All of my mods get warm, especially considering I have hot, sweaty hands.
But!
If I pop my batteries out at any given point during my mod being warm, the batteries themselves are never actually warm. They're always noticeably colder than my mod itself.
usually my batteries are cold when i take them to use, but when i use it, and the copper mod gets warm, the battery aint "cold" anymore, its not even close as warm as the mod, but its like hand warm

idk how to explain, or then im just overthinking this. Its just like handwarm, if there was any kind of problem, i think it would be warmer/hot. And i would be surprised if it didnt warm a bit when its on such a load
