http://www.alpkit.com offer one of the finest budget head torch on the market.
The Gamma LED Headtorch offers around 80 lumens from a single XPC Cree LED but also has the addition of green and red night vision LEDs.
I'd had used my head torch on a lot of trips for a number of years but unfortunately on one particular trip the battery clip door latch broke off when I was replacing the battery and rendered the whole unit useless so I've come up with a universal fix which should give you a little bit more use out of the left over torch.
The original unit was powered off 3 AAA batteries equating to 4.5v
I did a fair amount of research into alternate battery clips and batteries and in my opinion this was the the most efficient and cost effective.
Although my solution isn't technically kosher it works and will get you a little more life out of something that is bin-bound.
Parts & Tools needed In addition to a broken gamma head torch
1x Emergancy Mobile Phone Charger (EMPC)
1x AA Battery
1x 2mm Jack Plug (Several packaged with EMPC or you can buy separate)
Wire cutters/ strippers
Step 1: Preperation
Output voltage is 5v a little more than we need but not so much we need to bother with resistors. They run off a single 1.5v AA battery so the life expectancy is probably quite short, I imagine there's some joule thievery action going on in there. But AA batteries are cheap and light to carry, so that's not too much of a problem for me.
I bought a Rolson EMPC from Maplin for approximately £3 and they come in a variety of colours and normally include a battery.
2mm Jack Plug
The EMPC delivers its power via a 2mm jack and comes packaged with a number of jack-phone adapters, you need to pick one to sacrifice (make sure its not for your phone if you intend to additionally use the EMPC for its intended purpose).
Alternatively you can buy the jack and some cable separately.
Right enough blab lets get on with it.
Step 2: The Cuts
Take your Gamma head torch and cut the wire as close as possible to the battery compartment.
Now take your chosen 2mm Jack adapter and cut off the mobile plug end, again give yourself as much lead as you can.
Step 3: Strip & Test
From the head torch end there should be two thin wires mine were red and white, because we are working with diodes these wires are sensitive to polarity (+/-) so we need to do a quick test before we can continue.
On the jack there are also two wires pair these up with the head torch wires by twisting them together, plug the jack into the EMPC and make sure you have a AA battery installed and the correct way around. Press the button on your head torch and you should have light if not swap the pairs around.
Once you've got them working you can solder them up. I like to put a bit of heat shrink around each of my solders so don't forget to slide this on before you start soldering.
Step 4: Connect & Go
I had discarded the webbing so I didn't come up with a method for attaching the new battery pack but it works great as a backup flash light.