"nImmediateBatteryLevel" will return the battery level at the instant the function was called.
nAvgBatteryLevel will return the average of the past 20 samples (to smooth out sudden spikes).
Both functions return a value in millivolts. So a reading of 11987 represent 11.987 volts.
A nice way to keep that on the screen is to make a separate task that displays the average batter level every 20 milliseconds.
Hope that this helps,