I had a battery draining issue twice, and twice the core-module was the culprit.
First time: Process irq/288-wcd9xxx draining battery. The core-module was changed.
Second time: I suspected it to be the same type of issue (kernel wakelock), although the tests I had made for my first issue didn’t output any useful data. So I copied all my OS through TWRP, and put all my modules in the core-module of a second flawless FP2. The problem disappeared. Sent it in for repair, they put new core-module, and I’m still waiting for it to come back.
The thing is, I had these issues from the start, as soon as I got the phone.