Slim ecig batteries last at most 5 hours. If your unit is 120mm long with a carto on than it is probably a standard KR808D-1which is 280mah or about 3-5 hours of use, less if vaped heavily. That actually is about the longest charge life of a slim 120mm ecig. Many like the standard 510 are only 180mah or 1.5-3 hours of charge life (less if vaped heavily, and ditto for the short version of KR808D-1). Many people use chargers at work or use a PCC (a charging case you charge up over night then it can charge a battery 3-4 times)
If you want longer battery life then you need to step out of the slim ecigs and go to an Ego, Ego-T or Riva 510 which gives 6-8+ hours battery life.
If you want to vape constantly you can get a passthrough and vape off AC to USB.
Re cartos - You said you can pop off the carto end - if you keep the carto damp by adding liquid when the vapor seems light or a bit dry, then you should be able to get days out of one carto. I have one carto for another model (510) I have gotten 2 weeks out of but it's getting weaker now. Look for good PG/VG blend liquids so you aren't buying the Chinese PG juice which can be a bit harsh (PG lets throughthe most TH, VG smooths it a bit and adds more visible vapor).
You could keep V2 and refill the cartos. One downside in my opinion is they are probably auto batteries. Some people like them but I think most people here use manual batteries. Seems to be less work, simpler mechanism, more sealed against death by juice leak, won't turn on accidentally from wind or vibration.