WELL.
I opened up the oid Carbon Monoxide detector.
The battery is soldered in there, as I suspected, but the battery they used - a CR17335 - isn't cheap. 
You can make random number generator out of it -- https://github.com/bitplane/schrodingers-rng
But I wouldn't recommend that.
@avolkov @Jo @troubleMoney the poor man's hardware rng
@troubleMoney Eh, at the most, I might just harvest some bits off it, because I already bought a replacement.