Question about 18650 cells

[Sidecar Bob]

I have several 18650 cells that I salvaged from non-working items. I also have a power bank/charger that holds two 18650 cells and I have successfully used it to charge cells and then drawn power out, using it as a power bank.

Two of the cells I have came from similar products and seem to be identical. A voltmeter connected across either of them says approx. 3.3V but when I put either or both of them into the power bank they will not accept charge and I cannot draw power from them.

How can I tell whether the cells are good or bad?

 

[Martaine2005]

If these cells are from a smokers vape, check to see if a diode isn’t used to protect against charging or further use.
Failing that, give all the info you have.
are you checking voltage directly across cell terminals? Or via a BMS.
Sometimes, these cells have a BMS discretely hidden under the heat shrink.

 

[Sidecar Bob]

Sorry for the delay. For some reason I didn't receive an email notification.

I'm checking across what I believe to be the terminals: the flat end is negative and the end with the bump/cap is positive.

These cells came from failed emergency LED light bulbs (E27 base, light for several hours if the power fails).

 

[Delta Prime]

Your BMS has a pre-set low voltage battery protection threshold; as you have just discovered it will not charge the battery. It is too dangerous.
As well as over voltage & overcurrent protection thresholds.
The positive terminal of your battery has a safety vent that will release highly flammable gas that is toxic to human beings, this venting is done to release pressure, depending on the manufacture could be anywhere from 140 to 150 psi as the battery heats up from over discharging current from the load itself, or the charging circuitry is faulty.
That’s why you must know manufacture specifications of your particular battery and in your case, a single cell.
Nothing to play around with if they’re already used it is very possible that you will encounter an overload current conditions either by charging or discharging, will draw excessive current the battery chemistry heats up,pressure built up, sometimes the vents do not work properly then the phenomenon of thermal runway is engaged & cannot be stopped.
They can catch fire & they do explode!
But I must admit, I get too cells, fully charged connecting parallel with the under voltage cell to bring the voltage up to a point where the voltage is above the low-voltage preset level, but the low voltage cell will heat up and must be done under strict supervision even then there’s no guarantees..

BU-304b: Making Lithium-ion Safe

techniques for making lithium-ion batteries safe including the role of protection circuits

batteryuniversity.com

 

[Sidecar Bob]

Are you talking BMS in the charger or in the cell?

I put one of these cells into the charger with a different one that read about half a volt lower. The other one charged but this one did not.

They measure about 3.3V (should be well above the minimum) but when I put either or both into the power bank/charger its lights don't come on and I can't draw power from them.

I'm starting to think I should put them in the recycling.....

 

[Martaine2005]

3.3V should be a healthy LOW voltage.
A cut off voltage is typically 2.5V to 3V.
Some single cells have a BMS under the heat shrink but not all. So you might be dealing with two BMS.
Now, if you like to live dangerously, you could try using a known good charged cell and using it to jump start the other. A quick couple of 0.5 second jumps can sometimes bring the cell level up enough to be recognised by the charger.
But I use my bench power supply.

 

[Sidecar Bob]

I don't see/feel anything under the heat shrink.

When I had the 2 different cells in the power bank/charger the other one charged fully and I left them in there together for a while. I assumed that they were in parallel so if it was going to jump start it would have but maybe I should open the power bank/charger and see if there is something to isolate the cells from each other.

 

[Martaine2005]

This completely depends on the topology of the charger. A universal multi charger will be different to a dedicated charger for a particular type of equipment.
If you have a bench power supply, set it for 4.8V @ 0.8A for 20 minutes. DO NOT leave it unattended. The lithium cell could vent. After each 2 or 3 minutes, touch the cell to make sure it isn’t getting too hot.
Leave the cells that you have partially charged for 10 minutes to cool and try again in the power bank.

 

[Sidecar Bob]

Unfortunately the bench power supply is one of the projects that stalled and I never got back to. I'm hoping to start working on that in the next couple of weeks.

Just to clarify:
The cells from the lights have purple heat shrink and are marked "Hongli".
The good cell (from a failed power bank) has blue heat shrink with no marking.

I opened the electronics end of the power bank/charger and it definitely has separate charging circuits for each cell. I should have expected that.

With the 2 Hongli cells in the power bank/charger 3 of the charge indicator LEDs are on steady the 4th indicator is flashing as it would if the cells were nearing full charge. I measure 4.35V across each cell.
I have connected a USB volt/amp meter between the USB wall wart and the power bank/charger. It says 5.2V and 0.00A.

It has been plugged in for several minutes and nothing feels warm.

I will let this run for an hour and see if anything happens.

 

[Sidecar Bob]

OK, that didn't make any difference. Tomorrow I'll see if I can find a 4.8 volt supply.

 

[Sidecar Bob]

Well, that test didn't take long. I dug out a "5V 1A" supply, figuring I could put a resistor between it and the cell to drop voltage & limit current if necessary.
I started by connecting it across one if the Hongli cells with my multimeter (10 Amps) in series and measured zero current (0.0A)
Same for the other Hongli cell.
The blue cell drew 1.1A (I only left it connected long enough to measure).

 

[Sidecar Bob]

No further suggestions?