thegoogler
New Member
- Joined
- Apr 15, 2021
- Messages
- 108
I've seen a few posts about cell Ah ratings from various vendors, and whether they were true or if the vendors were stretching the truth. I wanted to share what I've found over the last couple months, in the hopes it might be useful to someone. The specs below are specific to CATL but I assume the energy densities etc are pretty generally applied across most of the prismatic cells out there.
AH ratings
From my research, sometimes cells which the manufacturer rates at 1C discharge rate will show up with a higher Ah with a lower (i.e. 0.5C) discharge rate on Alibaba/AliExpress. Also from my understanding you might actually get marginally higher Ah (+1% to 3%) out of brand new cells but after a couple cycles they will settle at slightly lower Ah. They may also be counting on people testing with equipment that can't discharge at 0.5C or 1C (anyone have a 400A heat sink handy?) so if you're discharging at 10-20A you might very well get a few extra Ah out of them at a lower discharge rate
Likely most grade A well matched commercial cells end up ~3% over listed capacity (kind of like buying 1# of bacon and finding it's actually 16.04oz), so I'm not shocked to see a major vendor like CATL sell a cell as 271Ah but then a vendor turn around and resell it as a 280Ah cell. For CATL they would rather guarantee 100% performance ratings at 271Ah than 98% at 280Ah with 2% of their cells not "making the grade". OTOH a secondary vendor who tests and rebrands the cells might find they actually get a bit more performance (Ah) out of the cells and can sell them as such. Or the secondary vendor might be counting on that 103% performance and not actually test. Or the secondary vendor might just be flat out lying because EVE sells 280Ah and they want you to find their 271Ah cells when you search for 280Ah and who knows they might end up being close to 280Ah anyway. In the US the FTC would step in and stop this behavior but it seems like in China this isn't monitored as closely.
Energy Density
When I was searching for the specs of my CATL 271 cells I saw a PDF which had all the CATL cells as of ~July 2020 and their respective energy densities. Interestingly, dimensions are NOT directly related to capacity. I do not know what the internals of these cells look like, but there is something going on (thinner walls? different chemical composition?) which allows for high Ah ratings without a linear change in cell size.
From https://rreport.einfomax.co.kr/report/ezmzkzzceccieilgcgccqgz.pdf. Note 6LH3L8 is the 271Ah battery (867Wh)
I found this data which shows the energy density all the way up to the 302Ah cells and it seems pretty accurate at http://www.iccsino.com/news/show-htm-itemid-13762.html:
And this (translated from Spanish) at http://www.poweroadbatt.com/lithium...ell/3-2v-lifepo4-cell-with-aluminum-case.html. Note that if you look at the total volume of the 100->125->150 cell (W x H x T) you'll see that the mm3/Ah (# of cubic mm required for 1 Ah) declines over time. The same thing happens with the 240 -> 271 -> 302 cells (the latter which aren't listed here in dimensions but apparently similar to the 271Ah cells)
Thus far I've not found any evidence of CATL producing anything larger than 302Ah. I would assume any vendor selling 310Ah CATL are stretching the truth. If I find any info on larger CATL cells I'll post them here.
I did find this doc which appears to be an actual CATL spec sheet for their 302Ah battery at
Note one really interesting thing that sheet notes is 3 separate BMS cut-off limits for OV/UV (3.65/3.8/4.0 and 2.5/2.0/1.8), which makes me think despite the BMS cut off's we use, the cells can likely (very temporarily) handle one or two cycles outside of those limits. Not that I'd try to charge/discharge outside of 2.5 to 3.65V, but it seems like 1.8V to 4.0V won't immediately and irrevocably destroy them.
Another interesting thing is that they say the BMS should stop charging after a max of 8 hours. I don't know how much rest time is expected between charges, but that does make me wonder (a) how does starting with a low charge state and a relatively low C rate on a large battery bank which is getting 12+ hours of solar come into play, and (b) the top-balance recommendation of "charge to 3.4V, rest, charge to 3.5V, rest, top off to 3.65V" would seem to make a lot of sense now. It does also say "stop charging at 3.65V and <= 0.05C rate"... I'm not sure what that means for those of us who top-balance or have small-ish solar configs where we typically charge at <=0.05C but that's probably best for a separate discussion thread.
