I've dug out my old Samsung S4. It boots up and runs fine, I want to give it to a friend. However the battery life on it is poor. Perhaps about 10 hours with no use. Just connected to the network and wifi. 

I have about six batteries for it with the dates that they first went into the phone. I went through a phase where I had the little battery charging kit and it was easier to simply swap batteries than plug the phone in. So in theory all the batteries should have had similar use. 

Is there a quick way of testing which battery is best? All I've tried today is letting it get to full charge, then looping a youtube video and starting the stopwatch, with the phone on full brightness. I figure the stopwatch will start when the phone dies!

Or just keep on the Android battery app and just change the batteries over during the next few days?