I've been studying lithium battery systems non-stop for several weeks sitting here on the dock in Whangarei, New Zealand. My catamaran comes out of the water in 3 weeks for an electrical system upgrade. I've got to commit to a system design and specific equipment. My boat is 12VDC/120VAC and I want to add a parallel 240VAC system. I have 20 shiny new Eve cells and 6 400W panels in a storage unit and I need to do something with them!
My first thought was to put in a 16S 48VDC bank and a large inverter and just design around that. That would allow me to ditch my humoungous diesel genset and add high-output alternators to the main diesels. Then I started to realize I'd never find a 48V alternator to charge that bank....and the selection of 48VDC marine grade equipment is very limited. Since my boat has a huge quantity of legacy 12VDC loads, I need to put in a 4S 12VDC lithium bank and ensure it's care and feeding. After a week I abandoned the 48VDC bank and started planning around two parallel 24VDC banks, each with a FET-based BMS. It seemed to be so sensible to have a self-balancing BMS that could separately regulate charging and discharging. Much more refined and civilized than an emergency-disconnect-or-die BMS with a contactor.
At the end of the day I had to face the simple fact that I have a 12VDC boat. 12VDC is critical for communications, raising the anchor, refrigeration, desalinating water. The 12VDC system deserves the highest standard of care. And the market offers up far more 12VDC equipment with worldwide availability. 12 Volt wiring, inverters and chargers are bigger and heavier but 12VDC high-output alternators are half the cost. Avoiding a separate DC system with associated bus bars, disconnects, and fusing also saves me time, space, and money. I will install one big honking 12VDC LFP bank.
My 12VDC LFP bank will be configured as one 5P4S bank with a contactor-based BMS. In my opinion, paralleling FET-based BMS units gets to be a hairball. Would these multiple battery systems, each with their own state-of-charge calc all dance in lock-step forever? And what prevents circulating currents between them? Everybody's in charge and nobody's in charge of the charging sources. In my case that's two diesel alternators, a diesel genset powered charger, 120VAC shore power, 240VAC shore power, and three solar controllers.
My off-grid home floats in saltwater, breathes salt air, and it's often hundreds of miles from anywhere. I can't have my critical power running through equipment that doesn't have proven reliability in the marine environment. I'm all for Chinese-designed electronics and electrical equipment for low cost, land based applications. On my boat, only my Sunpower solar panels come from China. That's not true actually. I do have a number of Alibaba USB chargers, flashlights, and other LED widgets - but they're mostly all dead or dying of corrosion.
I now need to chose between the few over-priced, barely customizable BMS products with established presence in marine. I'm holding out hope for something that will exchange more than just "charge enable" contacts with Victron and Wakespeed systems.