Originally Posted by
Haadkoe
Standard serial connection for batteries, as pictured.
If I'm not mistaken, the spc port on the rx (which was receiving 5s voltage direct from the esc battery terminals) is internally separate from the ch 1-4 ports, so I wouldn't expect any issues there, but what do I know, lol. You might be on to something.
If the +ve terminal of the 2S was wired to the -ve terminal of the 3S battery in that serial harness, then the effective voltage sent to the -ve terminal of the servo via the balance connector was the voltage of the 2S battery. The servo has a ground wire connected to the receiver (I doubt it's isolated from the ground wire of the power input), which is connected to the ground wire of the ESC BEC output. That'll definitely create a short (Edit: this would also explain why all the charring/melting you've observed is on the ground wires). It's irrelevant whether you had something connected to the SPC port in this instance.
If you had the +ve terminal of the 3S wired to the -ve terminal of the 2S battery, there shouldn't have been a problem, as everything would have had a universal ground reference point.