APC Back-UPS BX800CI-RS suddenly cuts power

[Anonymous user]

APC Back-UPS BX800CI-RS
y2014 manufaction date, including battery.

What happened before the issue:

There was a 1.5h hpower down with UPS and sleeping (suspended) PC connected to it.
Probably it completely drained out UPS. Well, at least PC was found shuted down.

The issue itself:

1) I cut power for UPS, UPS works well and PC connected to it too. I turn power for UPS on again. Everything works fine.
2) After 30-35 minues UPS suddenly cuts power for PC and produce constant tone. All light indicators are off.
Here is the video https://www.dropbox.com/s/34leydo29fwzgr7/VID_20190727_215732.mp4?dl=0

Note:
- I think 30-35 mins is a period of self-test after power outage. I think it always worked that way.
- Issue is reproducible even with tiny load (like 7W LED lamp)
- Computer runs on UPS well, so battery looks Ok.

Question is: is this 100% related to fact that battery is old and I just need just to replace battery, or it's some kind of UPS failure and it requires fixing?