First off, let me state that I love my VTR. It's my go-to machine and I'll probably add another to my collection soon.
That being said, these things are marketed as being able to adjust voltage automatically to generate the desired output wattage... I'm not seeing it. After a few weeks, I can tell when I pop a new battery in this thing. It hits MUCH harder with a fresh charge. Also, over the past few days I've noticed this thing hitting unexpectedly harder. As I check my coil resistance I can see that it's dropping. I normally keep it set at around 14/15 watts, but I'm having to back it down a little as my coil resistance drops. I can only assume it's going to die soon.
Maybe there's a problem with my VTR. I don't know... but I'm just saying, this thing doesn't hold a constant output.