Many digital cameras like the Olympus C-2100UZ have an external power jack that can be fed from an accessory supply. They can just as easily be powered by an external battery pack. Some web-links are provided below but be warned, some of the designs incorporate C or D cells with capacities far below that of today's AA cells. A 2500mah AA was unknown until recently and is a great weight saver. An easy alternative to an external supply is just carrying a couple of spare sets.
This is nothing new, radio-control car hobbyists have been making battery packs for a long time. When I asked at the hobby store for large shrink-wrap tubing the guy said "all we have is this here for making up battery packs" :-) Sheesh, half the time when I need a part for a diy project I have to explain why I need it and why I don't just buy the inferior product they have on the rack.
The nice thing about the external supply is the higher voltage. This makes for faster flash recharging and possibly faster lens movement and focusing. I can't vouch for the latter but it was reported on another brand of camera.
I made up two sets of five AA batteries and joined them in parallel for a nice ~6-volt 5000mah portable supply. The actual voltage when freshly charged is nearly 8 volts.
10 AA 2500mah NiMH batteries from Walmart ~ $22.00
1 small bag to hold the pack ~ 1.00
1 two-pack of Radio Shack power connectors 2.00
1 .75x.016" brass strip 1.00
2 >1 amp diodes 1.00
1 recharger jack
24 inches of flexible two-wire cable
24 inches of large heat-shrink tubing
2 cable ties, velcro strip, or tape
I cut the jack off an old Radio Shack 7.2volt r/c car battery and put that and the charger to good use. And cut the cable off an old transformer from the junk box. Walmart had a nice cheap camera bag for 99 cents and an 8-pack of batteries for $18. An 8-pack and a 4-pack even covered the two batteries that got soldered together in a very short circuit (man that solder melted fast!).
Weller 100/140 watt solder gun with new point
thin electrical solder (not plumbing solder)
Cut 8 1/8 inch brass strips 3/4 inch long to connect pairs of batteries. Tin both ends of each and tin each of the batteries to be connected. Tinning means to melt solder onto. Make sure it is a good tin by running the gun tip over the surface until the solder flattens. That is immediate on the brass, but a little longer on the batteries. Do not heat the batteries any longer than necessary. If the solder minutes instead of seconds to melt, put a new tip in your gun.
Clamp five batteries side to side in alternating directions. Connect them in series by soldering brass strips across the ends, positive to negative. Make two sets of five.
Run two wires from the positive side of each set, one from each set is joined into the recharger jack postitive side. The others are wired in series with a diode which is joined into the power connector. The two diodes prevent current from flowing from one pack into the other in case they have different voltages, or a cell goes bad.
Run another two wires from the negative side of each pair. Join two into the negative side of the recharger jack and the other two into the negative side of the power connector.
Cut heat-shrink tubing to fit about 1/2 inch on each end longer than the batteries, slip it over the pack and apply the blow dryer. As it shrinks, move the pack around and over to make a nice tight fit all around. I shrank the ends first to keep the batteries from being pushed out.
I used two cable ties to hold the two packs together, but velcro or tape would work fine. Drop everything into the little camera bag and you're all set. I just stick my little finger through the belt strap and let it hang below the camera. But the cable is long enough to reach my pocket.
More-or-less inspiring links: