solotripper wrote on Jan 19^th, 2014 at 4:03pm:

I did some Googling and from what I could gather it all depends on the device your trying to power and how long you want to use it.

You nailed it. 
I think for a particular chemistry you get more power per pound out of the bigger batteries but there is a lot of pounds there.  The AA’s are the only one you can get in lithium and lithium is lighter and more energy per pound.  Most of the detectors are made to operate on a automotive type battery.  Those detectors loose sensitivity when the battery voltage drops below 10 volts and they shut down at about 8 volts. You need 10 alkaline or 9 lithium in series to provide proper voltage.