Canon 50D battery grip

About how many shots should I get from both fully charged batteries? I want to use it when shooting a wedding in 2 weeks but don't want to run out of power.

And does using flash during most of the wedding, will that drain them any faster?

Thank you.

Flash won't affect the battery life of the camera batteries, unless you're relying on the camera's pop-up flash (I hope you aren't; it's not up to a wedding). You'll get the same number of shots, more or less, you'd get from using the two batteries one at a time in the body. Ballpark number for BP511s is 500-700 each, depending on battery age.

Jon wrote in post #10346745
Flash won't affect the battery life of the camera batteries, unless you're relying on the camera's pop-up flash (I hope you aren't; it's not up to a wedding). You'll get the same number of shots, more or less, you'd get from using the two batteries one at a time in the body. Ballpark number for BP511s is 500-700 each, depending on battery age.

Ok thanks..nope, I'll be using the 580exII..shucks at 500 shots per battery that is 1000 shots approximately..that will work big tiime.

Thank you. Now I don't have to buy 2 more and another charger to have in reserve.

I shot about about 540 shots at my sisters wedding and at the end of the day my battery still showd full

Don't forget you could also use the AA battery tray as back-up to the BP-511a's, then the AA's could also be used to back up your 580ex II batteries.

You'll get another 400 shots out of the AA's

You'll probably need about 2 sets of AA batteries for the flash if you expecting to take more than 500 images.

I have the XTI and 580 II also . I carry spare batteries for the 580 II anyway , so carrying the AA tray is no big deal .
I load if with fresh AA's [rechargeables] when ever I think I might use the flash . I recharge them every 2 or 3 weeks .