stienman
Mostly Harmless
- Joined
- Jan 6, 2021
- Messages
- 476
Compression affects lithium based cells. There are a number of mechanisms associated with compression, as each component of the cell is affected and must be taken into account separately and together.
This forum has many, many, many discussions about compression, and this isn't the first argument over whether a rigid method is better than a compliant method.
For those who really want to understand, here's a number of research papers that discuss the mechanisms that cause capacity degradation which can be reduced with mechanical compression: https://scholar.google.com/scholar?...apacity+degradation+lithium+compression&btnG=
If you choose to continue to assert your beliefs as true about why compression works and why your specific method is better/worse than another persons, please provide references, either to manufacturer's data sheets/application notes/etc or to published research.
Otherwise it's just a bunch of opinions based on second or third hand opinions from others, and you should be comfortable stating as such, with "I believe..." or "My anecdotal evidence suggests..." or "My opinion is...".
And for those who care, my opinion is:
All that said, I suspect we're spending a lot more time, individually, on this topic than it warrants. People who are using "rigid" mechanisms are probably compliant enough for their small group of cells. I haven't yet seen anyone on here show or describe a truly rigid mechanism.
Compress your cells, not too much, and move on.
This forum has many, many, many discussions about compression, and this isn't the first argument over whether a rigid method is better than a compliant method.
For those who really want to understand, here's a number of research papers that discuss the mechanisms that cause capacity degradation which can be reduced with mechanical compression: https://scholar.google.com/scholar?...apacity+degradation+lithium+compression&btnG=
If you choose to continue to assert your beliefs as true about why compression works and why your specific method is better/worse than another persons, please provide references, either to manufacturer's data sheets/application notes/etc or to published research.
Otherwise it's just a bunch of opinions based on second or third hand opinions from others, and you should be comfortable stating as such, with "I believe..." or "My anecdotal evidence suggests..." or "My opinion is...".
And for those who care, my opinion is:
I throw my lot in with those favoring compliant methods of compression.
As far as the pouch vs prismatic cell debate - prismatic cells are halfway between cylindrical and pouch cells. Cylindrical cells do expand, but are still under compression due to their mechanical construction. They don't need external compression.
Pouch cells are unconstrained, and may develop gaps between the materials in the cell which cause degradation. While the effect is significant under stress - such as the formation of gas bubbles due to electrolyte evaporation due to heat - this degradation will still happen under normal use over time, just on a smaller scale. There are many other aspects beside gas bubbles, that's just one example. Compression is absolutely necessary for pouch cells, as normal movement during charge/discharge will cause separation between materials in the pouch. Pressure also resists the state change from liquid to gas within the compressed area.
Prismatic cells are like cylindrical cells on the ends, and like pouch cells in the middle. The compression is NECESSARY on the larger faces for the same exact reasons you must compress pouch cells. The center of the cell is the only place where it's useful - the two edges of the cells on the large faces don't matter as much, and the top, sides, and bottom don't matter at all in terms of compression.
The only datasheet from a manufacturer I've seen regarding compression is EVE's 280AH cell, and compression is given in kgf - kilograms force. This is an old, outdated standard unit, but it is directly convertible to pressure given the size of the footprint. It was commonly used in applications involving spring force - such as bows, line tension, and, of course, springs.
After wading through these forums and the available research I've decided that for safety and cycle life, I'll compress to around 12psi at half charged state using either springs (ie, straps with known elasticity) or foam. Since I have to put something between each cell anyway, foam solves two issues at once. Given battery compression specific foams, this becomes trivial as I can dimension everything and not have to calibrate or measure beyond simple length measurements.
