Nexus 9 WiFi tablet permanently in awake state I've recently been observing heavy battery drain during idle periods on my Nexus 9 tablet. The key here appears to be the awake status of the tablet. The left hand side of the picture shows a solid line indicating awake permanently. I found this for a number of days before I fixed the issue. The solution I will outline below in an answer to my own question.
I found the answer was to turn off the tablet, then start the tablet in recovery mode (hold volume down and power). You will get an Android robot lying down, press volume up and power to get to the menu. You need to enter recovery mode, then wipe the cache partition. It took around 10-15 minutes for me, but I've seen reports of longer. When that completes, reboot and the issue should go away. As you can see from my original image, the battery life is significantly improved.