As I understand it, the standard DC plug for 3.7V 18650 (and similar 1S) is the 5.5mm x 2.1mm. I'm doing a few stunts with a 2S (7.4v nominal), and wanted to add a DC port. But I think that using the same plug for 3.7v and 7.4v is not a wise decision. What is an alternative DC plug? 5.5mm x 2.5mm?