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Very bizarre charging behaviour from 2012 N7!!
Ok I've not seen any threads quite like my issue, so I'll try to explain best I can. Ok, here I have an N7 (2012) wifi. Now, I work in electronics therefore know a bit about charging etc, so I've got the back off it to measure the voltage directly at the battery terminals. I have 4.2V across it - which is about as high as you want a 3.7V cell to reach at full charge. Also the unit (powered off or on, with screen off) is drawing about 17mA from my bench PSU (set to a 2.5A limit, @ 5.25V), which is what I'd expect of a full battery, along with the fact it's up at 4.2V.
However, and this is the bizarre bit - unplug it, and the unit bleats away saying there's 5% battery left. So, back in with the power, it says charging. Leave the screen to go off. Check it about an hour later, the unit isn't switched on. Boot it up, it bleats again saying 2% left. Take out the power and put it back in. Charging again (allegedly). So I look in the battery section of settings to see what had occurred over this last hour or so... and according to the graph, while I had left the unit to charge, it rose from 5% to 15% over a period of around half an hour, then steadily DISCHARGED for the next half hour right down to 0, and switched off !! Situation now - it's "charging", and has been sat at 2% for about two hours. However, as expected, on another battery check, guess what - still at 4.2V - fully charged, about where it has been throughout. Well the lowest I saw it go was actually 4.13V.
Any help would be appreciated here, as I can't see what the problem is, other than the battery is full and the os thinks it's empty. It was doing this on android 4.2, and I've since updated it to 4.3, and then 4.4.2. Same results, no change. I've also performed a factory reset via the os, and a hard reset (factory wipe/reset) via recovery. This is really strange, never come across this ever before. As I say, I've seen lots of battery issues experienced by N7 2012 owners, but nothing quite this odd!