i'd say rechargeable just aren't reliable enough. i also use lithium batteries. they are awesome, super light and reliable. Hi Matt,You can answer a question without deleting all the other preceding answers. It just a common courtesy. Accord to the Petzl website Lithium batteries are not to be used in this headlamp because the extra power could overheat and damage the unit. en.petzl.comOne more time like I previously stated, I've been using lithium AAA's in all my headlamps without any problems whatsoever. New lithium batteries measure to 1.77 volts as compared to 1.55 for alkaline. The extra voltage amounts to .1625 volts per LED. Petzl added their disclaimer just in case there was a problem. That way they are covered, CYA. The incidents where lithium batteries did cause a problem were so insignificant compared to the number of Tikka series headlamps sold that they are better than the Six Sigma quality initiative which is 3.4 failures per million units. Here is a slight twist on the above. Yes you can use rechargeable and lithiums. I've used both for a couple years without issues. You might want to consider the use your putting your lamp through. I've spent several months training and racing sled dogs and used this lamp as a primary chore light for everything at my training cabin (no electric lights).Here are some characterisitcs I've noticed from my own long winter nights:Lithium batteries- perform well in cold weather and have a flatter discharge profile. This means you get more consistent light over the life of the battery. Lithium batteries are expensive however.Rechargables- These tend (in my experience) to result in a weaker light in cold temperatures. They also tend to dim as they discharge (this may be due to them cooling as I work outside however, I'm not too sure here). But, the long term cost savings is significant.For me I use rechargables for everything except situatuions where I need a brighter light, then I pull out a set of lithium batteries.Hope that helps.