TommyHolly
New Member
- Joined
- Jun 24, 2021
- Messages
- 207
I'm only using the AC occasionally anyway. So thats fine. I don't mind using the generator which I've recently replaced every single major component so its now reliable.Reality check. With 2000W of solar, on a good day, expect about 600Ah of power. So that will only cover half of the 1200Ah battery, the rest will need to come from a generator. Running your A/C for 8 hours will consume 800-900Ah. So you will need the generator daily.
Realistically, you would need 6000W of solar to support a 1200Ah Lithium bank. 4000W to support 1200Ah, and another 2000 to recharge the batteries (over two days, while still supporting your load) after a day of rain. With your current setup you only have 600Ah of power you can use every day, anything more than that will come from the generator.
Just so you understand, with your current parts your goals cannot be met. Of course, you can still make all of your parts work, but expect to run the generator for several hours every evening and every morning as a primary charging source.
Your dilemma seems to surround whether you should go 12p4s, or 4s12p, or something between those. That isn't any easy question, as there are drawbacks to anyway you do it. My biggest issue with 12p is that you have old worn out cells. If one of them fails as a short, there is no way to know. The BMS won't be able to protect from it, and you will have 11 cells directly shorted though the 1 failed cell. That will cause a fire you will be unable to control, and you will loose the boat. 4s12p will protect from that scenario. There are lots of other arguments about keeping the cells balanced, and sharing current equally, keeping all the wires the same length, etc. With 48 old unmatched cells, you are going to have balance issues over time, so that really doesn't matter. You also have some active balancers to take care of those issues.
12p is a lot of cells in parallel. I wouldn't recommend that, even with new cells. But, it will work "ok." That is, It won't work well, but will be ok(ass long as a cell doesn't short). If it were me, I would build 4 packs of 3p4s. You will need 4 BMS's instead of 1, but that is better than needing 12 of them. You will have redundancy if a battery fails and the BMS disconnects it. It is much simpler to build and wire than 12 4s batteries. As for the potential fire issue, the only way to protect from that is 4s12p. But, calb are really well build cells, and shorts are rare. So it is unlikely, but could happen.
I've purchased four Overkill 120A BMS's and have finalized my system as a 3P 4S battery comprised of 12 cells each. This will make 4 batteries for a total of 1,200Ah... (Realistically 1,000Ah with my decreased capacity since the cells are 6 years old and each testing around 85Ah.) You and a few others convinced me that a 12P battery is dangerous. See pic of my latest design.
My question now is:
The previous owner had a QNBBM Lithium Battery Balancer attached to each and every cell in his system. It was 4 cells in series making 12v 100Ah and then 12 batteries in parallel making 1,200Ah. (See pic of balancer)
Should I use these in my 3P 4S battery system? Or is that the job of the Overkill 120A BMS that I purchased?