Wednesday, March 11, 2020

battery testing 3

Retesting after 6 months of daily use shows ~10% decline in capacity of my 18650 lithium rechargeable cells.

Last summer I tested three new 2200mAh cells. Initial average capacity was about 2030 mAh with my test which may or may not be accurate. After six months in service, I repeated the test with the same tester and method and found average cell capacity was now about 1822 mAh, or a reduction in capacity of about 10%. Raw data is here.

I attribute the reduction in capacity to my daily usage of the batteries although there could be other causes (* below).

The squiggle (~ tilde) before the 10 means "about" and that is really the theme of this post. Often in amateur science and engineering we need an approximate answer to check a hypothesis or opinion. Some quick measurements, even if approximate, are better than no data at all. Hence the "about". Sometimes repeatability and consistency may be more important than absolute accuracy when we are trying to compare.

In the case of these batteries I had tested for capacity about six months ago. Since then I had used these batteries daily in a vape pipe. They seemed to be performing well. Typically I would switch a battery sometime mid-day putting the partially discharged battery back onto the charger and inserting one of the two other charged batteries into the pipe. So I knew two of the three batteries went through some sort of discharge/charge cycle daily determined by my vaping habit with was pretty steady during the six months. I tried to randomize which battery I did not use each day but wasn't entirely careful about that aspect.

Since I had not used these PKCELL 2200mAh batteries before, I was curious if there was an effect on battery capacity and how much it might be.

Why go to the trouble of testing? I paid about Cdn$15 each for these cells. Some have been junk. I am accumulating things which use 18650 cells and had other applications in mind so I will need to buy more. The best way to determine capacity is to measure with a controlled test.

Initially I had tested each battery three times. Three data points. I like three. With only two measurements, if something goes wrong or varies, it may not be apparent in the data. The two measurements will differ always. With three measurements, hopefully two will be more similar than the one which varied. Ideally all three would cluster tightly. More measurements would be even better but we are after an approximate answer so three is good.

I tested with the same ZHIYU ZB2L3 v2.3a tester and the same 7.5 ohm resistor.

There is a new version 3 of the tester which I have ordered.

[click any pic to enlarge]

I considered changing the load resistor. I am simply using one of the two resistors that came with the kit. Readers will notice that I changed the arrangement of the load for the second round of testing with auto batteries to create a heavier load. I'd like to more closely match the 1C rate that the manufacturer has used for determining the branded 2200mAh rate. But at this point that is less important to me than using the same test and load to compare batteries from different manufacturers or different batches from the same company.

I have recently done initial testing of two new batches of 18650 batteries: four more of the PKCELL 2200mAh and two bright pink Samsung cells, the best I've tested so far! All tests with three repeats each.

Thank you for your interest.

George Plhak
Lions Head, Ontario, Canada

* Other causes - I knew you'd ask. Hypothetical causes really, but possible. Like maybe the batteries have some manufacturing defect that causes them to loose capacity independent of charge/discharge? Or maybe I stored them in a hot car in the sun? I did not, with these batteries, but four similar cells are installed in two emergency flashlights in my cars. Not used at all but subject to some extremes in temperature. I might retest those cells another time.

[The next day] Another possibility why the measurements this time are lower. I had forgotten but was reminded by the photo I used above: I wrote that I was not happy with the battery holder and had ordered a different type. After the new holder arrived, I installed it with shorter, larger gauge wire as you can see in the pic. This is the tester and holder used for this series of tests, on the right.

So it is likely that the resistance of the output load circuit is now less particularly because of the shorter beefier wire. The holder contacts also look more substantial and are well soldered to the wires. Lower load resistance would increase current (the C rate) and reduce mAh measured. Would it be ~10%? I don't know. I should measure! That's the original battery holder and wires to the left.

I won't go back and retest with the thinner wire. But when I retest the latest cells, I will be comparing them to the results from this tester. That's the importance of consistency.

As is my custom, this is not a paid review nor do I receive or solicit any product in return for writing. I am not selling these products. Just ideas. I don't get a commission. I don't show ads. I don't have a fundraising or patreon page.
I write in the hope that this is interesting or useful to you.
If you would like to show support for my independent work,
please consider commenting or buying one of my books? Thank you. George

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