On 03 Jun 2012, Jim Elbrecht wrote in
alt.home.repair:

2 c's will kill about a dozen mice was my experience with one many
years ago-- Victor says their 4aa model will kill 100 mice per
set-- 4 d's ought to kill a few rats.

there is a capacitor in there someplace.

Huh! I don't think of 4 D-cells in a flashlight being deadly like that.

I wish the Rat Zapper people made a larger version I could use on
squirrels.