First 12.5 seconds

[Gnat]

Please explain what happens during the following periods of time. When I press the power button on my UPS (BV800I-GR) and simultaneously the stopwatch, i have the following: 0-2.5 seconds (long beep, then relay <click>), 2.5-5.5 (just silence🤔), 5.5-12.5 ( <click> - power supply started from the battery, judging by the hum of the speakers - hello stepped approximated sine wave 😀 and then again <click> - power supply from the grid, since the speakers stop humming). From 0 to 12.5 seconds a “test of the power grid and itself” occurs, I believe. Is everything correct? In any case, I turn on the PC only after 12.5 seconds have passed.

P.S. The UPS is new, works without problems. There are no problems with electricity during startup.