On the DC side.. Normal practice is to tie the negative to the frame/body. But it is not needed, you can float it if you wish..
Can it operate w/o a battery.. Most likely yes.. I have not studied that line but most likely yes.. However if you find you get "Buzz" in 12 volt stuff you can add what is called a hardening capacitor from any high end auto audio store.
These high capacity "Cans" It looks like a long tube or can with two screws in one end) act as battery simulators... I do not believe any current converter needs one though.. Just a couple of very old ones.
