Last night I put one battery in a mf02 battery cage in reverse. It was the protected Keeppower 18550. The other three batteries were already in the cage and when I put this one in reverse, a little crackle/spark and quickly realized I goofed up and removed it. I removed it pretty fast, almost as soon as it sparked. The thing I don't get is that it didn't trip the protection circuit, no visible damage and everything seems to be fine. Is it safe to still use it?
it very likely Did trip the protection, it just has to draw more than about 5amps to do it and that was your spark. it should be quite safe to continue using 
Does it still have a voltage reading?
i don’t know what a mf02 cage is, but were the other 3 cells in series or parallel in the cage, and was there a load or a charger attached to the output terminals of the cage?
From what i have seen there are FETs with body diodes in the protection circuit. One of those diodes may have blown from an overcurrent condition. This could make it such that charging may not be possible in the future in the event of an UnderVoltage condition in which the Protection shuts OFF the Discharge FET.
I thought if it would trip the protection that I would have to reset it but not the case as far as I can tell as it's still has correct voltage
Don't be scared by a little :-D spark. Besides, everything checks allright and so it is. ;-)
Yes voltage is still normal and same as the other three cells. The battery cage is 2S2P, so two of the cells were in parallel and the other one would have had to been in series until I goofed up with the 4th one. No load or charger on the cage. If the diode did blow in the protection circuit, does that mean if I made the same mistake again that the overcurrent protection would not work the second time?
haha, not scared but a little disappointed I put it in reverse. First time I ever did that. Was watching tv at the same time and I guess I was too into the movie to realize what I was doing until it was too late.
[quote=klrman
I thought if it would trip the protection that I would have to reset it but not the case as far as I can tell as it’s still has correct voltage[/quote]
it only locks the battery out under a threat condition (reverse polarity in this case), remove the threat and it goes back to business as usual, the only time you may need to “reset” a protected cell is under-voltage conditions, some smart chargers will ignore the cell entirely, you need to put them in a dumb charger for a while to get them started again.
With 2S2P, then the cell that may be damaged is the single cell that was already installed, not the last one that was inserted backwards.
This was equivalent to connecting 3S in parallel to a 1S, which would cause an ~8volt OverVoltage condition and attempt to push current into the single cell as if it were being charged.
Interrupting the current could be what caused the spark, similar to the spark across the contacts of a relay that is interrupted while DC current is flowing.
As KA said above the chip would respond to the threat condition and return to normal when the threat was removed.
If it were left there a long time then there might be some damage, but sounds like you were quick. If the body diode were damaged then it would likely take out the FET channel too. Sounds like it’s working okay, and as Barkuti said so it is.
Good tech info thanks for the help guys! Lucky I didn't do that with my 30Q's, hate to think what could have happened there. I'll keep any eye out on the mf02 batteries for a little while longer then and if they don't play up I'll assume it's all good.