Judging the state of charge of a battery from the battery voltage is difficult to impossible. Too many parameters have an influence like:
- temperature
- capacity of the battery
- load current
- etc.
When can use the voltage as a coarse indicator. You'd need a comparator circuit with a stable reference voltage and some hysteresis to avoid flicker of the LED. However, you'd have to set the reference voltage rather low to avoid false indication of a drained battery when in reality the battery is still good for some work.
Probably Audioguru's method of observing the fan speed is much simpler and reliable.
To always have a fully charged battery at hand: with a quality charger and modern battery technology (e.g. NiMh) today it is possible to connect the battery to the charger whenever it is not needed without risk of damaging the battery (this is different from old NiCd batteries where regular charge/discharge cycles were recommended to avoid the so called
memory effect).
Harald
P.S.: Note that a charger that indicates a full batttery doesn't use the battery voltage or capacity as an indicator for the state of charge. A common method for a charger to
determine end of charge is to check for the
change in voltage, a change in temperature or absolute temperature when charging. These methods are obviously not available when measuring the steady state of a battery.