Just in the cost effectiveness, lithiums are a bit more expensive but the alternatives fall flat in very cold weather. So in cold weather, I use lithiums, in warmer weather I use low loss NiMH That goes for my GPS, Any camera that uses AA's, or a headlamp.
So after the research today I think I'm going to be moving to the NiMH route as well. Lot's of info out there about the lithium issues. In answer to the earlier isn't 1.5 volts 1.5 volts turns out the answer is - sort of. They are all nominally 1.5 with alkalines being the closest. NiMH rechargables are typically lower - between 1.2 and 1.5. Lithiums are typically higher - between 1.5 and 1.7. Petzl isn't the only headlight manufacturer that recommends against lithiums. In their case it appears to be because there is no voltage regulator built in and the overvoltage can greatly shorten the life of the LEDs. Not so bad by itself, but when LEDs fail they usually fail shorted which will cause the lithiums to generate excessive current and overheat. In the extreme contidion they can explode. Not very likely but the idea of something that is in close proximity to my forehead exploding isn't something I want to comtemplate. There are apparently several of the GPS manufacturers that have the same warnings against lithiums.
Here's the specific notice from Petzl: http://www.petzl.com/en/outdoor/news...es-information
And a few of the many online discussions on the subject:
Thanks for taking the time to research and report this. Another example of the cooperation that occurs in online communities. Something that never ceases to amaze me. Thanks again to all.
There are more dimensions to electricity than voltage. For example, 8 AA batteries in series would be 12 volts, but it wouldn't start your car. The Lithium batteries supply considerably more power than the alkalines, and are great for high power consumption devices ( high power lights, camera flashes, motorized toys, etc ) If the device being powered doesn't have some sort of limiter built into the circuit, and was designed for alkaline batteries, it could get more power than inteneded.
There were a number of cases where the 3 volt lithium batteries (CR123) overheated and in some cases exploded, but I have never seen or heard of that with the 1.5 volt batteries (AA or AAA) I think some of the warnings against lithiums come from those incidents and overcautious legal departments.
LED headlamp batteries
The problem is not the voltage of the lithum batteries. Many LED lights and laser pointers rely on the internal resistance of the batteries to limit the current through the LEDs. The lithium batteries have a lower internal resistance and may burn the LEDs out by forcing to much through the lamps.
Adding a series resistor between the LED and the batteries and the lamp can limit the current to a safe level if the correct size is used. An easy way to find this current is to measure it with an amp meter with a standard battery and then try different resistors inseries with the lithiums until you find one that gives you that current.
Start with a large resistor and work your way down until you have it. (Do so at your own risk!!!! you could burn out an expensive LED if you get it wrong!)
Correction to my post
I just re-read my post and found I had made it a little confusing...I typed:
"Adding a series resistor between the LED and the batteries and the lamp..."
When I should have typed:
"Adding a series resistor between the LED and the batteries..."
The "Lamp" is the LED!
I could see adding a resistor to a headlamp you were using around the yard, or some other non-critical application, but I don't think I'd want to be alone, at altitude, in the dark, depending on a light that I had to open up and solder parts into, and hadn't been tested over a long period. I also wouldn't want to be in that situation with a light that wasn't electronically regulated. Those aren't conditions for a low quality light, and I would think that most of the higher end lights would have sufficient circuitry and regulation to not have to worry about it.
There is a headlamp section on www.Candlepowerforums.com that will have more information than you could ever possibly need on lights, batteries, runtimes, brightness, etc. The forum is the same format as this one, so you will all be familiar with the layout.
I have not seen the insides of the headlight you are talking about, but suspect it is not overly sophisticated. To save battery power, very few lights will have anything more than a simple resistor to limit the current. Voltage regulators waste power and add additional cost. I always carry extra batteries and at least one additional LED type lamp when hiking on extended day trips or over night even if it only a small keychain LED lamp. The simple headlamps at Walmart have 400 hour run times on a single set of 3 AAA alkalines. Having a spare is a real good idea in any case...maybe one could be modified for lithiums and run ...what is it they say...10x or maybe 4000 hours!
I am a bit of a light freak, so my opinions might not be conventional wisdom, but I wouldn't go out on a hike with a light that wasn't a high quality light. I'd have a light that was definately an LED, would have a circuit to regulate it, and ideally would have multiple brightness levels. Any light that fit that description would have the ability to work on Lithium or Alkaline batteries. Just a quick look at REI's website came up with this:
If I did a lot of cold weather hiking, I'd make sure the battery pack was remote so I could keep it inside my clothing.
When you add up what you spend on your boots, coats, sunglasses, electronics, gas, and lodging....saving 30 or 40 bucks on a headlamp seems short sighted to me...but like I said: I am a light nut.