Unfortunately, there are no drop-in regulators currently being made, however there are high-voltage atomizers available designed specifically to provide the same wattage (heat) as a 5v source on 2x 3.7v
batteries. I actually spearheaded the development of these personally, and as far as I know the only supplier carrying them is Nhaler (Drewsworld on the forums). The eastmall 'HV' branded atomizers will perform poorly on 7.4v, they are of lower resistance and designed for 6v, however I have not personally tested these and cannot give an opinion on them either way. Drew is a good guy tho, and he'll take of you
