One decision that I initially postponed was the threshold to use for labelling a cell as “good”. I wanted to keep the storage ability as high as possible by using only cells with a high capacity, but I also wanted to be realistic and avoid “disqualifying” a big majority of the reclaimed cells. I decided to wait a bit and see what the data suggested.

After testing 20 cells I had an almost perfect dichotomy between dead cells (assigned a nominal capacity <1 mAh) and cells with capacities over 1900 mAh. The following histogram illustrates this situation, with the dead cells omitted as they are not interesting.

The histogram looked nice with 200 mAh bins, and that suggested a threshold for “good” of 1800 mAh. I still had plenty of cells to test, but adopted this as my draft categorisation to start counting how many good cells I was accumulating. Another nice thing about this threshold is that having an average cell capacity of 1930 mAh means that a 7s20p battery is almost exactly 1 kWh of storage (1.93 Ah × 3.7 V ×  7 series × 20 parallel).

By the time I had tested 100 cells, the picture was surprisingly similar. There were no longer any empty bins in the histogram, but the cluster above 1800 mAh remained. The chart below shows that a second smaller cluster had started to appear above a threshold of 1000 mAh.

I was noticing cells turning up with capacity between 1000 mAh and 1800 mAh which were otherwise perfectly passing all my tests, and it seemed a shame to throw them away with the dead cells. The appearance of this second cluster led me to introduce a third category of “poor” cells, and I can think about what to do with them later.

The clustering in the cell capacity distribution is fascinating and caught me by surprise. It is still clearly visible in my histogram (currently 172 cells have been tested).