I am building my new Tarot 650 and have started experimenting with FPV which I bought from 3DR. I power my FPV camera and transmitter directly from my 3S battery. The image is transmitted correctly but as soon as I start the motors the receiver monitor turns black. When I power the FPV system from a separate 3S battery the image remains more or less stable.
As I would like to have everything powered from one battery I bought a BEC from Hobbyking that keeps the voltage at 12V with input varying from 7-21V. That didn't work either. Even with the motors powered off the screen was blinking all the time. A quick measurement showed that the output voltage was 10.66V instead of the promised 12V. I have returned the thing to HK and before I buy a new one I am just wondering whether it is feasible what I want? Is a BEC able to keep the voltage stable with all the spikes and changes in the voltage when the drone is flying?
Let me rephrase the question: does anyone power his FPV from the same 3S battery as where the motor is powered from? How do you get stable 12V? Do you use BEC's or is there another way (apart from using a separate battery)?
Yes, I have done this.
First, you will not find a BEC that will not drop at least 1V, sometimes up to 2.
All the 12V gear I have actually works with 13.6V down to 9V. This allows you to connect the battery directly without using a separate BEC.
All that gear typically has their own voltage cleanup/voltage regulator, so no need to add a BEC in front in many cases.
Hoewver, if you see problems, using a separate small 12V lipo is your safest bet.
Well, I use 3DR FPV gear. That appears not to be able to stand the voltage fluctuations when connected to a powerline that feeds some powerfull motors as well, something I understand when I see the voltage fluctuations in the log. I understand your remark about an extra battery, I tried it and it helps, but it also adds extra weight.
There are step up/step down regulators that say to keep the voltage at a predefined level. I ordered one and am experimenting with it. As far as I can see it works. I have to do a real flight but that may take some months as winter has started :-)
As you found, yes, a step up/step down should help. A BEC cannot provide 12V from 12V, never mind 12V from 10V.
But if your voltage dips _that_ much there is a good chance your battery doesn't have enough uumph to run your motors. Either your C rating is too low or the battery is old. You can likely kludge around it like you did, but just a heads up that your battery is probably being taxed a bit beyond what it's meant for.
I had not considered that. Thanks very much for pointing my attention to this, I'll try order a new battery to test this.