Rechargeable NiCad battery pack

[Z80]

Hi can someone help me with this please
I have two rechargeable NiCad packs for an 18V electric drill. Recently one
of the packs would not recharge. At first I assumed it had reached the end
of it's life, but I thought I would take a look inside before I threw it
away as it is not that old. I found that there is a third terminal which is
monitored by the charger and connected between this terminal and the
negative side of the battery is a small component.
On the good battery pack when measured with a DVM this component measures
0.5R in either direction - whereas on the bad pack this component measures
disconnected either way.
My question is - does anyone know what this component is? and I don't think
it is a 0.5R resistor! it looks like a small diode, it has a glass (or
silicon) case about the size of a IN4148 - it is unmarked - maybe a zener?
Thanks for any info. Alan

 

[Art]

Pico Fuse??

 

[Z80]

Hi Yes it could be a fuse but it would not prevent excessive current at the
output when the battery is in use, and presumably the charger would have
it's own fuse if required.
It is somehow used by the charger to monitor the battery condition from
charging to completely charged, as the charger has two LED's which indicate
this.