Do you use Speedlites for events style photography? The sort of place where you don’t want to be changing batteries regularly and you want quicker recycle time?
If so, then it might be worth looking at the Canon CP-E4 battery pack. This pack has a slide magazine holder that takes 8xAA batteries and plugs into the side of your flashgun (550EX, 580EX, 580EX II, MR-14EX and MT-24EX).
By having 8 batteries instead of the normal four in the flashgun itself, the recycling time is reduced and you will get twice the number of flashes before you need to switch out the batteries.
If you go down this route, there is a flash custom function trick you should look at as well… on the 580EX II it’s Flash CFn-12 and on the MR-14EX and MT-24EX it’s Flash CFn-7.
This custom function allows you to set which power source the flash recycles from – the internal batteries and external power source (default) or the external power source only (setting 1).
You may think that the default setting is the best option – surely the flash will recycle a little quicker and you’ll get yet more flashes before battery change….right? Actually no. You may get a little faster recycle time but you can very well run into problems – the external power source will not power the LCD screen on the flashguns, it is only used to recycle the flash head. This means if your internal batteries flatten while using the default setting (for sake of argument FCFn-12-0) then no matter how much charge there is in the external battery pack, the flash will turn off and not function.
The solution? Set this flash custom function to setting 1 to only use the external power source for recycling the flash head. You’ll need to change batteries a in the external pack a little more frequently, but you won’t have the frustration of the flash turning off even though your flash pack batteries are brand new.
You can leave this set all the time as when you don’t have an external battery pack plugged in it will have no effect on the operation or recycling of the flashgun.
If indeed you don’t use the CP-E4 (E3/E2 etc) and prefer a third party unit like a ‘Digital Camera Battery’ or ‘Quantum’ pack, then again, this is a flash custom function you should be setting.