Default How to diagnose the battery properly?

It seems that lately my Nexus 7 can't keep up its charging rate with its usage... The battery stats show charging but it takes so damn long to charge. Watching a movie through usb otg on the charging dock drains 1% every 5 minutes or so. Even when the hdd provides its own power. Watching on the lowest brightness just improves minimally.

I am using Greenify on everything to stop apps from going nuts too.