Discharge problem

Rcdev

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Nov 14, 2020
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Hello I am new to the hobby of making 18650 battery packs and from the knowledge people in this forum have about battery related topics I have learned alot.

My problem
I have salvaged a couple of 18650 cells and I charged them to 4.1 and discharged them to 3.0 and I charge the cells one at a time. Everything worked ok the first time but the second time I tried to discharge from 4.1-3.0 the end voltage was 3.14 on the cell and after a day it raised to 3.6 why is this happening?
English is not my first language so I am sorry if its hard to understand the text
 

daromer

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Its called sag/bounce back and its normal. They bounce back if you end with high current.

Example if you discharge with 1a and there is voltage drop over the cell due to high internal resistance the voltage out will quickly reach 3.0V and when you stop you have no current or voltage drop and you suddenly meassures higher voltage.
 

daromer

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You have a voltage drop over the battery at the load. Therefore the voltage meassured during the load is lower than when you release.Therefore the so called bounce-back.

If you look at how you test discharge you do cc/cv where you start with a current of 1A as an example and then you stop at 100mA current and at 2.8V.
 

Rcdev

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Thanks for your replies, Why didnt I have a voltage bounce up the first time I discharged the battery and how can I discharge it again to 3.0v and it stays on stable 3.0v without voltage changes?
 

daromer

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How did you discharge? What was the end-current?

You need to ramp down the current alot if you want it to stay at 3volt after a discharge test. My tip is to google on how discharge testing is done :)
 

Rcdev

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I am using a smart charger and when the discharge reaches the final stages the amps decrease from 1 slowly to 0.2 amps
 

Cherry67

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Yes. Its No Bug, Its A feature.
Avoiding undervoltage.
 

Redpacket

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Rcdev said:
I am using a smart charger and when the discharge reaches the final stages the amps decrease from 1 slowly to 0.2 amps

That is a good way to discharge, you will get good test results this way.

Like daromer says when the discharge stops, a cell voltage may rise a bit & this is normal.
Cells which have not been used for a while may do this more for the first few cycles like you found.
 

100kwh-hunter

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compare it with a drain pipe.
When you did not use it much or not at all, it is will with mud, sediment and debris.
You flush it after a period, yes it will do its job, but the mud and sediment must get out first.
The second time you flush a lot of stuff is out, it will go quicker.
The third time the pipe is clean, water will rush into it.
But when it stops (plug it) you will get a shock wave and the water wants to level.

My two cents.
Best Igor
 

Rcdev

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Thanks for the great response!

I have now done a complete discharge/charge cycle with my 18650, on the second cycle can I charge the 18650 cell directly after the discharge is finished to test the capacity?

Thanks in advance
 

floydR

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After I check the IR, I do one complete charge test,{charge, discharge (results recorded on the cell), charge) cycle on my Opus's, then let them sit for at least 3 weeks. To check for self discharge.
later floyd
 
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