Great idea! Nice of you to do....
just to add to your tip, I've been researching detectors recently for my "sticks and bricks". Several nights ago we had one start squawking in the middle of the night for no reason. only the one was going off but they are wired so when one goes off they all are supposed to.
Anyway, in my research I re-learned that the suggestion is to replace these things after 10 years.
also, that when testing, the button only verifies the battery and horn. test with real smoke once in a while.
I did mine and found that 2 out of the 9 of them were very slow to alarm. They ultimately did, but still... Also, nver did the interconnect thing ever work.
so
i ordered a bunch, some combo CO and smoke, and now I have another project.
Oh, and one other thing to add... I came across some statistics in my research. I didn't even try to remember the numbers, but the point was that in a majority of fires (or was it fire deaths) there are smoke detectors present but they did not work.