I've concluded that compression isn't just a cycle life issue, but a safety issue. Uncompressed, gas bubbles may form, and that may result in hotspots that eventually could contribute to thermal runaway even under normal conditions. Overcompressed, the mechanical damage that occurs may damage the separator in a cell, allowing high current discharge and thermal runaway. As I'd rather suffer the slower thermal runaway of gas bubble hotspots than the fast runaway of anode and cathode contact, I prefer to tend toward undercompression. However, with the appropriate foam or spring mechanism, it's easy enough to maintain the correct pressure throughout the charge/discharge cycle that I believe the maximum safety and cycle life will occur under compliant tension. A significant impact such as a hammer hit is just a localized overcompression - if the separator fails, you get thermal runaway.
All that said, while many think of threaded rods and other constructions as rigid, in the world of 300kgf forces, they are surprisingly springy. I expect that as long as you account for maximum cell expansion and avoid significant overcompression at the extreme end, then you're going to be just fine. The forces are so large, and the expansion so small, that a common threaded rod will expand as well, and the pressure might not become too great.
Now if your design involves six 1" hardened steel rods and 16 batteries in a row, then I'd suggest you're risking a bit too much.
As far as the pouch vs prismatic cell debate - prismatic cells are halfway between cylindrical and pouch cells. Cylindrical cells do expand, but are still under compression due to their mechanical construction. They don't need external compression.
Pouch cells are unconstrained, and may develop gaps between the materials in the cell which cause degradation. While the effect is significant under stress - such as the formation of gas bubbles due to electrolyte evaporation due to heat - this degradation will still happen under normal use over time, just on a smaller scale. There are many other aspects beside gas bubbles, that's just one example. Compression is absolutely necessary for pouch cells, as normal movement during charge/discharge will cause separation between materials in the pouch. Pressure also resists the state change from liquid to gas within the compressed area.
Prismatic cells are like cylindrical cells on the ends, and like pouch cells in the middle. The compression is NECESSARY on the larger faces for the same exact reasons you must compress pouch cells. The center of the cell is the only place where it's useful - the two edges of the cells on the large faces don't matter as much, and the top, sides, and bottom don't matter at all in terms of compression.
The only datasheet from a manufacturer I've seen regarding compression is EVE's 280AH cell, and compression is given in kgf - kilograms force. This is an old, outdated standard unit, but it is directly convertible to pressure given the size of the footprint. It was commonly used in applications involving spring force - such as bows, line tension, and, of course, springs.
After wading through these forums and the available research I've decided that for safety and cycle life, I'll compress to around 12psi at half charged state using either springs (ie, straps with known elasticity) or foam. Since I have to put something between each cell anyway, foam solves two issues at once. Given battery compression specific foams, this becomes trivial as I can dimension everything and not have to calibrate or measure beyond simple length measurements.
I've concluded that compression isn't just a cycle life issue, but a safety issue. Uncompressed, gas bubbles may form, and that may result in hotspots that eventually could contribute to thermal runaway even under normal conditions. Overcompressed, the mechanical damage that occurs may damage the separator in a cell, allowing high current discharge and thermal runaway. As I'd rather suffer the slower thermal runaway of gas bubble hotspots than the fast runaway of anode and cathode contact, I prefer to tend toward undercompression. However, with the appropriate foam or spring mechanism, it's easy enough to maintain the correct pressure throughout the charge/discharge cycle that I believe the maximum safety and cycle life will occur under compliant tension. A significant impact such as a hammer hit is just a localized overcompression - if the separator fails, you get thermal runaway.
All that said, while many think of threaded rods and other constructions as rigid, in the world of 300kgf forces, they are surprisingly springy. I expect that as long as you account for maximum cell expansion and avoid significant overcompression at the extreme end, then you're going to be just fine. The forces are so large, and the expansion so small, that a common threaded rod will expand as well, and the pressure might not become too great.
Now if your design involves six 1" hardened steel rods and 16 batteries in a row, then I'd suggest you're risking a bit too much.
All that said, I suspect we're spending a lot more time, individually, on this topic than it warrants. People who are using "rigid" mechanisms are probably compliant enough for their small group of cells. I haven't yet seen anyone on here show or describe a truly rigid mechanism.
Compress your cells, not too much, and move on.
