Weird 18650 discharging behaviour

[Fallingwater]

I have a few 18650 cells I'm not using right now.
Three are new from DX, four more were salvaged from an old laptop battery pack. Surprisingly, their capacity only decayed from 1800mah to about 1100mah in the several years of life that pack had (or at least this is what my charger says).
Since I'm only using one of them in a flashlight and the others aren't currently seeing any use, I decided I'd discharge them to 40% and store them all in the fridge. No sense in wasting perfectly good battery capacity.

I discharged the three DX cells (500ma draw) and everything went fine. I discharged one of the laptop cells and that went fine too.
A few minutes after setting the second cell to discharge, however, my charger sounded the open-circuit alarm. I thought my battery clamp had failed, but upon checking with the multimeter the cell was indeed open circuit.
While I had the multimeter probes on it I saw its voltage quickly rise from zero to 0.2v, drop back to zero, go negative to about 0.04v, flutter for a while, and then go to zero and stay there.

This freaked me out considerably. I double-checked the multimeter, of course, but it was perfectly fine. I put some tape on the positive contact, put the cell in a metal pot and set it on the balcony. I don't think it's too likely to blow, but better safe than sorry.

I can understand cells failing, but they aren't usually perfectly fine one moment and open-circuit the next, and they most definitely don't fluctuate like that inbetween almost-empty and reversed.

The cell is unprotected, so there's no way faulty electronics caused this. What the hell just happened?

 

[VidPro]

there is (supposed to be) "mechanical" protection in the cell itself also, it doesnt do what the electronic protection does, but it ATTEMPTS to keep fires and such from occuring.

if gas formed inside the cell, the anode disconnect could activate, depending on the style of anode disconnect it could fluxuate, but the design is supposed to be a one shot disconnect, that disables further use of the cell.
the purpose being if pressure is building in the cell, the cell is trash, and so the disconnect makes it trash
also overcharging that forms gas, would disconnect it.
it is possible that corrosion rust or age could cause an anode disconnect to become faulty, but once again the cell itself is probably trash.

an anode disconnect might be like this
|( with the flexable concave part becomming flat || or convex |) when gas pushes on the reversed dome
or like this
||--- where when gas pushes on it the anode wire disconnects |( ---
or like this
|)--- where the same thing happens |( ---
the idea being the same, gas force against a movable dome thing , pushes the dome from the connection, or breaks the connection.

also there is that PCT thing, which changes in resistance when the tempreratures change, i dont quite understand it, but it can reduce connection when heating occurs. it is supposed to be for short protection, but could also help in disconnecting during heating. a dead short will heat the PCT itself , increasing the resistance and attempting to slow down the direct short.
the materials for a PCT are also a One time one shot deal, there is no intention for it to be some sort of permanent protection, and so it can break down on many triggers of it.

then there is just lack of seeding on the plates, dried electrolyte and stuff like that, where the resistance of the cell has gone to heck, and it stuttering and failing.

which ever it is, chances are good that , unless it was a fault with your interpretation of it (meter not connecting fully) the cell is bad and even dangerous.

that is all i know, i dont build them, and i learnd that from reading and dissasembly not from any facts.

 

[SilverFox]

Hello Fallingwater,

Interesting behavior.

Just for reference, Li-Ion cells are considered "crap" when they fall below about 80% of their initial capacity. Your cells started at 1800 and are now down to 1100 mAh. They are CRAP...

It sounds like you have a loose connection within the cell. More reason to recycle it. There are some Li-Ion cells that are not built for high current. If you force them, the internal connections come apart and they run a high probability of shorting out internally. Be careful.

Tom

 

[Fallingwater]

500ma is not "high current" by any stretch of the imagination.

And yeah, I'm aware they are crap, but they're still working for light duty stuff, and I hate to throw away things that still work.

This one obviously doesn't, though, whether it's because the disconnects failed or whatever other reason, so it's gonna have to go...

 

[SilverFox]

Hello Fallingwater,

While it doesn't seem that 500 mA would be considered high current, I have some Lithium primary cells that shut down at current draws much over 60 mA... High current to them is 100 mA. They are also 1800 mAh cells.

You mentioned that they were salvaged from a laptop battery pack. I agree. I am not aware of laptop batteries that are specially designed for extreme low current draws. They should be good for around 1 amp without problems, and probably more than that.

Tom