I wanted to open up a discussion about how we are powering our avionics, meaning servos and APM, on helicopters as they present a unique challenge in the APM world, and must be treated carefully. After crashing my Octocopter last weekend because I let the avionics battery drain, this has come front-and-center for me. I want to make sure it doesn't happen to my helicopter.
Helicopters present a particular problem because we don't have to worry only about the battery dying, but we have to worry about the servos doing something which drags down the APM supply voltage. While looking into the situation, I found this really interesting thread:
Long story short here, these guys are using a new Savox servo which is very fast and powerful, and it turns out it can draw a LOT of amps. A problem occurs when using a BEC powered by the main flight battery as the avionics power supply. If the servos move too fast, they draw a LOT of amps, it's a spike load, and the switching regulator cannot respond fast enough, and the voltage drops momentarily. It's not a big problem for the servos, but it is a problem for their FBL controllers. In some cases it causes a reboot.
Now, my servos are not that fast or powerful, but it's still a concern. Something to be aware of and watch out for.
This is one of the reasons I have decided to use HV servos, and power them directly from a battery. It does help the situation, but I think we should all consider using some capacitance in the system to prevent the problem. That was the solution in the case of these Savox servos.
This weekend I am going to be working on trying to improve the code to be able to monitor a second battery voltage, and also give some warnings about any problems