I have never used Yongnuo so I can't say for sure. Its a good bet you will still need 4 batteries in the unit. If you are talking about the SF-18 then this is what the manual says. "This product need to insert the 8 new AA batteries"The problem with external battery packs is two fold. Most of the OEM's use 8 rechargeable AA batteries and are moderately expensive. The ones that don't rely on rechargeable AA batteries are just plain expensive.I use eneloops with their included chargers with no issues.
If that's not good enough then you should be looking at external battery packs, not rechargeable AAs.
In addition you still need batteries in the flash to use a battery pack. The battery pack supplies the power to recharge the capacitor, the internal batteries provide power for the electronic operations of the unit itself.
Learn something new every day.. is this all flash units? Or just a certain brand?
I use yongnuo flashes, and love them. I've been thinking about buying their lithium ion external battery packs for shooting basketball games.
Flash battery Pack SF-18