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This actually started out as a comment, but I think it deserves an answer of its own even though it's more about whole-device behavior than electrical engineering per se.

I don't know if your particular UPS does it (it probably does), but a good UPS will occasionally do a **self-test**. Typically this will involve switching to battery power for a while even though mains power is considered to be of adequate quality, and in one way or another monitoring the discharge characteristics of the battery to make sure the battery's response is within acceptable limits.

You mention that you have occasional mains power failures, on the order of several times per year for more than a few minutes. A very rough estimate of the [power draw](https://electrical.codidact.com/comments/thread/9133) on your UPS would be on the order of maybe 100-200 W typical, and you mention that your UPS is specified to be able to handle a load of 450 W. A typical consumer UPS might be able to run for 10-15 minutes at load capacity. The upshot of all this is that in your situation the battery would be discharged at a reasonably high rate while there is a mains power outage.

When the UPS switches to battery power and starts drawing power from the battery, the battery terminal voltage will drop. This in itself is entirely normal; every battery has some non-zero internal resistance, and this internal resistance will manifest itself as a voltage drop while current is being drawn from the battery, increasing as more current is being drawn. Therefore any given battery will have some maximum current that can be drawn from it while keeping the terminal voltage about some given value.

However, if the UPS switches to battery power but the battery, being marginal, *can't keep up with the discharge rate*, then the battery voltage will drop below the requirements of the UPS' inverter, which is the part which takes the DC power from the battery and turns it into AC power for the outlets. That in turn will cause the inverter to shut off, either because of built-in protection circuitry or simply because it cannot work with the too-low voltage. (In a good UPS, you'd expect it to be the former.)

**The result: apparently random power failures on the nominally power failure-protected outlets of the UPS in the presence of normal AC mains power.**

**Therefore**, apparently random power loss on equipment protected by a battery-powered standby UPS is **a good indication that the battery is overdue for replacement.**