The vast majority of our users have ESCs with BECs built in. Indeed this is the first time I've heard of a user without that. I actually wasn't even aware that it was possible to buy an ESC without an BEC. Why do people choose them?
And in three pages no one answered this question.
I have been through this problem with an oscilloscope.
When the electric motor in the servo accelerates from rest to fullspeed, it draws many times its average current - often referred to as a current spike. Extra servos compound the problem. Your average current draw might be 1.3 amps, but you need way more than 2.25 amps to allow for peaks.
We tried to overcome this by adding some 100 uF capacitors, but this was not enough. We now use a good 5 amp average BEC for small aircaft. We like the Castle Creations BEC. It is switching regulator that gives 5 amps average and 10 amps peak.
For bigger aircraft we have sparate batteries for the receiver, APM and servos.
Jack: Well explained. Thanks!
This is the correct answer. Just to expand a bit, this is due to (I suspect) a combination of the inductive load of the motors + the fact that they just draw more current when moving quickly from one setpoint to a new one as opposed to holding a position. The large load (force) on the servos due to the big flaps on a flying wing doesn't help either. A sufficiently large capacitor will usually fix such a problem. If you put a scope on the power rail and see how long the voltage droop lasts (and/or measure the current with a scope as well, if you have the right probe), you can get an idea of how large a capacitor you need. I wouldn't expect a 100uF to be nearly large enough for current spikes on the order of an amp. At I guess I'd say you'd want closer to 10,000uF. As an aside, this is the same phenomena that causes your lights to flicker when your air conditioner turns on in the summer. The big inductive load of the motor causes a similar voltage droop until the system reaches steady state.
Chris, most big powerful ESC's have no BEC. The reason is because they are opto isolated. There are two circuits on the board. There is the power section, that is powered by the flight battery. Then there is the Rx side. The Rx side consists of nothing but an LED, I think. The LED is the transmitter stage of the Opto-isolator.
There is no electrical connection between the power section, and the Rx section. There is an "air gap". The main reason to do this is so that there is NO WAY for electrical noise to propagate from the power section to the Rx side.
They also do it because big models using big powerful motors also have pig powerful servos, and you are expected to use a big powerful stand-alone BEC .