The Good If you can install a battery, you can install the Roost Smart Battery into your existing smoke detector, and it will reliably send you push notifications when your alarm sounds. Both the battery and the app are attractively designed and easy to use.The Bad Roost offers the option to “snooze” your alarm, but that won’t work if your alarm is hardwired and the 9V is acting as a backup. Roost doesn’t offer any interconnectivity to other smart devices.The Bottom Line You won’t get as many smarts with the Roost Smart Battery as you would with a wholesale connected replacement, but at $35, it’s a viable and cost-effective option for simple, retrofit smarts.While raising money on Kickstarter in 2014, a big part of the elevator pitch for Roost — a startup based in Sunnyvale, California — was that its Wi-Fi-enabled smoke alarm battery would forever banish the middle of the night “low-battery chirp” that’s the nightmare of homeowners and apartment dwellers everywhere.Does the Roost deliver? Well, we won’t know if its smartphone-based low-power notifier works anytime soon, since its battery life is rated at five years. But we can tell you that the $35 unit — which replaces the 9-volt battery in your standard smoke or carbon-monoxide detector — works well on nearly all other accounts, delivering notifications to your smartphone wherever you are when a smoke or CO event is detected in your home.At $35, it’s an easier splurge than the similarly aimed $100 Nest Protect , and a more cost-friendly way to smarten up your whole home’s worth of detectors as well. The Roost Smart Battery isn’t as smart or feature rich as the Nest Protect 2.0, so it’s not the perfect option for everyone, especially not those looking to build an interconnected smart home as Nest plays better with others. But for those looking for basic connectivity without hassle, I like the Roost Smart Battery. It really only does one thing, but that’s all it’s promising, and it fulfills that promise with polish and gusto.Ruling the RoostI found the Roost Smart Battery refreshing in a few ways. It’s not flashy, but it is attractive. It didn’t have to be attractive, since you’ll never see it once you stick it in your smoke detector, but I appreciated the touch.