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LiFePO4 capacity in series
#1
I'm ready to buy some used LiFePO4 prismatic cells to make a 24v s8 pack. How much of a variance in capacity can I have in series. I know in parallel it's less important with more parallel cells.

4 would be 170-180Ah and 4 will be 180-190Ah. The battery will be expected to push 80 maybe 90 Amps at times in my RV. Should I wait for a closer grouping in capacity?
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#2
If you're going to buy them, why don't you buy eight identical ones? Given the size you've stated I guess we're talking about Winston or Calb? They are manufactured to a certain spec for a reason, exactly this reason to be precise, so that they are not out of balance when being put in series. Which is what the cells will be over time when using different capacities.

Worst case using your numbers, 170Ah and 190Ah cells, 20Ah difference. That's huge. It might cause you some trouble, it prevents using the full capacity of the larger cells and it's especially bad on LiFePO cells because of their flat discharge curve. They will start drifting and because of their very stable discharge voltage you will only see it very late, if at all.
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#3
Thanks, I'll keep looking. When you say 8 identical ones, do you mean new ones? It doesn't seem like I could expect to find 8 used ones within a few mAh correct?
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#4
Yes, I thought you meant new cells when you said that you are going to buy cells. If you're buying used cells try to find eight ones that have spend their lifes together and are of the same age. They won't be identical to a few mAh either but on 180Ah cells you don't need a few mAh. 1Ah or 2Ah might be alright.
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#5
You can have cells with 10Ah and cells with 200Ah but you wont be able to use more than the lowest one can supply....
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#6
(02-07-2019, 04:37 PM)daromer Wrote: You can have cells with 10Ah and cells with 200Ah but you wont be able to use more than the lowest one can supply....

Or you need an active balancer with a really high balance current Tongue
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