiFePO4 battery off of the alternator, it's generally possible to do so as long as the alternator is capable of producing enough current to charge the battery. You may need to use a charge controller or other special equipment to ensure that the battery is charged safely and efficiently. It's also important to make sure that the battery is compatible with the charging system in your vehicle and that you follow the manufacturer's recommendations for charging and maintaining the battery.