HighTechLab
AKA Dexter - CTO of Current Connected, LLC
- Joined
- Sep 23, 2019
- Messages
- 1,691
Does anyone have an exact reason why brand new perfectly flat cells slightly swell on their first charge? No guesses please, and please cite your source or whitepaper.
I have a theory but I am really trying to get to the bottom of this. I'll omit the theory for now and stick to the facts of this particular test.
I just took 4 brand new perfectly flat CATL grade A+ cells from a sequential production batch of a few hundred cells that I just received. These cells literally were perfectly flat in the beginning. I began charging them up without any fixture or compression. I left ~1/8" gap between cells for this test. I charged at a relatively low current, around 40a in 12v series. As I got closer to 3.4~3.5v the cells were NOT flat anymore, and between two cells they were now nearly touching. I hooked up to the electronic load/precision shunt and as we have been doing this c/5 discharge test they have slowly but surely been returning to their original flat state.
From my experience, it is best practice to keep cells flat from the very beginning by using the 300kgf fixture (or as close to this as possible). Then the cells never have a chance to move/grow. Many suggestions lately have been that the compression is pointless and only prolong life cycles.
This is very different from the other sequential production batch of CALB L173F230 230ah cells, which also arrived PERFECTLY flat, but even at 100% SOC & floating for 4 hours at 3.65v, never showed the slightest growth.
Are there any studies that cover the growth/expansion of even perfectly new grade A cells anyone has shared lately? Are there any battery chemists out there in the crowd being buried by the guesses and uninformed knowledge floating around the internet?
I have a theory but I am really trying to get to the bottom of this. I'll omit the theory for now and stick to the facts of this particular test.
I just took 4 brand new perfectly flat CATL grade A+ cells from a sequential production batch of a few hundred cells that I just received. These cells literally were perfectly flat in the beginning. I began charging them up without any fixture or compression. I left ~1/8" gap between cells for this test. I charged at a relatively low current, around 40a in 12v series. As I got closer to 3.4~3.5v the cells were NOT flat anymore, and between two cells they were now nearly touching. I hooked up to the electronic load/precision shunt and as we have been doing this c/5 discharge test they have slowly but surely been returning to their original flat state.
From my experience, it is best practice to keep cells flat from the very beginning by using the 300kgf fixture (or as close to this as possible). Then the cells never have a chance to move/grow. Many suggestions lately have been that the compression is pointless and only prolong life cycles.
This is very different from the other sequential production batch of CALB L173F230 230ah cells, which also arrived PERFECTLY flat, but even at 100% SOC & floating for 4 hours at 3.65v, never showed the slightest growth.
Are there any studies that cover the growth/expansion of even perfectly new grade A cells anyone has shared lately? Are there any battery chemists out there in the crowd being buried by the guesses and uninformed knowledge floating around the internet?