18650 battery

[evthunder]

Hey I was wondering how long you let your 18650 batteries sit on the shelf to test the self discharge. Also how much of a drop in voltage you allow before calling a cell bad. For example I had a batch of cells tested and charged to 4.2 volts. They were left for a week and after that week they had all dropped by a bout 2 to 3 10ths of a volt. would you still keep these cells?

[Daixiwen]

The self discharge is higher at high charge levels so it is better to test it at about 50 % state of charge.
IIRC in the cell factory I used to work at, after the initial cycle they were charged to 3.7V and stored for 48 hours at 25 °C. The measured voltage drop was then something between 8 and 12 mV. Those were brand new cells of course, and used cells will discharge a bit faster.

[beanflying]

From 4.2 they will drop off a bit initially but then that rate slows a lot. I have some LiPo's that have been uncharged for years and still sit close to where I left them for storage generally 3.6-3.7V. For R/C use (non contest) charge the day before use at most or generally a top off on the day is still a good idea for peak performance.

Have a good read here https://batteryuniversity.com/learn/article/elevating_self_discharge

Also do your cells have BMS boards fitted it would explain a higher discharge?

As to keeping them the best test is a load of some sort and look at capacity and voltage while that happens as a proper guide to health. There is some good low cost options for logging that some of them you will find here https://rdtech.aliexpress.com/store/923042 I like the bluetooth one I have as part of my kit in spite of having some much better gear it works really well.

[mvas]

Hey I was wondering how long you let your 18650 batteries sit on the shelf to test the self discharge. Also how much of a drop in voltage you allow before calling a cell bad. For example I had a batch of cells tested and charged to 4.2 volts. They were left for a week and after that week they had all dropped by a bout 2 to 3 10ths of a volt. would you still keep these cells?

I store my batteries at 50% State-of-Charge, not 100% SOC.
I charge them just before I need them, I use them, and then put them back on the shelf = Longer Life

[Siwastaja]

Note that after suddenly stopping a charging current (by removing the cell from a charger), initially the voltage drops a little bit, this phenomenon is called "surface charge", I don't know if it's a scientifically valid name or not, but anyway it exists.

You can see how the voltage drops considerably during the first second or two after removing from the charger, but this goes on possibly for hours. So if you want reliable self-discharge measurements, you should wait to make your "initial" measurement, I'd recommend going as far as waiting 24 hours to be sure.

Self-discharge itself is minute enough that measuring it requires long waiting times. I studied this extensively, and my waiting period was 1.5 years. It paid back, since I was actually able to properly quantify the self-discharge of fully charged cells stored at room temp. Had I tried to do it over just a few months, I would only have read some noise due to too little self-discharge. Any measurement method will have noise.

The self-discharges in my tests varied between 0% and 15% per year, depending on cell brand, storage temperature, and SoC. 2-6% per year is fairly typical range for a fully charged (4.20V) cell at room temp (23 degC). Cells at 50% SoC have almost no leakage at room temperature. In most cases, it was impossible to measure at all with 1.5 year waiting period, so most of my self-discharge data is from the 80% and 100% SoC samples.

Sony cells were the best with lowest self-discharge.

You could measure the voltage every week during a month to see if the leakage continues, or if it was just the after-charge surface charge dissipating.
If you they leak more than about 1-2% per month at room temp, fully charged, they have likely gone very bad.