What's the precise model number?
Though there aren't much service manuals/precise data about UPSes.
One thing which comes to mind is circuitry needed for powering UPS.
At least some UPSes need battery power to even connect into mains with default state being mains relay in disconnected state.
(and when presence of mains voltage fitting insdie tolerances detected UPS connects to main only then)
For example if that circuitry has any power regulation, some failed capacitor could have cooled during battery change, if UPS was disconnected from power for longer time during that.
Also that fuse of battery connection is easy to check.
(or more likely it's multiple smaller fuses in parallel)