diy solar

diy solar

Cat Owner - Too Many Batteries? Plz Brainstorm With Me

manly

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Jun 23, 2021
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Long story short, I recently purchased at 39' Catamaran that came with 10 DragonFly LiFePo4 batteries (essentially BattleBorn but with a different sticker). They are 100 ah each, so that gives me 1,000 Amp-Hours at 12 volts. They were purchased in 2018. I'm planning to capacity test each but haven't yet.

The kicker: before purchasing this boat I had already been putting together a DIY LiFePo4 pack with 280ah Eve Cells. I have 16 cells and am making 4 12V batteries with OverKill Solar BMS's. That gives me 1,120 Amp-Hours at 12 volts.

Together, that's 2,120 amp-hours, which is a lot of battery! The boat only has 960 watts of solar over the dinghy davits. No generator. I'd prefer not to have a generator. For about $3,000 I could add another 1,000 watts of flexible panels (the ones Will recommends) on top our cockpit canvas enclosure. I measured already and that much will easily fit.

Our largest load is the air conditioner off the inverter. We have a 3,000va Victron Quattro inverter.

Scratching my head on how to manage this much power. Options I'm thinking of:

A) Run all loads from one bank and keep the other charged via a DC-DC charger. Then if we run the first bank down too far we can switch to Bank #2 as a backup. This is nice from a redundancy standpoint, but I have a feeling we would hardly ever use Bank #2 so it would just be sitting there doing nothing.
B) Put the whole bank together in parallel, but that is 14 BMS's in parallel! 10 of them 100ah while 4 would be 280ah. Is this even a good idea?
C) Run the inverter off one bank (that stays topped up with a DC-DC charger from the other bank) and all other DC loads from the other bank that would stay topped up from solar.
D) Sell one or the other bank.
E) Use the Dragonfly batteries to convert our 11.5' dinghy to electric? It currently has a 15hp yamaha.

Thoughts?
 
You really have 5 reasonable or better solutions to choose from!

Your hesitancy to put 14 BMSs in parallel makes sense to me but I really do not have a technical reason not to.

Seems you could more easily implement A without limiting your ability to change to B or C. I would consider D and E as fallback solutions.
 
sounds like your boat’s going to have a lot of power????
I didn’t know Will recommended any flexible panels. What panels are they?
Thanks
 
The first thing you need to do is calculate how much power that A/C unit will use. How many hours will you use it? Then you can size your solar and battery. If you want to run it 24/7, you will probably need 2000Ah of battery, and about 5000W of solar. If you only need it 8 hours per day, then you can divide those numbers by 3. But that is only a guess. You need to start with what power your unit uses. 300Ah is probably more than enough for everything except the A/C.

Another option, sell one or both banks and build a bank that is uniform and possibly 48 volts. You could then have the same Wh, but with only 1 or 2 BMS's.
 
@Rocksnsalt Yep, Will recommends the RichSolar CIGS panels.



@wholybee Good idea to start with the energy audit and work backwards.

With our current 16,000 BTU monster pulling some 120 amps, I'm not going to get very far even with 2,000 ah. At this point solar is more my limiting factor.

I'm considering upgrading to a DC air conditioner/heater such as this one which only pulls about 45 amps at 12 volts:

The O'Kelly's recently installed one of these units and it looked pretty snazzy (albiet expensive at $5,500).

If I double our solar to 2,000 watts and assume 5 hours solar per day and plan to allocate 60% of our daily capacity to climatization, that yields about ~450 ah per day towards heating/cooling. With this Mabru unit pulling 35-45 amps, that would give us up to 12 hours per day, which isn't bad.

You're right it would be easier if I just had one combined homogenous bank instead of these two mismatched ones. But the DragonFly's were about $1,000/ea, so the previous owner must have dropped about $10,000 3 years ago. If I sold them now used I bet they're worth less than half that so it probably makes sense to keep them.

I debated long and hard between 12v, 24v, and 48v, but in the end have decided to keep the boat entirely 12v for simplicity's sake. The windlass is 12v, I have two 12v fridges, and the boat came with a 12v 3000va Victorn inverter. I'm plannng to upgrade air conditioning to 12v too. The only large load left that is non-12v is the watermaker which is 110 so runs off the inverter. I'm not a fan of that because it makes water supply dependent on the inverter. When we're ready for longer term cruising I plan to buy a backup inverter.

Maybe I'll dedicate one bank to the air conditioning entirely, and the other bank to everything else.
 
We just ordered our Mabru unit for our Lagoon 380. We have 8 of the Big Battery Seal 12v batteries 228 Amp/H. We will be able to us the unit for heat also. This will be nice while we make our way south in January.
 
I am switching my boat over from AGM to Lithium as well. Plan is to build system similar to your DIY system. I also looked into 48v bank but opted to remain 12v as all systems on my boat are 12v and to many power switches or upgrades to change that over. I would be more inclined to sell the older bank and build a large arch and place a solar array on it. This will keep everything adequately powered. all the potential for large storage of power without anyway to refill it will leave you with a bunch of empty batteries. We have just over 2000W between flexible and mono panels. Already holding the AGMs, I can't wait to see what its like to have the lithium in there!
 
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