I tried something similar with my 5v box mod, my Li-Ions had not come in yet, so I used 4xAA's in a box attached to the underside of my mod box. They lasted me almost 36 hours before I had to change them, but I could definitely notice a difference in quality after 2 hours of use. Put a fresh set in, good to go. But with the Li-ions, they have a very flat power-output curve over their 12 hour (avg for me) charge span, and quality only seems to suffer on the last 2 or 3 drags.
If you are used to a constant 5v vapor, then you won't get the same experience with NiMh. For me, 3.7 was okay but above that was better. 5v with the 901 makes a bit harsh vape for me. With this box and the 901 atomizer, the quality stays pretty constant until near the very end of battery life. You know when it's time to change the batteries when you are holding the button down longer and the dead givaway is when the LED turns green.
With other lower resistance atomizers, I could imagine that the quality could change but I only use this with 901 atomizers and believe me, it stays very constant until the very end.
The only thing I don't like is when you first put in a fresh set of batteries, there are 4 to 5 puffs that are hotter at first then it mellows out and it is good until the end of the battery life. That's my only dislike. Everything else is a plus.
The Puck supplies great smoke up until the batteries drop below 3.7 and thats when it's time to change them with a newly charged set.
It is true that lions will supply the constant voltage up until the very end of their charge and can hold a static charge longer but since I can get more than 3.7 volts out of this using 4AAA NiMh batteries for over a 24 hour period with my average vaping, I don't need lion batteries. Lion also seem to wear out faster. I was (and am) tired of buying them. That's why I created this mod.
Like I said before, on my worst day when I had chain vaped constantly, I still got 12 to 14 hours out them. That still makes it worth to me to use NiMh.
The Puck is simply an alternative to using your standard old lion batteries and using batteries you can go out anywhere and buy when you need a new set.