Needs a base plate, but alarm life valid for another 3 years so great if you have an existing model expiring and can swap out for another 3 years’ life.
This is the ST-750. Many FireAngel alarms with built in batteries use similar base plates, eg. ST-622 so this will happily replace one of those.
Alarm is fully working. We had it replaced when a new connected alarm system was fitted as part of an extension to our house.