2P4S LiFePO4 config

[wfdTamar]

I have 8 x Eve 304Ah LiFePO4 cells and a 4S BMS. I'm going to do a 2P4S configuration for 12 volt 608Ah. This means pairing up cells on one BMS lead.

I've tested all the cells and they range from 311-317 Ah. When I'm pairing cells, am I best to put similar Ah cells together, or the worst with the best so they even out? Or would you match them by resistance?

 

[Partimewages]

Match by resistance if you have that. Since it's one pack the lowest cell will determine the overall pack.

 

[MisterSandals]

or the worst with the best so they even out?

Definitely this. Label your cells and take good notes.
I have doe this several times with very good results.

Match by resistance if you have that.

I've not heard this. Can you teach me what you know about this?

 

[Partimewages]

Matching internal resistance is like buying matched cells. The closer the IR is combined with AH the better the pack performance.
If you didn't purchase with matched and batched sheets then it's probably not going to make a difference for this one pack.
The lowest AH cell will dictate the overall pack AH because it will run at each curve and trip the limits.

 

[MisterSandals]

Matching internal resistance

Are you saying one should put similar IR cells together in the 2P pairs? Or to make the sum of all the pairs equal or ??

 

[Partimewages]

All cells in a pack should be as close as possible. Lowest cell and most odd IR will end up dictating the whole packs total capacity. They will start running out at the edges of the curve on each end and hit the cutoff.

 

[MisterSandals]

All cells in a pack should be as close as possible

Yes of course but OP was asking how to match the pairs.

 

[wfdTamar]

Exactly. Of course you want all 8 to be as close as possible, but as each pair of cells is treated as one by the BMS (due to only having a 4S BMS) my question is which cells to combine in the pairs.

If I match the best cell with worst do the pair equalise somewhere in the middle, or does the weak one draw the best one down to its level (so wasting the benefit of the better cell?). If the latter, is it better to pair weakest cell with second weakest, best cell with second best. so that overall the whole battery might have a few Ah more capacity? Or if you do that, does the weakest pair win out again and draw all other pairs down to its level (or rather - trip the BMS first so effectively all pairs match the weakest)?

This is all about 6Ah (1% of total capacity) so not a big deal, but it's interesting.

 

[rodrick]

Best cell with weakest then 2nd best with next weakest ect

 

[Partimewages]

No matter what you match the weakest cell will rule the pack.
For the amount of AH you are dealing with it's probably a waste of time.
Just make sure your BMS settings are correct. Don't try to eek out every bit of AH. Try to keep the pack running between 3.0 and 3.4 volts that's where the manufacturer bases their AH rating.

 

[MisterSandals]

No matter what you match the weakest cell will rule the pack.

No, with 2P4S, the weakest pair will "rule the pack".

For the amount of AH you are dealing with it's probably a waste of time.

No, this makes a huge difference if there are significant single cell differences. If pairings can be made close to having similar capacities, you will have a FAR better battery than a randomly paired battery.