Most of Canon's high-end flash units have sockets on the side which can accommodate external high-voltage (270 volts) battery packs. These packs have two basic functions - they speed up the flash's recycle time between shots to a second or two (critical for news or wedding photography) and extend the time you can go between changing batteries. They're also useful in cold weather (battery performance always drops precipitately at freezing temperatures) since you can stuff the pack inside your jacket to keep the cells warm if necessary.
The Compact Battery Pack E requires 6 regular AA alkaline, NiCad or NIMH cells, but the newer Compact Battery Pack CP-E2 can also accept lithium AA cells. Either compact pack can be attached to the bottom of a camera using the tripod mounting screw. The much larger Transistor Pack E can use either 6 regular C cells (with Battery Magazine - lithium cells are not compatible) or nickel-cadmium rechargeable cells (with Ni-Cd Pack) and obviously has much greater capacity than the smaller Compact pack.
A number of other companies also sell high-power battery packs compatible with the Canon Speedlite high-voltage connector. These products include Quantum Instruments' Turbo (lead-acid) and Turbo Z (NiCad), Lumedyne's Cycler and Dynalite's Jackrabbit.
Unfortunately, the packs are all fairly heavy, bulky and inconvenient, (especially the huge Transistor Pack E and third party products) and require that the flash unit be tethered to the battery pack via a coiled cord. Note also that the flash unit will not work with an external pack if the flash unit's internal AA batteries are dead or missing - the high-voltage power is used solely for recharging the unit's capacitors, not for powering its control circuitry.
A number of manufacturers also sell generic battery packs (such as the Quantum Bantam) which can be connected to most AA-powered EOS flash units - even those which don't have special power sockets. They work by replacing the AA batteries with a plastic shell and running a cord to the power pack. However, as they aren't highpower they can't speed up the recycle time as dramatically - they're more useful for extending the number of shots you can accomplish between battery changes.
Keep in mind that portable flash units were not designed for continuous high-power use. You can damage your flash if you fire too many high-power bursts in a short period of time; something an external battery pack may let you do. So try not to fire flash bursts for longer than a few seconds, especially at full power manual or small aperture TTL firing. Remember that smoke emerging from your flash unit is shorthand for "stop immediately."
Flash units with high-voltage sockets:
Speedlites 430EZ, 540EZ, 550EX, 480EG*, MR-14EX and MT-24EX.
* The Compact Battery Packs are not recommended for use with the 480EG. Flash extenders.
If you're doing nature photography of wild animals or are stalking wild celebrities for a tabloid and need to use flash photography across great distances, you might consider a flash extender, such as the Better Beamer. These accessories are simply plastic Fresnel lenses you can attach to your flash unit's head with tape or velcro. They concentrate the light much like a zooming head and give you an extra couple stops of light, at the cost of coverage area. They're only really useful, therefore, when using very long telephoto lenses - say, 300mm or so or longer. Michael Reichmann's "Luminous Landscape" Web site has some example photos of how this works, and Arthur Morris' "Birds as Art" site sells them.
I've also seen the term "flash extender" refer to devices which let you mount your external flash unit higher up off the camera hotshoe, but that's something different altogether.
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