The StenLight battery is meant to be mounted outside the rear of the helmet. I opted to try and mount mine inside the top. I realize there is a risk of injury should the helmet be called upon to stop a falling rock, but I doubt I would sustain any more injury from the battery. It fits perfectly inside the top of my Petzl Ecrin helmet. There is even room for a second backup cell on the other side. Heavy duty velcro holds the battery in place.