This should have been one of the first log... better late than never.
During this phase a learned a bit about how that UPS worked, Especially how the old battery where wired internally.
here is a look of it after the little modification. I added the cable on the right that go to the back of the UPS where there screwed to a terminal. Ready to connect the external battery.
Originally the four cable on the left went to theses two fuse pannel (for lack of a better decritpion) and from there where wired 4 pack of batterys. each pack contained 24 battery wired in series for a total of 288V
I believe that there pack where put half in serie and half in parallel and that a -288V was created with the two 288V pack put in serie . Then the juice went to the internal circuitry. from there i guess that a H bridge created the Sinus but this is only an educated guess as i didnt dive deeply into the circuitry...