I dont currently have one, i may get one, however working on the tracks, walking back and forward from media center i just carry a spare battery and card, its lighter on the hands and i dont currently shoot alot of portrait. However with some of the areas im moving into outside of motorsport i think one may be on the shopping list.
Your doing well to go through a battery in a day on a 40d, i probabaly use 1.5 batteries each day when shooting motorsports, thats shooting about 1500 photos each day.