Odd battery issue

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http://forums.androidcentral.com – Hey all, my wife's Nexus 7 is doing something odd, she didn't use it for a few days do left it on charge. Took it off and after 2 hours the battery dropped to 0% and stayed that way until she noticed it 5 hours later. The status bar icon registers when its on charge but the battery app says 0% Unknown. Have reset it, done a "fastboot erase cache" and even completely run flashed the stock ROM. For now I've told her to use it as normal until the battery dies and see what happens when she charges it. Any theories aside from a duff device? Sent from my Nexus 4 using Tapatalk 2 (HowTos)