I would go with at least 6AWG stranded wire. You maybe able to find butt splices that big but you need a big crimper to connect them. I use split bolt connectors, Lowes has them, You just need two wrenches to make a good connection and you can undo if need be. Just cover in electrical tape and your good. to connect to the battery I use ring terminals. Again you need a crimper. There are ring terminals with screws I just do not like those.

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8-10 Gauge Wire also look at NAPA part number 751-1090 or 751-1091, with part number 751-1127 for your connectors, that is what I used when I put a Trolling motor on my Jon boat that way I could remove it.