Get battery percentage over web bluetooth

Thanks - yes, I'll have a go at that :)

Do you know if it's possible to connect multiple devices at once via Web Bluetooth?
It is, yes! It depends on your platform, but it's around 5 or 6 maximum

Do you know if I can connect to devices automatically via Web Bluetooth? (Let's say: Remember a device and if it's reachable, Web Bluetooth should automatically connect to it)

I don't believe so - yet. That's one of the things they're looking into though.

Using the characteristicvaluechanged event, I should be able to get the real time data, every time the battery percentage changes, right?

Yes, absolutely - as long as notify:true in that Puck.js service :)