I wonder if you can modify your usage behavior to eke more juice from the battery....FWIW, and this may not be nearly the most efficient way to do things, I usually do a session per temp setting, then turn it off. When I want another hit, whether it's 15 min later or a few hours, I just turn it on and set it to the next temp, use it, and shut it off. I run out of weed well before I run out of power, BUT I also haven't used it exclusively as an intensive driver for the whole day (i.e. I'm not vaping continuously).
Similarly FWIW, I've wrapped a small fat rubber band around the pax2 and its dock and tossed it in my bag to charge, it worked perfectly and charged fully while I was hustling through Boston.