Any Protection Required for Rechargeable Battery Powered Device?
Luckily, the NiMH batteries are relatively tolerant to abuse, compared to other battery chemistries.

Fuse in series with the battery.  If could be a non-resettable fuse, or it could be a resettable PTC fuse.  If the load (i.e. circuit powered by the battery) develops a short, it will draw excessive current from the battery, which in turn will cause the fuse to open and break the short circuit.

The type and rating of the fuse would depend on the design and operation of your circuit that consumes the battery power.

Check the peak current consumption of your device.  Electric motors, for example, can draw a large current for short time when they start-up under nominal conditions.  This is normal, and it shouldn't trip fuses.  But if there is a short through the motor, that has to trip the fuse.  One way to address this is to use a slow fuse, which opens only after a high current has been sustained for some time.