Do you know the external battery pack called MyBattery Pro? It's a Lithium Ion with 4400 mAh capacity , 4 times larger than regular rechargeable batteries. so you can use it for all day long, once you charge it full. It's worth to check the compatibility to your camera that you are using now. In addition, compatible with some Canon Digital EOS using optional DC Coupler. Look at website and take an advantage of it. What do you think?
www.mybatteryplus.com