As I said, some of them have no overly obvious battery compartment. Either it's on the backside, or internally and you have to unscrew it open. Not all have an obvious battery compartment.

And it's incredibly annoying when you have to climb onto a chair/table/whathaveyou because your ceilings are so high you can't naturally reach it.

The one in my last apartment even had a big button in the middle. I thought I might be able to press it in order to turn off the alarm temporarily. That was not a good idea.