Why not use the BEC - LI-POLY battery on the Caslte speed control to power the receiver on the Elecrtic Chrysalis?

From : Don Stackhouse

It's generally a bad idea to use a BEC ("Battery Eliminator Circuit") to power the radio from the motor battery on a self-launching sailplane.

On a model with a separate battery (including the case of a gas-powered model, and a pure sailplane) it's usually fairly easy to figure out how long the battery will power the radio. The power consumption of the radio is reasonably consistent. Once you know how long that radio battery lasts, throw on a reasonable safety factor, and you know how much flying time you can safely get from that battery.

On a non-soaring electric model, the speed controller is set to shut off power to the motor while there is still enough power left in the battery to run the radio long enough to land the plane. Since it is a non-soaring model, in most cases it will not stay in the air very long after the low voltage cutoff shuts off the power to the motor. The model will be safely back on the ground before the radio shuts down due to lack of power from the battery.

However, in the case of a self-launching electric sailplane on a day with reasonably available lift (thermals, slope lift, etc.), the model could stay aloft for long periods (perhaps hours) after shutting down the motor. Since motors typically use relatively large amounts of power very quickly, it becomes very difficult to predict how much electric power (and therefore radio operating time) is left after a few climbs to altitude. It's possible to run the battery down without realizing it, till there's only a few minutes of radio time left and not realize it. The end result could be having the radio go dead while the model is still hundreds of feet high in a thermal! Using a separate radio battery makes the radio operating time predictable (just as it is with a non-electric model), and avoids this problem.

Note, this applies to sport flying with no specified flight task. If you are flying in competition such as the LMR ("Limited Motor Run") event, where you will be flying with a freshly charged battery on every flight, where the motor run time is regulated by the rules and the flight time is also limited by the tasks for that event, then it's possible to be sure that the battery will be able to power both the radio and the motor for the entire time limits of that task. In that case it would be OK to use the BEC to power the radio. It's the undefined motor run times and unpredictable flight times typical of sport flying that create the problem.

Don Stackhouse
DJ Aerotech