Have you ever wondered why newer Canon DSLR battery covers have a small rectangular hole punched into them? It’s more than just for style:
Take a look at the cover. Does it have a small cut-out a few millimetres in from one edge? This is not just decoration. It is designed so that you can tell at a glance which of your batteries are fully charged and which are not. The batteries that come with this cover have a blue stripe down one side of the back. When you remove a charged battery from the charger, you can attach the cover so that the blue is visible. When you remove a discharged battery from the camera, you can attach the cover so that the blue patch is not showing.
It’s a simple and useful tip that those of you who don’t read instruction manuals may have never learned.