This battery pack was made with the Fuji S602Zoom in mind, I've no idea if it'll work with any other camera. Please also note I take NO responsibility if you fry your camera - please check the voltage out before connecting to your camera.
The battery pack I've produced uses a 6V battery I use for my bicycle lights in the winter - seemed a shame to have is sitting around doing nothing for 6 months. The measured output is 5.3V - the IC is rated at 5amps +- 5%, you might be able to find a better one than me. It's not the lightest pack around but I didn't make it to be light - I carry it in a rucksack with other stuff so don't really notice the weight.
I've have a wire coming out of the container which plugs into the battery and a wire coming out the other end which fits into the camera. These wires are fairly thick as I didn't want to use speaker wire in case it frayed.
Parts used are below. Tried creating a picture of the circuit I used but gave up and took a photo of the paper I worked off - see pictures below.
Parts used;C1 = Capacitor 1000 micro farads/25V eloctrolytic.
C2 = 0.22 micro farads (ceramic type)
C3 = 0.1 micro farads (ceramic type)
IC LM2940CT 5V 1A low dropout regulator
Heat sink for IC above
2Amp fuse and fuse holder
Switch - to switch power on and off
5V LED - to show that the pack is on!
1K ohms resistor to restrict current to LED
Container to put it all in and 2amp rated wire
Various connectors (plus the one from radio shack discussed in earlier threads on DPReview).
I know the IC is only rated at 1A but I've had no problems so far (2 weeks testing). Using an ameter the amps used with the AA batteries in didn't go above .92 amps which was when I switched from record to replay. Haven't tried this to download pictures to the computer or using the internal flash - those jobs might be above 1A so may not work. I haven't tried them as I didn't want to fry the IC chip!
The circuit is nothing special, you can find plenty of these on the internet, I had help from someone at work to ensure that the circuit was okay, didn't want to fry the camera.
I'm now in the process of creating a 12V version for use in our motorhome (RV to some) or car, it's the same circuit but just change the IC for a 12V to 5V regulator.