Yes what you want to do is / was commonly done. I would suggest that you do the research first. You have a good idea of what the numbers should be and Wiki should do nicely. Since you have some part of a controller available you could also monitor battery condition by discharging the Ni-Cd battery at a set rate/time and noting the voltage at the end of the test period. While not terribly accurate the method can be interpolated to give a rough estimate of charge/condition. Charge the battery properly (I like to sense the voltage rise at EOC) but follow the MFR's rec's and then discharge it at a known rate for a fixed time. The End of test voltage will decrease as the batteries age, more or less in a linear manner until nearly end of life, a rapidly decreasing step in the change of the voltage indicates battery issues. I did this with SLA batteries primarily but it worked well with a couple of NI-Cd powered devices I built several years ago as well. I stored the values in EEprom when the device was powered up along with the time/date from a DS3231 clock and graphed it with custom chars onthe new a 4 X 20 LCD display against the new values.
Bob