I bought 2used ipads and want to keep one. They are 16gb wifi. I charged both to 100% and running a video on the browser. After 10 minutes one is at 97% and the other is 100%. The one at 100% is built in March 2010 and 97% built April 2010. The April one has 8 days left on warranty. Looks like a 3-4% difference so far. Is this a good way to determine the battery? Are there any other tests I can run? I do have the same apps running with the double click. Advice and info appreciated, thanks! PS-anything else I should compare to pick which one to keep???