Thanks MP + KF.
Some more dumb questions  :-[

I understand that if you hook 2-9 v batteries in a series you get 18 volts?
I also see they now make 9 v in Lithium
  (You need to Login or Register

If you hook a fish finder to a 18 volt source will it damage it?
Now that they have 9 v Lithium, would it possible to use X amount hooked in a series to get the same run time as those 10 AA Lithium batteries.

I know your a "electrical guy" MP, so if you could get the proper voltage/ run time with using 9 v Lithium, is there a formula for comparing the run time at required voltage against the 10 AA set-up?

I don't have a Fish Finder and may never have one, but I always like to know/understand my options.