On my 20’ and 23’ used Blue Seas mini add-a- battery ACR. One battery for engine and one for the “house”. The relay switches over to charge house battery once the start battery is at 13v. Has worked well with no issues on both boats. If you go that route suggest installing the toggle switch they mention to isolate relay when boat is l stored / tied up as there is a small current for circuitry that , over extended time, can drain down the battery.
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