I recently pulled apart a laptop battery with some 2600mAhr LG cells in it. 4S2P. Three sets measured around 3V, and the 4th measured 0V. I was able to charge the first 3 pairs, they seem decent enough. The 0V pair is interesting. I cut the 2P and tested each cell. 0V. I applied 3.6V current limited to 0.1A, and the cells measure 3.6V with no current. It looks like an open circuit. I would have thought that laptop cells would not have a protection circuit internal to them, and even if they did they would be reset if voltage was applied. Anyone have an idea what is up here?
The cells are marked LGDBB31865.
BTW, these cells can deliver 15A into a load if coaxed, don’t ask how I know . . . :person_facepalming:
Let’s just say the one that showed that characteristic is spending the night in the bar-b-que
That one has very very high internal resistance, meaning both the cells is probably near dead and has the PTC activated.
It is probably the PTC (Pressure, Temperature, Current) switch which was probably activated. Once turned on permanently, it makes the cell have extremely high resistance, and charge to nominal voltage extremely quickly once voltage is applied, but goes down quickly after being in open circuit.
Thanks for the feedback. I bet you are right. Looks like there are a couple of protection features in “unprotected” cells. I did not realize that. It’s always good to learn something new.
PTC stands for “positive thermal coefficient” which is essentially a resettable thermal fuse. When the PTC gets hot its resistance rises and the battery current is throttled. It isn’t permanent and returns to normal after cooling. It has nothing to do with pressure,
There is a pressure safety called the Current Interruption Device (CID). It is permanent and when the pressure inside the battery is too high the CID blows the battery becomes an open circuit.
Guessing this is what happened to the two cells in parallel that measured 0V. Those cells don’t come in my house, I figured something bad was up when they measured 0V.