Those Red/Yellow/Green 3-LED Battery Level Indicator Voltage State Of Charge
This isn't as easy to do with NiCd and NiMH cells. These chemistries
have a much flatter voltage-discharge curve... a bit of droop at the
beginning, a nearly constant output voltage over most of the remaining
time, and then the voltage "drops off a cliff" when there's only a few
percent of the charge remaining.
** Most users do not run cells flat in a *single* discharge session - they
turn the load on and off many times.
NiCd and NiMH cells recover voltage after a time and if you plotted a curve
of typical usage - it would look like a saw tooth wave. At each peak of the
wave, the voltage is very similar right up until the cells are quite flat.
Users will switch the device on and *then look* at the 3 LED charge
display - and never see any change no matter what the state of charge.