[Espeefan]

Joe, speed controllers with internal BECs have limitations in what current they can supply to the other loads you are powering. Often times the BEC voltage and current is listed in the spec sheet for each speed controller. The catch is the max current rating is usually decreased, as input battery voltage increases. A speed controller might be able to supply 6 volts and 5 amps, running a 7.2 volt battery pack, but when you bump the voltage up the BEC capacity decreases. Or under a heavy load, the internal BEC just can't keep up, especially when the speed controller is already driving some power hungry motors, consuming a lot of current. Your ESC either thermals and shuts down, or the servos get glitchy.

I'm all for running external BECs in such situations. It's the best way to solve the problem. By doing so, you are relieving the ESC of two jobs. Running a motor, and supplying power to your servos, via the internal BEC. In high performance applications, where every little bit counts, running an external BEC will allow the speed controller to put everything into the motor(s).

In the case of running dual (or more) speed controllers on one model, people sometimes recommend you pull the power wire out of the additonal speed controller's plug and let just one speed controller's internal BEC handle powering the servos. I think otherwise, and here is why. If your speed controllers are identical, then the internal BEC voltage and current output of each speed controller will be identical too. The internal BECs will work together, just like dual battery packs in parallel, which basically means the current draw will be split between the two internal BECs. Make sense? I think it does, and I have run dual motor models this way with no ill effects, and no external BEC. The key is to make sure you turn the power switches 'on' for both ESCs, even though turning just one 'on' will power up the other automatically. I used to run my dual motor, dual ESC rock crawlers like this. It worked just fine for me, and that was powering two power hungry high torque steering servos, plus the drivetrain motors.

Now in the case of running an external BEC, I definitely agree that the power wires for each ESC should be pulled out of the plug. The ESCs will all power up together, as soon as the external BEC is switched 'on'. No big deal. Or you could run a separate rx battery pack too. Either way, the results are the same. I would not recommend trying to use the speed controller's internal BEC along with a battery pack, or an external BEC. Reason being is the voltages could be very different, and you might burn something up. Some external BECs are voltage adjustable, and some aren't. Best to just let the external BEC work alone in either case.

I hope this helps and makes sense.