New project 1999 gtx. Charging issue.

[mograndfront]

I bought a 1999 gtx as a project. It had TWO batteries in it and an extra rectifier in the storage compartment.
I got it running. I took out one battery. When it runs, the volts drop to about 3 volts.
When you cut the ski off, the volts go back up. Would this be a stator or a rectifier issue? The batter is about a month old AGM battery

 

[mograndfront]

Also, this is a fuel injected ski if it makes a difference.

 

[Mraw]

I would get the battery load tested. Then To test the rectifier, rev the ski to 5500 ish rpm. The voltage at the battery should be between 13.5 to no more than 14.5