Mixing different capacities of Lithium Batteries in parallel

[jerryk]

I'm doing an autopsy on LiFePO4 battery failure.  The packs of are arranged in a bizarre way that I have not seen before.

There are two packs in parallel.  One is 4S and the other is 4S2P.  There is a significant capacity difference between the two main packs. I know you can mix capacities to a degree and I have done it.  I usually have some sort of uniformity in the arrangement and avoid such a huge capacity difference.   Is this kind of practice normal?  Anyone ever seen this done?

Jerry

[BrokenYugo]

I don't follow, isn't this no different than 3 4S packs in parallel? The cells look to be the all the same size so I would think all 3 4S strings would be of similar capacity. 

[james_s]

As long as they're all the same chemistry and configuration it makes no difference.

[amyk]

If you conceptually cut a battery in half, you get two cells of half the capacity of the original, and if you put them in parallel, they become one. If you cut a battery into thirds and put two of them in parallel, then the third, why would it matter? Each piece will discharge at the same voltage, and if they're all the same chemistry, then they'll be at the same (percentage) state of charge.

Having the same capacity is only important for cells in series, as otherwise one would become completely depleted long before another and be damaged by reverse-charging.