When I go on a photo session, I sometimes take a few en-el3a batteries for the session. I Just dump them all in my bag. When I finish using a battery, I put it back in the bag. How do I know which battery is empty and which is full? – The solution is easy, when a battery is charged, I wrap it with a rubber band before I put it in the bag. The battery with the rubber band on can not be placed inside the battery compartment. So before I use it I have to take the band of. After I am done using it, I put I in the bag again. Of course without the rubber band now.