I noticed that the pinout for cheap Baofeng connectors has +V available. I'm interested in powering some electronics directly from this connector.
When unloaded, this seems to have about 3.3V on it, but when I try to power my load (an ESP32), the voltage drops to 0.8V.
How much current can this pin actually provide without significant voltage drop? Or, what does the voltage/current curve look like?
The math works out to a 10kΩ series resistance between +V and ground.
As such the maximum power that can be drawn is at 1.59V/10kΩ load, or 0.25mW max power. Short circuit current would be about 0.3mA.
I would first measure the current draw of your ESP32 to rule out a fault there.
The +V pin on the Baofeng microphone connection is probably intended to bias an electret microphone. These are essentially a capacitor, with sound pressure changing the spacing between the plates and thus the voltage. They contain a FET buffer since the capacitor could not drive a cable directly. The buffer requires power, but very little: the bias current is probably far less than 1 mA.
Thus it's quite likely the +V pin is unable to supply even a small current. Adding a series resistance on such a pin would have no impact to the intended use of biasing a microphone, but is a cheap and effective way to protect the radio from all kinds of faults that could happen on such a connector.
