When a charged battery is inserted into an UV-3R, the radio powers up automatically. This doesn't occur when the battery is inserted, radio is OFF and the charger is plugged: the radio begins charging the battery.
The good news (for me) is that the radio (UV-3R mkI) powers up automatically also when there is no battery inserted and an external power source is supplied.
I am planning to install an UV-3R in the car to access the local UHF repeater. I don't need to change frequencies or touch the radio controls: just wear the headphone/hands-free and talk. This way I will get the radio up and running together with the car.
In order to reduce 12V of the car battery to about 4V I will use a recently acquired DC-DC switching step-down module (variable voltage, 2 amps) and throw everything into a small box under driver's or passenger's seat.
And REMEMBER: on UV-3R tip is NEGATIVE!!!