My rule of thumb for batteries is put in the largest you can afford and that will fit. That way you have plenty of reserve for when charging systems fail, lights are left on, bilge pump has to work overtime, etc. Right now I’m using a 27 series marine battery for an electric trolling motor.
The further down the cranking voltage goes the more amps are going through the starter. Make sure your cables are large enough as well.
Kenn
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