I’m confused about how the charging system works

Ok. Bought a 09 grizzly 350 that didn’t have a battery in it. It runs fine. I figured before I install a new battery I will check the charging system. I connected a volt meter to the battery leads and sure enough, only 6 volts with no battery installed.

When I connect a new battery, the voltage reads 12 volts with the atv off, and 14 volts with the atv on.

Why is it only 6 volts without a battery in it? I don’t get it.