This type of cloud key does not have a battery, if you also look at the official website it is not mentioned and mine which is from 2019 does not have one. You have to power it via a UPS.

Ubiquiti stopped using the battery from model year 2022 onwards because the batteries became very warm due to the high heat in the housing and inflated.
This is a safety risk and also caused many RMAs, so Ubiquiti solved the problem by removing the battery.
You now need an external UPS with the new models if you want to operate the device without interruption."

This is also confirmed here:

community.ui.com/questions/Cloud-Key-Gen-2-Plus-Battery-Issu...

You've already got the answer about the battery: Power via PoE.
If my CloudKey gives up the ghost at some point, or my security gateway, then I'll switch to the Dream machine. As far as I've seen, the controller is also included there and the Cloud Key is no longer needed. This simplifies the setup. But my information comes with reservations, you would have to take a closer look

Hello,

I bought one in January 2020. It doesn't have a battery, but in terms of security, that doesn't change anything because your router or switch doesn't have a battery either. For good security, it's best to connect all devices (cameras, router, switch, Cloud, etc.) to a backup power supply or UPS system.