How to position different IR cells in the battery

[llubi]

A conversation of this weekend with a friend, talking about small lifepo banks (4-8 cells). He told me that he tested IR and cells that when top balancing goes down faster than the others, he then take this cells and put them in the middle of the package, so they need to work more and then all the battery kind of auto balance.

I think, that if you have cells that go lower faster, this should be at the beginning and the end, so current is always flowing throw them and they charge to the same as the middle ones (not so fast lowing voltage).

Too many beers?

Thanks!

 

[vtx1029]

I've wondered the same thing but got no answer for you other than I think I would side with your friend on putting lower resistance cells inside the pack.

 

[wholybee]

In series it shouldn't make a difference where the cell with a different IR is, it will still go out of balance. Consider that if you include both the positive and negative path, the total wire distance distance and number of bus bars to for current to flow is exactly the same for every cell. If you are building a parallel then series pack (ex 3p4s) then you can group cells such that the total IR of the 3p cells would all be equal:
1/Rt = 1/R1 + 1/R2 + 1/Rn
To make it simple, lets say in a 3p4s pack you had 8 cells with the same IR, and 4 with too high an IR, each group of 3 cells would get one cell of higher IR. If the IR is different with all 12 cells, you could use the formula above to calculate the total IR of 3 cells, and arrange them so that each group of 3 cells totaled the same.
Probably only worth the effort if you had poor quality cells with a very wide range of IR.

 

[llubi]

Thank you so much guys!! Understood