Finding out capacities of 18650s

ank

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I got a bunch of 18650 from a laptop battery and I put them on a Opus charger.
The charger showed one with a voltage of 2.8V and the others with 3.4V

Then I charged them at 500mA them and the one that had 2.8V finished with 2200 mAh capacity (full), and the others with 1600 mAh.

Is this because the other 3 were not completely discharged and the 1600 mAh is not the battery capacity, but the amount of mA that the charger put in from 3.4V to 4.2V?

If I do a discharge refresh will that give me a read of the real battery capacity?
 
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ChrisGarrett

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Charge them all up to 4.20v and then discharge them at 1000mA/1A and see what you see. That's a good indicator. My Opus 3400 v. 2.2 discharges down to 2.80v and the newer v. 3.1 discharges down to...wait for it...wait for it...3.1v, which isn't a huge difference, but upon discharge, the 2.80v threshold should yield a higher capacity reading.

Remember, the Opus 3100/3400 charges can be all over the map with their numbers, so take them with a grain of salt. Sometimes I think mine is off by 5-10% and at other times it seems to be nuts on.

Chris
 

ank

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I discharged them at 1A and after 1900mah I see 2.8V. Then the discharge current went down to 0, but voltage started increasing to 3.1V in 30 seconds.
So I guess the real capacity is 1900mah
 

ChrisGarrett

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I discharged them at 1A and after 1900mah I see 2.8V. Then the discharge current went down to 0, but voltage started increasing to 3.1V in 30 seconds.
So I guess the real capacity is 1900mah

Yeah, so you did a moderate discharge of 1A and got 1900mAh/1.9Ah for a 1 amp draw. This last part is important to note and understand, since things aren't linear.

Had you discharged those cells at only 200mA, you might have gotten 2300mAh, as an example.

Generally, the higher the current draw, the lower the capacity and vice-versa.

After the charger shuts charging down (no trickle on li-ions) at 2.80v, the cells will start to rebound, especially if you're discharging them at 1A, or higher.

That's normal.

Chris
 

fivemega

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I discharged them at 1A and after 1900mah I see 2.8V.
Just make sure when voltage goes down to 3.0 or 2.9 volt (before test termination), current draw is still 1 Amp and no less. Another world, current draw should remain same from start to end of test.
 

Arizona_Mike

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That charger reports actual current so after cutoff it would be 0mA although only for a minute before it starts charging again.

Generally official capacity ratings are at 0.2C, but I usually never test below 500mA for a Lithium Ion. I test 3400mAh and above at 700mA (for NiMH I use 700 for AA and 500 for AAA).

There really should be no need to use the Discharge Refresh mode with Lithium ions Just use Charge Test.. I have however been able to bring NiMH cells back to life with Discharge Refresh.

Mike
 
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