Don’t use car battery in UPS. They are made to start car, high surge current. Under deep cycle in UPS, battery will be ruined in a dozen cycles. Get a 12v 50-100ah LFP (LiFePO4) battery. Wire it to UPS. Should do the job. I retrofitted multiple UPSes with quality LFP. Much better
Copper in those heatsinks is worth more than the server