I have a Fenix headtorch here from Mike Moore, unsure what model, if he is around he may care to comment.
It packed up and on examination water ingress to the battery box was the issue
Frankly, the 4 series AA battery box was unfit for purpose. mike is no sump diver and yet water managed to get in. The end rubber seal was easily peeled away even when secured with the screw. The intention now is to mate it with a 4 AA Petzl Duo battery box.
If anyone has done this and there is something I should know about please tell.
A Li Ion pack would be better but voltages don't match. 4 series AAs will give 6V off charge, depleting. Li Ions are single 4.2V depleting, or 2 series 8.4V depleting. I could probably do something with a DC DC converter but its not worth him spending the money.
An idea that has just occurred is to put a USB plug on the end. It would run off any 5V power bank then.
Ideas in action.