Hi,
I have a dual motor, inboard gas boat which currently has 2x dual purpose AGM batteries for start and house functions. The boat only has 2 battery circuits: port battery and starboard battery (there is also a parallel switch). There is no “house” switch, the house loads are just fed via the same switches as the motors. I purchased the boat with 2-65Ah group 34 AGM batteries, which don’t last long at all (I think it originally came with 8Ds). I want to add more house capacity, but I don’t want to deal with the weight, and I also want the deeper discharge and lifespan benefits of lithium.
I’ve been researching various DC-DC chargers, battery combiners, isolators, etc, but I can’t figure out what setup I need because I don’t have a dedicated house bank. Ideally I would like to:
- Charge house bank from alternator of one motor.
- Charge house bank + starters while on shore power (I have an existing Promariner 3-bank 1220P charger which has a lithium mode).
- Discharge from house bank while at anchor or larger loads underway.
- Must be ignition protected (gas engine room).
- Bonus to also be able to isolate the AGM starter batt. from getting too discharged.
I can’t wrap my head around how the charge and discharge would work. Obviously a DC-DC charger between the AGM and lithium could charge the lithium from the alternator. But then I can’t figure out how to get power from the lithium back to the boat. Any sort of DC-DC charger or isolator would see the charge voltage coming in, connect, and just create a charging loop.
Any input or suggestions would be welcome!
Thanks.