How long have you had it for? Mine lasted 6h on reactive mode at the HPM (battery change on bleaklow; even turned it off), last time I used it from fully charged I got the death flash at about 100mins - and it's not been used that much!
My main issue is that my laptop didn't recognise it when I plugged it in so I couldn't tweak the settings.
I have used it quite a bit this winter, but never tried plugging it in and tweaking. It lasted for over six hours at the weekend with no death flash. But with all the other lights around it spent quite a bit of time turned down.