I have about 5 batteries for my 5D and about 6 for my Leica. Very rarely I require more than 1 battery for my shooting and I bought extra batteries for my extended trips. Between me and my wife we may rotate Leica batteries since they fit in her camera but Canon ones do not get much workout. I usually pop one out of the charger throw it in the camera and shot with it until it dies. Than I grab another and cycle goes on so I really only use 2 batteries for my canon storing the rest charged. How do I make sure that those batteries are good? Any test I can perform on them to see how they hold up? I have extended trip to patagonia comming up.