The DC's are finicky at best! I found somewhere around 4.5-4.8v was a good spot for my taste. Which doing the math, 1.5 x 2= 3 + 2 = 5v doesn't make much sense, but these burn hotter, and the top coil is closer to your mouth and at 5v it would pop and boil
juice sending it into my mouth. Not a good taste!
In theory, you should be able to go up to 6v since it's in a tank and cools faster, but more than 4.8 was too hot for my taste.
Not sure I helped much, but I'd start around 4.5v and go from there to find where you like it at.