Here's a product I can get behind called the Roost Smart Battery. It's a Lithium battery in a 9-Volt form factor designed specifically for smoke alarms. First, it will last five years. Secondly, it has built-in Wi-Fi that allows you to monitor and communicate with the battery from your smartphone. That includes the ability to identify WHICH FREAKIN' SMOKE ALARM IS BEEPING FOR THE LOVE OF ALL THAT IS HOLY MAKE IT STOP!!! Oops, sorry. I just went through an epic hunt for a chirping smoke detectors, and it was absolutely the most idiotic event I've participated in this year. This product would have made it so much simpler to handle. Back to the Roost Smart Battery: the Lithium core is replaceable. It's priced at US$34.99, and just began shipping. You can also order it through Amazon.