The alternator becomes the ground reference when you pull the battery. It outputs a voltage between its output and the case.
BUT just don't do it on a modern car. On an old points style system, a brief 18-20 volt or higher spike would not be noticed.
A computer would notice big time.
Among other things, the battery acts as a buffer and a load. The regulator on an alternator is not very "tight". For one thing, you are regulating a mechanical object.
As the voltage out begins to rise, the regulator starts reducing the rotor voltage. As the voltage begins to go above the battery voltage, it starts to draw current. The higher it goes, the more current it draws.
This sucks down the excess voltage and gives the regulator time to work.
Without the battery, if the car is not drawing a heavy load, there is nothing to limit the voltage.
All the regulator can do is reduce the drive, with little load, the excess electrons don't get used, and the voltage can go really high.
And that is if everything is perfect.
If you have an external regulator, and it's ground reference is slightly different from the alternator's due to a bad ground wire or connection, it might be seeing 14 volts while the alternator is putting out 30.