..and what is interesting for a low power design is the system can measure its battery voltage in the way it will not consume much energy. So you need an op-amp (sourced from i/o pin or switched on/off when required) with fet intputs, and a high impedance voltage divider (not to discharge the battery too much). I am using MAX1724 3V3 stepup (great chip -sot23-5)and my system is powered from one nimh cell.. From time to time it measures the battery voltage.. The 1.2V cell is charged from a sollar cell.. It works from 0.6V (it shall work from 0.8V input voltage according to the datasheet)..
P.