Monitoring NiMH charging is a bit tricky issue. The usual way seems to monitor the battery temperature rise, and the battery voltage: When the battery is full, the temperature starts to rise, and the voltage has a tiny dip. In practice the stop criteria is that dV/dt turns from positive to zero (and actually a bit negative for a moment). A temperature sensor is a good security measure, NiMH cells can not take too much overcharge without damage.
To monitor discharging is even worse: You can not in practice estimate the remaining charge by any simple measurement. I have worked with a cordles phone solution where the processor simply "gestimated" the remaining charge by monitoring (and integrating) the power consumption; it seemed to be the only way.
You can find some basic info about NiMH charging in:
**broken link removed**
The article has also some diagrams illustrating the issue of charging. Other charger chip producers have similar papers.
In practice it is easiest to use a ready-made charger controller. The next easiest is to use a microcontroller (specially if you happen to have one present anyway) and use it to monitor the voltage and temperature.
Good luck,
Ted