Test 18650 cell capacity at different rates: what's the optimum rate of discharge?

Today I'll be testing the LG 3350mAh 18650 cells at different rates: 200mA, 300mA, 500mA, 700mA, and 1A. I want to find out which rate is the most accurate, yields the best result, and at the same time takes the least amount of time. Warning: this contains graphic material. Tread with caution.

[Alex-usvw]

Discharging slow or fast is not anything like driving a car fast or slow. It all depends on the battery, some are meant for high (fast) power draw, others are more suited to slower usage. But all will provide a 1 amp draw no problem, testing 18650 at 0.2amp or even 0.5amps is kinda crazy, that’s for testing much smaller li-ion batteries like 14500 or even AA/AAA NI-MH cells.

[dillonmccormick]

Nice scene with the dead rat and maggots. Exactly what I was looking for in a 18650 capacity testing video.

[MrMaxeemum]

You should have discharged them all at the same rate in the same unit and in the same slot for a base result then repeated at different rates and produced the results in a percentage of the original base result. Your theory of discharging faster giving less capacity should be true as a higher discharge rate will produce more heat / wasted energy.

[mahdixn]

the issue with that test is that you are testing different cells, even if you discharge the at the same rate, you'll get different results.
You need to take the same battery and test it with different discharge rates, sure it's going to take longer but it makes more sense.

[Zain.Shikari]

It's all about cell internal resistance. Means, that cell gives you more discharge time who having more internal resistance. Sometimes people get confused when they noticed that same capacity holder sells gives different discharge time. You should check internal resistance of those cells. For example you have two 2000 MAh cells. On same load . One cell discharged in 10 minutes and other cell discharged 1 hour, it means that faster discharging cell has less internal resistance, and other cell has high internal resistance . Thank you.

[marbesky]

I opened this charger today and I noticed when charging 4 cells at 1 amp the cells, springs and motherboard inside gets very hot and that fan is pretty much useless in cooling the whole unit so I mounted a bigger fan right underneath in the middle of unit extracting air thru the cells on the top and makes a big difference

[jasonbrown]

i appreciate the video, had you considered redoing it so that all batteries get shuffled through the different discharge rates? i would like the know if your lowest capacity battery maintains the lowest rating through all the tests, or if there is something else going on.

[rb-ex]

your intuition and experience is generally correct that bigger loads reduce capacity. this is because more heat is usually generated by higher loads. the trouble with this test is that due to equipment limitations the discharge loads are too small to generate the kind of heat you get from real loads, and in the range of testing, you are probably in the flattish part of a convex temperature curve. (by 'convex' i mean it increases at an increasing rate). with temperature rising very slowly from 200-800mAH, total thermal energy released (measured in an energy unit like calories) was largely a function of the time the cell spent in discharge rather than the discharge rate, so you saw increased capacity up to 800mAH because you had big reductions in time spent in discharge and only small increases in temperature. then you had a little drop in capacity at 1000mAH because the temperature curve is moving into a steeper slope here, enough for energy released as heat to overcome the shorter discharge time. if you tested the cells at increasingly higher discharge rates above 1000mAH, e.g, 2A, 4A, 6A, 8A, 10A, you would get into steeper portions of the temperature curve, and i predict you would find the trend you expected. but in the low range your equipment can test you're seeing this unexpected effect due to convexity of the temperature curve.

[ErusNotitia]

I used to drive for a living, and my understanding is that gas mileage is determined more by revolutions per minute (rpm’s) than by speed. There are sweet spots at different gears, that if one drives at those rpm’s in those gears, they will burn less fuel, also a sharp increase in acceleration, especially on an incline, will burn a lot of fuel. So, an increase in speed doesn’t necessarily burn more fuel.

[thecelldoctor]

Different cells are built for different tasks. The best way to choose the current for discharging is by reading the datasheet to understand what is the rated discharge rate. We are doing a giveaway for 4 megacell chargers for testing cells in case you want to do more automated testing for multiple cycling etc

[thonatim]

In order to have accurate data, you will need to test all 5 batteries in all discharge conditions. Then you can compare each battery to each other based on the same discharge rate. Only then can you determine what discharge rate provides the highest capacity. You are assuming each battery has the exact same capacity but you are only guessing. Peukert is your friend.

[Chaosfury]

Every battery has an internal resistance which could even vary depending on how much charge it has and thus the slowest one could have been discharging at a resistance just high enough to see those results

[keokio]

what you could've done is retest again, take the 200ma discharge rate battery and change it to fastest 1amp and see if you get more capacity. Same with 1amp discharge rate battery and discharge it at 200ma and see if capacity is lower. Would tell you if its the battery or discharge rate that is affecting your capacity

how to test 18650 battery total capacity without lowering the cut off voltage too much?

[freedomunrestricted]

You would have to test the same cell multiple times at each discharge rate to get a more accurate average. Testing the same cell twice at the same discharge rate will give slightly different results. Comparing one cell discharged at 200mAh to one discharged at 1000mAh isn't a very reliable test.

[robertmotion]

Great video.
Question - can you recommend a lithium battery C-rate tester to purchase.
I build ebike batteries and need a solid c-rate tester. thanks.

/Bklyn👑

[felonebike]

Dude great video! Why can't other people make it so simple? Now I have to check your other videos out!

[Ziplock]

After capacity testing, do you charge back up to nominal voltage for storage?

[LeeFall]

The voltage cut off for this charger is 2.8 volts but cut off for most 18650 is 2.5 so you lose a bit there.

[gustavinus]

During the 15 hours of slow draining you also had more passive loss of energy. Like when you leave a cell unplugged and it slowly loses charge.