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 Canon BP-511A Battery Pack 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 Canon BP-511A Battery Packs (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.
Recommended Shopping Resources (Using these links to make your purchase supports this site)
Canon BP-511A Battery Pack at B&H Photo
Canon BP-511A Battery Pack at Adorama
Canon BP-511A Battery Pack at Amazon.com
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