18650 discharge test for mAh rating

[ag273n]

i now have a SkyRC D100 charger and i want to test the 18650 cells i collected for their mAh rating. I already have them all fully charged. and i just tried the discharge option in the charger. started first with 1A discharge rate on 1 cell - it suddenly stoped after about 20 seconds and it says its done - it showed me 9mAh so i pulled out the battery to measure with a multimeter, and it showed 4.10V - so almost nothing really happened on the discharge test.
i tried it again on 500mA discharge - this lasted for a minute and it showed me 37mAh. i measured again the battery with a multimeter and found 3.89V.... goodness gracious....
should I use 100mA discharge? whats a good Amperage to use for discharge tests?.

 

[amberwolf]

Well, if you want to know what their actual capacity is at their manufacturer ratings, you have to test them the same way they did. But this is not very useful to know, as it doesn't help you know how they will perform in your EV.

But if you want to know what you're actually going to get out of them, you have to test them at the rates you're going to use them at.

Most likely, given the results you've already listed, they're either recycled junk/garbage cells (like ultrafire/surefire/etc use), with high internal resistance, low actual capacity, etc., or they are simply very low rate cells, and unsuitable for powering motors/etc unless you parallell quite a lot of them together (up to a few dozen in each parallel group. perhaps, if you need high current out of the pack).


Most of the low-rate and/or recycled junk cells require very large and heavy packs to supply current without extreme voltage sag and heat generation. They will usually be a lot larger and heavier than packs made from high-rate cells (though the high rate cells may have less capacity than a similar low-rate cell, they will be able to deliver that capacity at the higher rate, while the low-rate cells can't).

If you read around the many 18650 build and test threads there is plenty of information to test and sort cells, and how to build a pack, and what kind of stuff you need to know to do all that.

 

[Sunder]

Those SkyRC chargers detect a lower voltage than is actually there. This is exacerbated if you use the leads that came with it. I have no idea what gauge the wire that is inside the leads, but they must be thin.

Anyway, enough about the problem, here's the solution.

1. Configure the Skyrc charger to discharge NiMh
2. Configure the termination voltage to 0.0v. No, I am not mad yet.
3. Crank up the discharge current to what you want it to be in the real discharge (this is just a calibration step).
4. Start the discharge.
5. Allow the current to hit the value you set, and give it another 30ish seconds or so to stabilise.
6. Record the voltage at the battery and on the charger screen. Compare the two values, and record the difference. (In your case, it might have been 4.10v vs 3.2v, which is default discharge voltage, I think)
7. Recharge the battery to full using the normal LiIon/LiPo profile.
8. Configure to NiMh discharge again, but this time, set the termination voltage to 3.2v minus the <difference>. if it was 4.10 vs 3.2v = 0.9v difference, then set the charger to terminate at 2.3v.
9. Do a full discharge.

The number should be much closer to accurate now.

If you really didn't care about the cells, you could set the end voltage even lower (even 0.0v), and when you pull it off the voltage "cliff", the charger will stop anyway, and it will bounce back fairly quickly. However, you are probably damaging the cells doing that, only to a small degree if you charge it straight back up, but probably noticeably if you leave it completely discharged for any time at all.

 

[ag273n]

Thanks Sunder!
a question... On Step 8, what Amp rating do i discharge the cell?

 

[Sunder]

Thanks Sunder!
a question... On Step 8, what Amp rating do i discharge the cell?

Your choice. I'd start at 1C (whatever current it takes to discharge in 1 hour, not discharge at 1 amp) for eBike cells, and if it copes well, move up from there.