Everybody over fills them and drowns the element inside. It is only a fine element wire and with 'gallons' of smoke fluid to contend with, they don't succeed. The latest units are far superior to the originals. I would suggest 6v or 7.2v direct.
Have to disagree with you there, that may be the issue sometimes, but I measure out the fluid with a pipette for my Bulldog, it still only works occasionally. Works better on my sons Tiger I, never worked despite being able to hear it on my other sons Pershing, and never worked on a 2nd Bulldog. The other issue I know of is that the piston that pumps the smoke out often doesn't fit properly, so doesn't pump properly/at all. Someone did manage to fix theirs by packing it out with something, but for the cost of the TARR unit, it doesn't seem worth it. They seem most effective at reducing battery life.