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18650 at 0V and open-circuit? Answered

Something odd happened today. I went to use my Skyray king flashlight, but the output was very dim. I pulled the batteries out, 3 were at 2.7V, and one of them was completely dead. IDK how they discharged as I charged them not too long ago. They have protection PCBs in them so that is the first thing I suspected that tripped in the dead one. Sticking that one on the charger did not reset the protection PCB.

So I pulled the insulation of the dead battery, However it is the battery itself has failed high impedance / open circuit. Even bypassing the protection PCB, I still only measure 0.001ish volt measured across it (the real measurement varies a large amount) and only uA of current short circuit. I applied 4-40V to it and measured no current flowing into the cell. Certainly open-circuit. I just thought this was peculiar, what is going on with this cell? I figured if it discharges all the way the internal resistance will go up a little, but to the point of becoming a complete open circuit!!! It is possible that a internal bond broke?



Best Answer 2 years ago

I ran into this same situation on many cells and I figured out what was going on.... I've pulled apart a lot of laptop batteries (mainly Dell branded as I got my hands on a bunch) and I ran into a LOT of cells that were open circuit, this struck me as odd because there should at least be SOME resistance. So figuring the cell was dead I cut one open and in the process shorted out a piece of material under the top, I was surprised to see it short out and pass a good deal of current, proving the battery was far from dead.

So I began looking into it more and found that there is a current interrupt device under the top of the cell. When pressure inside the cell grows too high this aluminum button (kind of like the pop top on a glass bottle) pops up and breaks the circuit. It seem this is a one time use device as something has gone pretty wrong to cause an intense pressure build up in the cell (it got really hot).

So I'd first check into why that might have happened... do you ever notice your cells getting hot? or did you notice this issue after charging it? In my situation I work on these dell computers (6410 and 6420) quite often and I've personally witnessed one machine I had in overcharging a pack to the point it was WAY too hot to handle continuously..all these failed packs I have with the open circuit cells are from the same type of machine, I figure they must have an issue that causes them to overcharge and pop the protection disk in the cells... I don't see any other reason for that to trigger.

Many would call this not smart, but it is possible to stick a stiff little poking screwdriver under the cap of these cells and reset the device. I think I'm going to post a video on YouTube about this as I don't really see any talk about this out there. FYI, if you do tamper with the cell do it outside so you can toss it if things go wrong.

On the previous campout I used my flashlight (skyray king) as a hammer for steaks but the flashlight worked for over a week afterwards. All the cells internally are connected in parallel. Also I did shave away some solder on the contacts on the inside of the light with a knife, I think some of those shavings may have fallen into the top of cell shorting it. This would have shorted all the cells, and bypassed the protection PCB on that one cell. The protection PCB was able to save the other cells. Although I think its time to get some new 18650's. The panasonic 3400mAH batteries are real nice, and only $25 for 4 on amazon. But they are unprotected flat top cells. Protected 3400mAH cells are like twice as much >:(.

I've had an 18650 break, And be able to close the circuit only when I pressed hard on the contacts.



2 years ago

Well I might still be able to wrap a shin new xxxxFire brand on it and sell it in eBay! lol