If you are using canon batteries on an SLR like BP series for DSLR you should not let them discharge as this will shorten the life of them... These batteries do not have memory problems like on some mobile phones, and can be topped up at any time, to keep them in prime condition... If you are using a bettery Grip, it is worth noting that the batteries when fitted in pairs will discharge at different rates, unless they are both exactly the same power, if not the higher rated one will discharge first...

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt