Ni-MH pack topology [semi-OT, I know, sorry...]

I was wondering whether anyone here could help me on this one: I have a battery pack of a mobile phone I'd like to replace for a set of AA Ni-Cad batteries (reason: cheaper!). The mobile phone even has the necessary mechanical/electrical pieces necessary for the mobile to work with 4 AA batteries. The question is a couple of extra small terminals the pack provides. I dismantled the pack and I found out that one of them was connected to the pack (-) terminal, and the other one was connected to a weird component, which is connected to the (-) terminal. My question is: *what* weird component is that?

Some clues: It looks like a common 1N4148 diode: glass capsule, two small copper-color cylinders, and something really small in between. However, it is not a (common) diode! However, I suspected it were a zenner pair (in series, symmetric polarity), and therefore I tryed to measure the zenner voltage by connecting the component in series with a 1K resistor, to a variable voltage power supply. In fact, varying the voltage does not affect the voltage drop across the component, remaining at 9.1V. At this time I was pretty sure it was a zenner pair, maybe for protection or something like that. However, the mobile does not charge the batteries (and does not recognize their presence) if this component is removed! The mobile charger has about 7V (open circuit), so I was expecting the supposedly zenner pair to be "open". What I did was to measure the extra pin voltage: without the component it measures about 2.5V, *with* the component it drops to about 1.2V. So the component cannot be a zenner pair. And this was the time I had the idea to post this message... Thanks for any help/clues/(flames?) given! Cheers,