Minimum cells and mah for MMMv3??
 

I know the max is 6s But what is the minimum amount of cells and milliamps this setup can run on,just out of curiosity I'm not gonna try it because I know better:lol:

technically any.. Im pretty sure it will run on 1s. and the MAH can be anything. but ur BEC will only run as high as the battery.

Since it is a switching BEC, it might have trouble at anything below 2s. But yeah, pretty much anything works as long as you don't exceed the voltage or current specs of the ESC, and the motor/battery are chosen appropriately.

What is the reason for castles external BEC? Is it required to run on their esc even though one is built in ? Or is it meant for non BEC esc's?

Just a stand alone switching bec. Plane guys usually use them, and it is versatile enough to be used with any setup, and is programmable for output voltage.

You know my setup linc,is it mandatory or beneficial for me to have one of these?

If your running a MMM currently you probably dont need an external BEC