Very odd finding on recycled laptop batteries

A

Ant?nio Pereira

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Jan 20, 2018
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Hi, I'm new to this forum.

I'm planning to build a ebike battery from used laptop batteries, on the process of charging, testing, sorting, etc. I've found something that is completely different from anything I found on the internet.

Basically everyone says that cells over 3.5V are fine and under 3V are most likely damaged.

In my findings I found the completeopposite, for example, I have a Panasonic CGR18650CA that had an original voltage on the pack of 4.06V I thought it would be a very healthy cell.

I charged it to 4.2V and tested for capacity at 1A discharge, to my great surprise in less than a minute it dropped to 3.0V showing a 20 mAh capacity, the cell quickly recovered the voltage to 4.07V after the test finished.

On the other hand my best testedcell so faris a Panasonic NCR18650 that wasa 1.81V cell when removed from the pack which has a capacity of2647mAh at a 1A discharge rate (91.28% capacity).

Anyone care to comment?
 
Welcome!

The one stating that cells under 3V is damaged or not working dont know what he/she is talking about Im afraid.

Many LiIon are fine down to 2.7 or even 2.5 before any damage. With that said the damage depends on how they have been cycled an I have had 100s of cells recover perfectly fine below 1V and 1000s below 2.5V.....

With that said: ALWAYS test all cells that will take charge and then holds the charge: (Below image from my webpage)

image_ynbbdb.jpg
 
Very helpful diagram! Thanks!

The wierd thing is that one weird cell did hold the charge for weeks at 4.17V but as soon as there is a load it dips down to 3.0V really fast! It then recovers again after the load is removed. I've never seen that before. And the battery was 4.06V in the pack, I was sure it was going to be a good cell but that's not always the case, I also have a couple of cells that were around 3.8V in the pack that turned out to have only a a few hundred mAh capacity.
 
It can be the CID that have popped. That can show voltage when measuring but cant deliver any current. It can also have HUGE resistance and just broken. I bin all those cells personally.

The voltage they hold are no indication of the capacity im afraid. But with that said cells under 1.5V are generally not as good as cells around 3.7V but that's just totally dependent on what type of cells or batteries you have gotten hold of.
 
I had a similar experience, I had a pack that measured 3.98v that was new. I put it in a replacement pack for a cordlews drill, tried to charge the pack and it indicated dead pack. toock the cells apart and tested them all for charge and discharge, the bad cell measured almost 4v but looked like an open circuit when testing cell resistance. This cell will not take a charge nor discharge unsusually. Orobably a manufacturing defect...
 
Sounds like brooken CID. High pressure inside but not high enough to break contact 100% so you can still measure voltage. Seen it happens rather often!
 
This could be an old cell with a high internal resistance.

I have had many that hold an excellent voltage but are incapable of supplying 1A during the test.

This is especially true if the resting voltage recovers significantly at the end of a CC discharge.

You may find that if you test at 500 mA it will last much longer as the

If you have an imax B6 you should be able to do a CC/CV discharge test and find it takes forever once the discharge reaches 300-400 mAh.

These batteries are nearly useless except if you want to power very low current devices..
 
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