AH ratings
From my research, sometimes cells which the manufacturer rates at 1C discharge rate will show up with a higher Ah with a lower (i.e. 0.5C) discharge rate on Alibaba/AliExpress. Also from my understanding you might actually get marginally higher Ah (+1% to 3%) out of brand new cells but after a couple cycles they will settle at slightly lower Ah. They may also be counting on people testing with equipment that can't discharge at 0.5C or 1C (anyone have a 400A heat sink handy?) so if you're discharging at 10-20A you might very well get a few extra Ah out of them at a lower discharge rate
Likely most grade A well matched commercial cells end up ~3% over listed capacity (kind of like buying 1# of bacon and finding it's actually 16.04oz), so I'm not shocked to see a major vendor like CATL sell a cell as 271Ah but then a vendor turn around and resell it as a 280Ah cell. For CATL they would rather guarantee 100% performance ratings at 271Ah than 98% at 280Ah with 2% of their cells not "making the grade". OTOH a secondary vendor who tests and rebrands the cells might find they actually get a bit more performance (Ah) out of the cells and can sell them as such. Or the secondary vendor might be counting on that 103% performance and not actually test. Or the secondary vendor might just be flat out lying because EVE sells 280Ah and they want you to find their 271Ah cells when you search for 280Ah and who knows they might end up being close to 280Ah anyway. In the US the FTC would step in and stop this behavior but it seems like in China this isn't monitored as closely.
Energy Density
When I was searching for the specs of my CATL 271 cells I saw a PDF which had all the CATL cells as of ~July 2020 and their respective energy densities. Interestingly, dimensions are NOT directly related to capacity. I do not know what the internals of these cells look like, but there is something going on (thinner walls? different chemical composition?) which allows for high Ah ratings without a linear change in cell size.
From https://rreport.einfomax.co.kr/report/ezmzkzzceccieilgcgccqgz.pdf. Note 6LH3L8 is the 271Ah battery (867Wh)
I found this data which shows the energy density all the way up to the 302Ah cells and it seems pretty accurate at http://www.iccsino.com/news/show-htm-itemid-13762.html:
And this (translated from Spanish) at http://www.poweroadbatt.com/lithium...ell/3-2v-lifepo4-cell-with-aluminum-case.html. Note that if you look at the total volume of the 100->125->150 cell (W x H x T) you'll see that the mm3/Ah (# of cubic mm required for 1 Ah) declines over time. The same thing happens with the 240 -> 271 -> 302 cells (the latter which aren't listed here in dimensions but apparently similar to the 271Ah cells)
Thus far I've not found any evidence of CATL producing anything larger than 302Ah. I would assume any vendor selling 310Ah CATL are stretching the truth. If I find any info on larger CATL cells I'll post them here.
I did find this doc which appears to be an actual CATL spec sheet for their 302Ah battery at
Note one really interesting thing that sheet notes is 3 separate BMS cut-off limits for OV/UV (3.65/3.8/4.0 and 2.5/2.0/1.8), which makes me think despite the BMS cut off's we use, the cells can likely (very temporarily) handle one or two cycles outside of those limits. Not that I'd try to charge/discharge outside of 2.5 to 3.65V, but it seems like 1.8V to 4.0V won't immediately and irrevocably destroy them.
Another interesting thing is that they say the BMS should stop charging after a max of 8 hours. I don't know how much rest time is expected between charges, but that does make me wonder (a) how does starting with a low charge state and a relatively low C rate on a large battery bank which is getting 12+ hours of solar come into play, and (b) the top-balance recommendation of "charge to 3.4V, rest, charge to 3.5V, rest, top off to 3.65V" would seem to make a lot of sense now. It does also say "stop charging at 3.65V and <= 0.05C rate"... I'm not sure what that means for those of us who top-balance or have small-ish solar configs where we typically charge at <=0.05C but that's probably best for a separate discussion thread.
