Li Ion Float Charging - Final Results for Crappy Cells after 500 Days

I started float charging low quality Li ion cells to ensure my testing process was appropriate. What I found was the need for a "control" cell and recording of the date of measurement were needed. With that change a test began on a brand new quality cell with a second cell acting as a control.

However, curiosity on my part meant I kept the first batch of low quality and recovered cells on a float charge. Those sacrificial cells have now been on float charge for up to 500 days. During that period ad-hoc testing of cell capacity took place. 

The results are clear for these sacrificial cells. Something happened 12 months ago that I didn't notice but resulted in all but one cell starting to deteriorate, regardless of how long they had been in the testing process.

Battery B

 

Run 27 is around the start of Summer 2022. Up until then no adverse impact of float charging had been noticed.

Battery D

Either the deterioration started 12 months ago and test 18 is an aberration, or this cell took a little longer before deteriorating. There was some conjecture at the 180 day mark that this was a newer, but still rubbish, cell.

Battery E

Right at the outset this "pouch" cell was deteriorating. Cheap android tablets must have cheap batteries!

Battery F

The second pouch cell recovered from the cheap android tablet.

 

Battery C

12 months ago I appear to have accidentally swapped out the test cell for a different one. This cell has not deteriorated in the last 12 months! On the other hand, it was a really bad cell (120mAh) so it was hard to deteriorate from such a low value any further.

 

Conclusions

It is unclear what triggered the deterioration. 

• A change in the measuring equipment over time?
• Hot summer weather?

Battery C uses the same test apparatus and since the "new" battery C shows no deterioration that suggests the measuring equipment has not been the cause of the shift. 

The end of Summer just prior to this 12 month point had been hot, however the summer just finished was hotter still. Battery B showed no signs of determination over the earlier summer, nor did the "new" Battery C  deteriorate over the more recent still hotter summer.

I suspect it is a case of garbage in garbage out. All of these sacrificial cells were considered rubbish to begin with. As such, these cells were used purely to refine the testing process and they have served their purpose well. 

It's about time I built some automated testing equipment with a detailed log.