LiFePO4 Capacity vs Temperature

Just John

Photon Sorcerer
Joined
Aug 15, 2020
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I'm wondering how much cell capacity is affected by cells being just a bit cooler. Here's my story:

In early Aug-2021, I received 8 Eve 230Ah cells, purchased via Michael B Caro (I was on one the lucky ones). I purchased one of the EBC-A40L testers (this one) and went about testing each cell. The tests were done in our basement, and although it was still pretty hot outside, the A/C kept the basement pretty cool (too cool for my wife's taste) at around 65°F-68°F. Of the 8 cells, one tested out at 237Ah, and the rest all tested at 238Ah or 239Ah. Obviously I was pleased.

Last week I received 8 Eve 280Ah cells, purchased from the famous @Amy Wan (Luyuan) at Shenzhen Luyuan. These were the new Eve LF280k cells that are supposed to have up to 6,000 cycles. Amy provides a report as the cells are shipped, and the report includes a cell capacity for each cell, although I know nothing about how the capacity is determined. The report indicated the capacities were 290.3, 292.0, 290.7, 288.5, 290.8, 289.5, 291.7, and 291.2. All good, with an average capacity of 290.6.

I used the same tester to do the capacity testing. I set it up for each cell with a multi-step program, so it would charge the cell to 3.65V at 25A, stopping the charge when the current dropped to 2.0A. Then the second step would start, discharging at 25A until the voltage reached 2.5V. I'm pretty sure this was the same process I used for the 230Ah cells I tested in the summer.

The test results were somewhat disappointing: 278.5, 284.0, 282.0, 275.7, 278.7, 281.8, 282.5, 283.5, 280.8 That's three cells that didn't even meet the 280Ah spec, and an average of only 280.8Ah, less than Amy's report by 9.8Ah.

I'll admit the results are not horrible, and I'm not likely to raise a stink. The person I'm building the two 4S packs for is perfectly happy with them. However, it is disappointing, and - given Amy's reputation here - pretty surprising. So I tried to figure out what could have somehow biased the test. It was the same tester and the same test methodology, so I don't think that's it. Then I realized that the spec for the LF280k states that the test conditions are at 25°C (77°F). Although it is December, here in the Denver area the daytime highs have been generally in the 50°s while I was doing the test, and our garage has been in the mid- to high-50°s. I'm guessing it may have gotten down into the mid-40°s at night, and I did run some of the tests overnight, since each test takes over 11 hours.

So here's the question: Can capacity testing at say 50°F - lower than the standard test conditions by 27°F - result in the capacity showing 10Ah lower? Can anyone think of something else that could have caused these results?
I have documented in my testing 5 to 7 amp hours difference in capacity at about 60 degrees versus 70.
 
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