Annual Battery Capacity Test

mrbill

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Silicon Valley, California
I've written up a page that summarizes my annual battery capacity test on my e-bike batteries (Samsung INR18650-29E cells) that see no more than 50 cycles each per year. The main take-away from is that capacity declines most significantly during the first year of use, then declines more gradually thereafter. I intend to update this page annually.

http://mrbill.homeip.net/hybridBike.php#batteryCapacityTesting
 
Jon NCal said:
Wondering if the capacity tests were temperature controlled? ie done at standard 68/70 deg. F.

Hi Jon:

I performed the tests in my workshop where the temperature varied from a low of 10C to 17C. I took notes but didn't feel the ambient temperature was relevant since the batteries self-heat during the test. They got to about 30C on their exterior when the BMS cut off the current.
 
For an accurate comparison the internal cells need to start at the same temperature, suggest standard 68 deg. F.(20d C) Otherwise difficult to say how much variation is due to temperature.
 
Jon NCal said:
For an accurate comparison the internal cells need to start at the same temperature, suggest standard 68 deg. F.(20d C) Otherwise difficult to say how much variation is due to temperature.

I'm testing the pack as I'd use it on the bike. Each battery was tested the same way on my workbench and self-warmed during discharge to about 30C at the end of the test. Ambient temperature was usually around 15C year to year. Although I didn't specifically control temperature, I tested under similar conditions each year.

My notes for 2016 have ambient temperature of 15C for S1, 14C for S2, 14C for S3, 15C for S4, 17C for S5 and 18C for S6. In 2015 ambient temperature was slightly lower, 11C for S1, 11C for S2, 13C for S3, 11C for S4, 14C for S5 and 13C for S6.

The tests ran over a number of days. So, it's possible that S5 and S6 benefitted from being tested at slightly warmer ambient temperature in both 2015 and 2016.

I purposely do this test in winter when ambient temperature is lower because I want a pessimistic estimate. That way I can expect to get similar capacity in summer when temperatures are warmer and the pack a little older.

One thing I did notice was that on most of the packs cell 7 (the most positive) was consistently higher voltage than the other cells, and that cell 6 was consistently lower.
 
A few years later.. It seems like your batteries have lost no capacity at all after the first year 🤔 Have you done any tests on internal resistance?
 
Hi mighty82:

I haven't tested IR under more controlled conditions, but I have observed the IR stat on my CA3 using various configurations. I find that figure varies enough with temperature and state of charge that it will swamp any change due to age.

The only thing that seems certain to me is that I lost capacity in the first year, then not much thereafter. The variation in the data from 2016 to 2019 could be noise. I should add that I don't cycle these batteries daily. More like once per week. And I store them at 50-60% charge, charging to 3.7v/cell prior to letting them sit idle. So, I'm now around 250 95% to 10% cycles on them. Still pretty young as far as cycle life goes.
 
Okay. I don't give my batteries enough cycles to cause a lot of capacity loss either. But I have noticed increased IR with resulting voltage sag after a few years.

Also when removing cells from old laptop batteries etc they are ofte close to their rated capacity but with very high ir. Could be because of prolonged heat during use.

It sounds like you are doing everything right if you want to make your batteries last a long time :thumb:

My primary Lifepo4 pack on the other hand has lost a lot of capacity in 6 years. Even with what equals to less than 100 full cycles. The 20ah pack now only has 12ah. That is probably because of 1 or 2 bad cells though.
 
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