As is the case with smoke detectors, some states and cities require combination smoke and CO detectors to use a non replaceable battery with a 10 year lifespan. If that applies to you, the Kidde P3010CU Est. $55 is a model to consider. It's a stand alone alarm with voice warnings to signal the nature of the threat. The smoke detector uses a photoelectric sensor. We didn't see any credible expert reviews and user feedback is only so so 3. 9 stars based on more than 130 reviews in total split between single units and twin packs at Home Depot, and even worse scores at Amazon, though that's based on fewer than 20 reviews. Unfortunately, we didn't spot any other combination smoke and CO detectors with 10 year batteries with significant feedback and better scores. Like the First Alert SCO501CN 3ST, the Nest Protect is a combination CO detector and photoelectric smoke detector. Its feature lineup includes a night mode that prevents low battery chirps from disturbing your sleep and a humidity sensor that helps the unit tell the difference between smoke and a steamy shower. It forms a network with other Nest Protect units, and can pass information on regarding the location and nature of a detected hazard.