2 BMS in Parallel

[Moonwalker]

I am building a house battery for a boat consisting of 24 (280K) cells. It is a 12V system and onboard inverter is rated at 3kW so the max current is around 300A.
I was looking at the JK smart BMS but the max current they have is 200A.

Can I separate them in two batteries and connect them in parallel?

 

[Steve_S]

Batteries in Parallel divide Load & Charge between them.
You can have several battery packs in Parallel but there are some basic rules as such.
The most efficient way of setting batteries in Parallel is to use a "Common DC Bus" which is to use 2 Busbars and attaching the batteries to them with equal-length cables from Batteries to Busbar. Quality Busbars by Pike Industries are an excellent option https://www.amazon.com/Pike-Industries-Copper-Busbar-Terminal/dp/B08JKXKF3Q/?th=1
Each Battery Pack must also be Fused, there are several options possible but for 12V & Marine use, MRBF Fuses are an excellent fit and a good value.

NB: With a 12V Inverter/Charger the limit is pretty much 3000W which is 250A Draw from the Battery Packs and this will require 4/0 AWG wire to carry such loads from Battery Posts to Busbar and from Busbar to Inverter.

MARINE: Use only Tin Plated Battery Terminals & Lugs, high quality Lugs such as Selterm Products and marine grade fine wire battery cables. Hydraulic Crimping with adhesive shrink wrap is most strongly recommended.

Battery Pack Notes:
Battery Cells in parallel within a pack should be properly Matched & Batched so they have the same IR (Internal Resistance) across the working voltage range that delivers your actual Amp-Hour rating. If not matched the paralleled cells may imbalance and result in stress on teh stronger cell over the weaker cell which cannot be managed by a BMS as paralleled cells are seen as one cell. Fully Matched & Batched cells are more expensive as these are "proper grade A" and not just simply Voltage & IR Matched as 95% of vendors provide. One of the very few Vendors that sells EVE Factory Tested Matched & Batched Grade-A with Test Reports is Luyuan Technologies and one of the Known Good Vendors supplying cells & more like JKBMS etc.

An A-Typical component system with paralleled battery packs.

 

[chrisski]

An A-Typical component system with paralleled battery packs.
View attachment 126981

The only thing I did different was add an additional master fuse after busbar and before the master on switch.

Looks like class t fuses are pictured which I used. Those are huge, about the 6 inches by 2 inches. MBRF fuses as mentioned take up much less space. My fusing and busbars took up as much space as my batteries.

 

[Moonwalker]

Thanks for the detailed explanation and fuse ideas.

Note about parallel cells - As you said 95% of market cells are not proper grade A. However with cells in parallel there is very little risk of stressing one cell unless they have a lot of capacity imbalance. Problems may occur if the total current draw is way over the cell max current though.
In my case I will be using 280K batteries while the max current should not exceed 300A. For this "standard" market batteries should do the job.

 

[Steve_S]

Below is a picture of my Pike Busbars with MRBF's for each pack. These busbars are superior to many (I got a lot of scrap bars from over the years). MRBF Fuses use 5/16 bolts, as do the Pike Bars I use, so I the lugs are sized accordingly. The MRBF's fit very nicely and with a 3/4" offset for the top the wires can run tightly together. Makes for a "neat" installation.

I've been at this quite some time, you are making an assumption on cell quality and issues... Paralleling cells is not the way to go about this unless they are fully matched, honestly, I do NOT kid about stuff like that BUT it is your money, risk & liability - plus work to build the packs and setup everything... There are a number of folks here who built packs with paralleled cells only to later have to undo the same and make straight series packs due to issues.

BTW: The Only Fuses I have is for the battery packs, everything else is using DC Breakers from Midnite Solar, as I use Midnite Gear like their E-Panels SCC's etc.

Please Download the Linked Resource PDF, it contains more than just "build a pack" info and ref info + links to help you.
Luyuan Tech Basic Lifepo4 Guide V1.0A