I have several of the older style Black Diamond Ions. They use 2 5mm LEDs but the newer ones use a 1/2 watt Nichia. I have them as backups for when my real headlamp gives out or if I unexpectly stranded after dark and have to get out. They are part of my first aid kit, and like most items in there, I don't ever plan on using them. They fold over to lock out a stiff clickie switch, they weigh virtually nothing, and the strap folds up nicely to a low-bulk package.
The battery in the BD Ion headlamp is just a bad size for anything other than emergency use. It is essentially a stack of 4 AG13 sized cells wrapped up in a tube. You can add a spacer and run the headlamp on the cheap AG13 Alkaline batteries that laser pointers use, but battery life is rarely more than half an hour. Don't even think about using hearing aid battereis of the same size as they can't put out the current and will self discharge once exposed to air. The silver oxide batteries BD sells or the lithium cells you can get in the Duracell brand cost about $5 each and last about 2 hours if you are lucky. If you use it for more than a few minutes, you really need to change the battery when you get back home to be sure it will get you out of a jam next time. That makes it very expensive light to feed.
The Princeton-Tec Scout and similar Petzl headlamps both use CR2032 battereis, which are better suited for somewhat unexpected use, but not great. I have a Scout which is a decent light. It takes 4 cells, but will run fine on 2. The Petzl came out towards the end of my healamp binge and I still don't have one. It seems like a nice light.
The Ultralight I'd like to try is the Coleman that has a Cree LED and runs on a CR2 battery. It seems to me the best battery for going light when you may need a headlamp. Something that runs on a single AAA battery might be just as good if lithiums AAA's are used. Just make sure it is from a reputable company.
