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