Depends on your mod as well, I have some 3400 mah Efest protected 18650 batteries that don't fit in some of my mods because of the increased size due to the pcb and side wire, not only the length is a problem but the diameter is slightly bigger causing problems with some mods, I try to stick to name brand batteries AW IMR, Samsung for the evic. Batteries can pack a lot of power in the bigger sizes so I will stick to the big names in term of safety/quality control