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.

 
 
 
 

 
 
 
     

                 

 
 

 

 

 

 

 

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