The external battery packs are power converters using more AA cells, but which output about 325 volts directly to the flash capacitor via a special high voltage port. The flash already contains one such power converter, and this adds a second one. And the flash still requires the original AA cells to power the rest of the electronics. Of the CLS iTTL flashes, only the SB-800 and SB-900 have this HV access.
Like most things, there are less expensive ones than the Nikon brand external power units.
But if the need is specifically for those flash without access, there is another choice.
The Al Jacobs models are two versions using a conventional 6V gel lead acid battery, with dummy AA battery size plugs which connect 6V into the regular AA battery terminals (instead of the AA cells).
BATTERY PACKS WE OFFER | [url]www.ALJACOBS.COM | Alan Jacobson[/url]
Lower internal resistance than NiMH, so it also can provide recycle some faster, and much longer battery capacity. Will use a different charger than AA of course.