A first name on you profile might help.
I have no idea what you vehicle is or what type of generator you have.
I am no Auto electician, but a 12 volt geny should run up past 50volts un regulated. 20 volts sounds a bit low?
It should also "motor" in other words, it should run as an electric motor if you earth the case of the geny, and put the battery feed to the "A" terminal. These are basic checks.
I cant help with the fuze.
Can you post pictures?