Yeah, I do that today (2S lines connected directly) using a PowerSafe Receiver with two lipo inputs. But I agree with others that you really want a regulated voltage so that the servos perform the same before, during and after flight even as the main pack is getting heavily used by the motor. I've wanted to go regulated HV for about a year but haven't found a weight efficient way to do it, was hoping the Jaccio to 8.2v + 3S balance tap would be the answer for the secondary side of my Rx (with the Rx bat on a reg set to something like 8.1v) so that the normal circumstance is pulling from the main pack and the Rx battery is just a backup.
Peter+
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Peter, have you got any data that shows what your back up pack is doing? It would seem to me that without isolation you would be draining the small lipo every time you pulled high current in your motor. I know my pack drops to about 35 volts under high loads late in flight, and that says the voltage would be 7.0 volts at the 2s tap. If your small lipo is over two volts current will flow from it into the motor circuit draining the back up. Not good.
Or does a "PowerSafe Receiver" provide the isolation?
Jim O