Ni-MH battery voltage
Individually, it's 1.4V max charge, 1.3V ~ 1.2V nominal for most of its drain cycle,  1.1V to 0.9V dead,  0.8v you broke it.

Since this is a pack, of I'm assuming series cells, multiply these values by the number of cells.

And since you want to use an external, make sure to use an appropriate resistor voltage divider to bring the voltage range down to acceptable levels for it, as most will not accept a direct 10+ Volt input.