Hi Amir,
Usually, external BEC is needed only if you fly an
aircraft with big servos and/or a lot of servos (maybe
you should give us more information on your aircraft and
servo setup)
Anyway, you must power the Apogee with the
battery in order to have a correct internal supply
production (voltage range 5.5V min → 17.0v
max, so 2 to 4 Lipo cells)
So you will get a correct reading of the battery
voltage.
For servo power supply connection, you ave two options :
1) servos are supplied by the Apogee internal 5V (no
BEC, default configuration for small aircraft), in this
case
--> connect battery on Apogee supply inputs (+ and
ground)
--> connect all 3 wires of your servos directly on
Apogee, nothing else to do except be sure that the total
5V current consumption do not exceed 1.5A