A test on Self Discharge of 18650

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Budda
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A test on Self Discharge of 18650

As many other flashlights addicted, I worried about li-ion self discharge.
In the literature there are different tables that show certain rate of self discharge. However, as the time progresses, technology and chemistry changes and the same cell model bought 1 year later could be very different from the one bought today.

According to a table present on batteryuniversity, the self discharge ratio at 25° is about 20% for each month.

So I decided to test it for myself:
1) Take some cells, test them under the same conditions to assure they are working fine and are very similar.
2) Fully charge them and store them at RT in an open container.
3) Measure the voltage every few days for each cell
4) If the voltage drop isn’t something massive (for example after a month it drops from 4.2 to 3.5volts), and the resting voltage is not always so accurate for indicating the actual energy left in the cells (it is an estimation that differs from chemistry to chemistry), I will discharge them at the same conditions to obtain a more accurate representation of the capacity.
This will be done periodically, so I will have data from a bigger timespan, I hope.

Please note: I use a SKYRC MC3000 with firmware 1.11 both for charging (li-ion algorithm, 1 A charge) and discharging (li-ion algorithm, 2 A discharge cutoff at 2.8 Volts). Why the cutoff at 2.8 when the cells can safely be discharged more?
A) Discharging them at the lowest Voltage recommended by the producer should give me the rated capacity… however I am interested in using them in flashlights that don’t usually work around voltages lower than 2.7-2.8 volts.
B) Because there is a lot of testing present in literature where the curves stop a 2.8 volts
C) Because I have done a lot of testing (more than 50 tests) with li ion and that is now my standard.
D) Discharging them at the lowest Voltage recommended by the producer should give me the rated capacity… however I am interested in using them in flashlights that don’t usually work around voltages lower than 2.7-2.8 volts.

I knew that the result from the discharge test are precise from a previous test: I discharged the same cell in different days with the same discharge currents, and the result are consistent.


Of course, I am not pretending exploit differences of few mAh.
I am running this test to see if there is a substantial change. I am in the shoes of Mr LG or Mr Panasonic, where 1% of difference means millions of dollars.
All I want to know if that there is some difference that will concern flashlight guy.
The test could be better if I tested 10 cells for each model, and even better If I tested 50 cells for each model. However, this is what I got at the moment.

Also, If I could I would do the discharge test at higher current to really push the cells and hopefully maximize the differences making them visible. However, the SKYRC MC3000 has a max discharge rate of 2A.
So, let’s see how this goes.

1) I took 4 slightly used LG MJ1 (with the same age) and 2 practically new Panasonic NCR18650B (with the same age).
I tested all of them with SKYRC MC3000 and showed no issues.

2) I charged them and let them rest inside an open battery carrier, kept at room temperature.

3) Then I measured the voltage every few days for all cells.


After a few days the voltage dropped and reached a plateau for both kind of cells

After 34 days I remembered about this test and discharged the first cell, the LG MJ1 1.


According to this test the battery has lost 2,7% of the initial charge at a 2A discharge current.
The test will continue.

All my reviews, in italian and english, here: Lumenreviews.com

Budda
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Budda
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Last seen: 12 hours 17 min ago
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Location: Italy

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All my reviews, in italian and english, here: Lumenreviews.com