Here's a comment on a reply in this thread.

It is not recommended to run the car with the battery removed.

The sensing circuit will tell the voltage regulator to kick up to the max, say 19 - 20 volts. Any light or electronic gear on the car may fry due to overvoltage.

I once fixed a car that had been on a railcar and had to be driven off, but the battery was dead. They borrowed a battery from a passing taxi (not in the US) and used it to fire up the car. They turned on the lights, got it off the railcar, and then with engine running pulled the battery and put it back in the taxi. As they were removing the battery, they noticed all the lights burning out. The alternator also failed, developed a short in the windings. So we got to replace all the bulbs and the alternator on a brand new car.

Profit from my misadventure, don't do it.

R.