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Old 03-13-2019, 02:35 PM   #1
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City: Raymond NH
Country: USA
Vessel Name: Mischief Managed II
Vessel Model: 1992 Tollycraft 44 CPMY
Join Date: Oct 2018
Posts: 79
Planning to add solar and extra house battery capacity

My Tollycraft 44 only has 400AH at 12V of brand-new FLA golf cart house batteries and I plan to increase that capacity. I also plan to add roughly 1000 watts of solar to the aft deck hard top and use an MPPT controller to charge batteries.

Since I'd like to some day upgrade to lithium-based batteries when they are truly safe and reasonably priced (solid state electrolyte looks really promising to make both happen), I was thinking of putting the added battery capacity somewhere other than in the engine compartment, where the other batteries reside. I realize that this is a no no if the house batteries are all directly connected to each other. However, I was thinking that a great way to get around this is to place an MPPT controller between the added house batteries and existing house batteries to act as a DC to DC battery charger. That got me thinking that the added capacity could be any voltage I want, so to save money on cable runs, I could use 48V instead of 12V on the added batteries. MPPT controllers are perfectly happy to work with 48V in and 12V out. I'll always need some 12V house battery capacity regardless, because I have no intention of upgrading the boat to run on 48V. The solar panels would only be connected to the 48V bank.

My battery management plan would be to make the 12V batteries the priority and attempt to keep them charged until the 48V batteries are depleted to a safe level, UNLESS I was planning to operate the main engines or genset soon. In that case, I'd stop all draw from the 48V batteries to let the solar panels bring them up to charge while I drained the 12V batteries to a safe level.

Any thoughts on this idea? I know it may sound needlessly complex, but I don't really think it is.

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Old 03-13-2019, 03:10 PM   #2
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City: Litchfield, Ct/Punta Gorda, Fl
Country: USA
Vessel Model: Atlas Pompano 23- outboard
Join Date: Aug 2012
Posts: 4,414
It is needlessly complex and maybe unsafe. With 48 volt nominal batteries, that means that they will be charging at as much as 58 (4*14.5) volts and the solar panels would be delivering 68 volts to the charge controller. You need different wiring techniques for that kind of voltage to make it safe.

How far away is the location of the new batteries from the existing batteries? It isn't a no no to connect two separate banks of batteries if they are wired correctly.

I would use at least two solar controllers and feed each bank from separate controllers. I would connect the two banks in parallel with good sized wire, size depending on what the distance and maximum loads could be, but probably 2/0.

The new bank will probably be discharged less as well as charged less (from non solar sources) than the existing bank due to the tiny voltage drop in the connecting wire. But you can monitor this and it should be a small effect that should be resolved with the independent solar sources.

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