The last wedding i did i got through 3 camera batteries for the whole day, approximately 2000 shots, The D7K is suprisingly economical regarding battery life, Obviously i never used the screen, only the LCD display.
The SB700 flash however got through 5 sets of Eneloops.
I always take the chargers with me, at a wedding reception there is (usually) always somewhere to plug in and charge up, and if not, I have a power inverter (converts my cars 12v cigarette power socket up to 240V so i can always charge up there if i need to.
Power inverters (often used for caravans / campers etc) have become a lot more affordable recently, you can get a 300amp unit for less than £25 (approx $35) and is ideal if you travel a lot, 300amp is more than enough to power a couple of battery chargers and run a laptop at the same time.