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Battery to battery charging question

Posted: Sat Jun 22, 2019 9:15 pm
by Kommy000
Probably a simple question but here is a background story on what I'm trying to do.
Have solar panels and midnight solar charge controller I'm planning on installing. Sometime in a future I will switch from yanmar diesel to full electric motor.
My idea to make things simple, is to create a separate bank of 2 batteries that will be lithium or similar in performance batteries that I will use and expand on to run electric motor in the future.
For now I will have those 2 batteries connected to solar panels and I will use them as a complete separate bank wired separately from my existing wiring. I will get an inverter and run that off these new batteries and probably wire separate ac outlets on one side of the boat to run refrigeration and small appliances when I'm not on shore power.
Again, these batteries will be able to take that load better and discharge well below the level of my current wet cell batteries.
So I was thinking of connecting them to my current bank with some kind of switch of course in between? This way, my "better" house bank that also being charged with solar, can charge my old bank when I'm on anchor.
This way I pretty much will keep both systems separately for now till I do the electric motor switch.
I know I can run battery charger from inverter that will be on that "better" battery bank but that means buying a new battery charger in addition to the one I already have that charges my existing bank when I'm on shore power.
Can I just connect new power bank directly to the old one via some kind of device to charge the old batteries?
Also how would I wire that? I have 3 batteries with 2 connected together and one that used as starter battery.
I guess I can follow the wire from my charger and see how they wired it to all 3 batteries.
So basically can I use one bank of batteries charge the other bank of batteries without frying both or old one and if there is a device in between that can prevent me destroying both banks.