I have not seen one of those since my PUP days (I had one in the PUP) this is a power supply not a converter, It does NOT charge the battery, You basically have a 3 amp unregulated 12 volt supply that can run a light or two.. I would (Did) use it to control a ceiling light (one) in the PUP. The switch let me use it off AC (via the power supply, just a transformer and didoes and I think a small cap but not sure) or battery, which I charged with a outboard charger.
But man, that is a piece of HISTORY.. I mean 30 years at least, it is a genuine antique.
My recommendation is to upgrade to a more modern Progressive Dynamics unit.
IT DOES NOT CHARGE THE BATTERY in any mode.. Mine was modified to do so (By me) but I had major issues with the breaker tripping, It is simply not designed for that job.