I believe the Canon BP-511A Battery Pack was first introduced on the Canon PowerShot Pro 1 and is now featured in the Canon EOS 20D, its accessory grip - the Canon BG-E2 Battery Grip, the Canon EOS 5D and its accessory grip - the Canon BG-E4 Battery Grip.
The BP-511A holds about 27% more energy than its predecessor, the Canon BP511 Battery.
The Canon BP-511A Battery Pack is a Lithium-Ion battery - as maintenance-free as the current crop of digital camera batteries get.
My advice is to have at least two BP-511A batteriess (4 if you are using battery grips). Keep one (or a set) in the camera, keep the other(s) fully charged in your bag or pocket - ready for immediate use.