Ah, ok. I think this was one of the odd things of the U21; the light wasn’t pushed hard at all, nothing like the E14 for example. The light was ok, but nothing more than that…
As far as the XHP-35 goes, 2.6A is the sweet spot, output drops off after that. That’s emitter amperage though, I don’t know what tail current should be on this particular light to achieve optimum emitter current.
Edit: This is both in my experience and with testing TA is doing.
Amps at the cell is like 4X the LED, roughly, maybe little more. Should be doing over 5A because stock, the U21 was like 1300 lumens, so in the 1-1.5A range, think it was spec'd at 1.3A at the LED. Wonder if he had it on true turbo... I've made that mistake before.
Since these boost drivers are a bit more finicky, don't think you can crank them up much, plus it's a huge pull of amps for a single cell - 2.5A at the LED is at least 10A from the cell. Good reason to convert the driver over and go with a conventional single cell LED, like the XPL. I got just over 1,700 lumens on a dedomed XPL in this light.
Thank you for the review so far, the amp draw is very low, I have a X1 with a boost driver it pulls 7,34A on a rested King Kong 4000 mAh ICR26650 at the tail for 1,47A at the LED. I think it should be pulling at least 5A to get 1A at the LED.