The 1.5 ohm dual coils work fine at 5.0v. Some people run them higher - and I have too, but 5.0 loaded volts is as high as I care to use them. After 5.0v, it all depends on the
juice if it can take the extra heat without scorching. I have a couple of juices that are OK at 6.0v, but I only run them at 6.0v rarely.