Battery Carbon Monoxide detectors and Smoke Alarms - Battery or mains

[grantr37]

Anybody here offering CO and Smoke detectors to customers? could fit them at cost when doing other work in domestic locations....even just Battery operated ones...
There's a lot of dodgy gas heating etc out there and also the Holiday season in December with the candles etc....

 

[Fin170]

They tend to be a 'free' upgrade on many jobs when we are having a decent amount of floor boards up, I have little faith in any of the battery units on sale (probably because all the ones you see are tobacco stained non runners with the words 'What's wrong with the battery ones?' audible when pressing the test button)

Have a Co one near the boiler but haven't much experience with the mains operated ones other than the ones I'm given never seem to be interlinked with the smokes