dsteding wrote:
The temperature thing is a backup in case the voltage doesn't switch or the charger doesn't pick up the negative deltaV-without that the battery could heat up and (I guess) eventually go boom.
It won't go "boom" but you could eventually damage the cells. The vent tabs should pop and that's it for the cell.
I don't know if the X has a series-parallel string, as mine hasn't arrived yet (next week...whee!) but for our 9Ah dive-lights, they use a really bad, at least from an engineering point of view, combination of 10 cells in parallel and 10 in series. The internal resistance of all the cells are not equal, and so some will tend to heat up more than others. It's that kind of run-away from a shorted cell or one that's kind of gone nuts, that the thermistor is there to pick up.
However, if everything is working right the negative voltage drop check should be the actual triggering event to shut off the pack's charger.
In the case of the similar NiMH dive light packs, particularly the Halcyon chargers were notoriously unreliable with regards to shutting off prematurely, and before I replaced it with something better built, I would just remove the thermistor line and jumper it with a 10k fixed value resistor, which seemed to do the trick. With the replacement pack I soldered, I didn't even bother to install the thermistor in place.
It's a good backup, and will keep you from destroying a pack in the case of a malfunctioning charger (well, maybe...), but honestly, if the thermistor is what causes the pack to shut off, I think you have bigger issues around the health of your batteries (or your charger, assuming it's dumping too much charging current but the protection circuitry is still good).
