I have the small model, works on a 9v battery. Works best when the
solution is mixed correctly! (Read the bottle.)
I worked for someone who must have used it full strength - lots of
fizzing, but the silver came out with a grey sludge look and had to
be hand-cleaned. Also, theirs was the larger model that had an AC
adapter, so was always at the same voltage.
My success could also be that the battery was rarely full-strength,
and the sludge wasn’t deposited at lower voltages.