4s2p LiFePO4 Leisure battery

[beemac]

Hi all,

I'm almost certain that this will have been asked before - but I've had a search to no avail so apologies if so...

Am building a replacement for a pile of lead acid I have in my campervan. I've ordered 8 LF280K LiFePO4 cells from fogstar (https://www.fogstar.co.uk/products/lifepo4-280ah-prismatic-cell-grade-b) and I plan to box then in a 4s2p configuration for a drop-in 12v battery replacement.

Final capacity will be c.500Ah with a continuous discharge of 250A. BMS is a 250A Daly board.

I plan to use copper busbars for all cell-cell connections (38mm x 3.6mm) = 136.8mm2 which gives a current capacity of 1.2 x 136.8 = 164.16A (apparently 1mm2 of copper busbar supports 1.2A of current from the research I've done).

Question is about arrangement - which is the better of these two arrangements (or is there a better one) - my assumption is that A would mean less voltage difference across cells because it doesn't have the long busbar connections across 4 terminals. But A is less flexible and has to be a long, thin battery wheras B can be arranged into a squarer final battery if required...

In A all inter-cell connections are a single busbar of the spec above (except the first/last cell which would have a doubled-up busbar) - for B all connections are a doubled-up busbar.

Any obvious gotchas or things that might cause me to burn down my campervan?

 

[ColinB]

It looks pretty similar to me, but I don't know anything about building batteries. I will ask though -
- It looks like your BMS rating is higher than the bus bars you are planning to use. Is that ok?
- When you parallel the cells, the BMS will watch the paralleled cells, right? If one cell starts to go bad, how will you know?

 

[beemac]

It looks pretty similar to me, but I don't know anything about building batteries. I will ask though -
- It looks like your BMS rating is higher than the bus bars you are planning to use. Is that ok?
- When you parallel the cells, the BMS will watch the paralleled cells, right? If one cell starts to go bad, how will you know?

Busbars are doubled-up or there are two current paths where necessary to meet the amperage requirements.

BMS generally only protect each parallel group as a single entity from what i've seen - and what I've built previously (14s4p ebike batteries) - they are typically sold on the 's' number - so for 12v you use a 4s BMS. I also can't see how a BMS would measure individual cells in a parallel group - by definition they are voltage tied...

Really my only concern (I think) is whether I get any benefit from using arrangement A over B. Since A means I can only lay the cells out in one pattern - long thin - which isn't optimal.