It's mainly due to the utilized chemical composition of the cells. I'm no expert on battery chemistry though, but my post is based on Mooch's comparison of the HG2, VTC6 and 30q in a stress test, which can be found here: Bench Test Results: Which lasts longer? VTC6, 30Q, or HG2? But as a rule of thumb, the cooler a battery runs, the longer is its cyclelife. And the safer it is in overall use for vaping obviously.