Is your Nexus 7 no longer in charge of its battery?

Betteridge’s Law might not apply here. It didn’t to me.
I was the owner of a new Google (slash-Asus) Nexus 7. And I rather liked it. I liked it a lot more once I found out the previously suggested lack of USB OTG capability turned out not to be true- a review later on.
I liked it right up until the point that my N7 stopped charging. Or did it? The battery meter was suggesting that I could not get off the 77% mark, despite also showing I was on AC power. But then it also didn’t drop below 77%.

I turned it off and on again, as if it ran Windows, and now it alerted me to a totally flat battery- a recreation shown here.  Yet it carried on working. So it seemed as though it was charging, and it had charge. It just didn’t know.
Googling for answers pointed me in a few directions, eventually leading to a factory reset- not too traumatic as so much is backed up online, and all my apps were automatically downloaded.
My problem persisted. Looking at the battery info page I could see that the system no longer had any idea of how much charge was in the battery.

So I reached for a guitar pick, and got the back off. A nice soft pick is ideal for this, and shouldn’t leave any marks. If you don’t have one, take an old credit card or similar, and cut that up. You will probably need to slim down a corner with sandpaper to get a thin enough edge to get between the seams.
Nothing looked wrong at first glance, and I liked seeing all the aerials built into the rear case- one for the GPS, one for NFC, and one for WiFi- no 3G in my model. But I tried to re-seat the battery connection regardless, and lo and behold, it moved. It had come just loose enough for the battery monitoring lines to become disconnected, but not the power lines, so the N7 kept on working. I’ve tried to show approximately what I would have seen then in this photo. Note the visible copper.
Something that only struck me upon opening the device to take these photos today is the missing screw in the top right of the photo. There’s a metal threaded insert, so it certainly seems like there should be, and it is present on some other teardowns. Ah well, mistakes happen.
That’s the before. Here’s the after. Now you cannot see any copper, just black plastic.
The problem is three-fold. Firstly, the compartment surrounding the Li-ion cell is quite a bit bigger than the cell. This is likely on purpose, to allow them to pick other cells with other dimensions; secondly, the adhesive keeping the cell held down has too much give (an upside- you can more easily swap out the cell for a different/new one); thirdly, there isn’t much slack in the wires, so any sufficiently forceful movement allows the cell to tug on the wires.
Given these issues, and given that it actually took very little force to occur (it has never been dropped or mistreated in any way, and I have not left it alone with any clumsy friends), it was bound to happen again without some remedial work. I did two things. I took up some of the space in the compartment with some firm closed-cell foam,  and I used a couple of pieces of kapton tape to secure the connector and cell. Kapton is thin, heat resistant, electrically non-conductive, and the adhesive won’t get unpleasantly gummy, unlike electrician’s tape. It is also an amber-yellow, which looks more professional. Make sure you don’t cover up any of the aerial contacts!
I’m hoping this will work. A few test thumps provoked no issues. Perhaps I should have kapton’d the foam in place too. Maybe I should have used silicone to glue the connector together- although definitely nothing more hardcore than that.
Even if you’re not having problems, you might consider performing this yourself as a pre-emptive measure- because if the monitor lines could come loose, so could the power lines, potentially losing some data.
Whilst I have your attention, something you might have seen already, the hidden extra button. On the side of the N7 is a power button nearest to the top, then a volume rocker with a volume+ and a volume-. But under the case you can find a fourth button. On my N7, as opposed to some teardowns I’ve seen, the buttons are labelled KEY2, KEY3 and KEY4 (the first button, power, isn’t labelled, or at least isn’t readable). This is a 32GB Nexus 7 and therefore probably possesses some hardware revisions since the earlier models.
I understand it is a reset button, and perhaps Google felt it was unnecessary, given there are button combos to do resets? I haven’t pressed it. Why don’t you press yours if you’re so damn curious?