I've seen inside drill batteries before (NiCad ones) and there's usually just three terminals to the pack, -ve, +ve, and one for the temperature sensor. But this one has a 4th pin, which is connected to the -ve of the batteries via a 10K resistor. What is its purpose?

https://www.dropbox.com/s/1np3y4ubr4....jpg?dl=0&s=sl

Bottom left large pin is connected to -ve of cells and one end of the temperature sensor (a little black blob) and one end of the 10K resistor.
Right large pin is connected to +ve of cells.
Top left large pin is connected to the other end of the temperature sensor.
Small pin is connected to the other end of the 10K resistor.

I don't have access to the charger for this pack, I only bought this pack to take the cells out to replace them in my own battery pack, as this one was half the price.