Roost calls the RSA-400 a smart smoke alarm, but it's really just a 4-in-1 alarm where the battery contains all the smarts.

We gave the Roost Smart Battery high marks when it came to market late last year. Equipped with a removable Wi-Fi module and a tiny microphone, the battery will send an alert to you and as many “watchers” as you care to designate the instant your smoke...