Default [Q] Nexus 7 zigzag-like battery graph

I have been using my Nexus 7 2013 for few months and the battery is really strange.

The battery graph is often zigzag-like as attached and I really don't know where the issue is.

I have been using purity rom, franco kernel with smart cover.

I am also charging my Nexus 7 2013 with my Nexus 4 charger and external powerbank.

Is this a battery problem and should I get it back to ASUS for warranty?

Thanks everyone
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