I appreciate your Pacific Yachting Magazine columns and online advice. My boat has a conventional 1-Both-2 battery set up. I use bank 1 as a house bank with 370Ah, Bank 2 is essentially a reserve starter, group 27. I just purchased some pricey L16 AGM batteries and would like to add an external regulator to my 80 amp alternator. Since the voltage and temperature sensor will go to the Battery 1 house bank, what will happen when I switch to Battery 2 to do a start? Will the depleted house bank trigger the alternator to overcharge Battery 2?
When charging multiple battery banks from one alternator, we recommend always attaching the alternator output directly to the largest bank, in this case the Battery 1 house. You may to also consider an ACR to automatically combine the house and starter while underway to reduce the risk of leaving the switch in the “both” position while at anchor.
Balmar makes good external regulators that can even come with two battery temperature sensors. https://www.pysystems.ca/shop/power/external-regulator/