I replaced the (original) battery in my trusty N9 today, with a new PolarCell N9/Lumia 800 battery. Remarkably, the old battery was still ok-ish, but I figured it had to be due for replacement after 4 1/2 years! I also worried that if I didn't replace it now, I might not be able to find a suitable battery when it becomes urgent; even the devil's bastard child Lumia 800 is by now long gone.

Phone seems to be working well when I test it, but the third party Battery Usage app (v0.8-0) has caused the phone to reboot twice, and just now, with no apps running and the lock-screen activated (no charger connected) the phone just switched off on its own - when it came back on it asked for date and time again. Something's clearly not right here; the battery shows as fully charged, I have re-opened the case and checked all connectors and cables - under the microscope even - no signs of anything untoward. The phone boots normally, and all appears to be well - then *poof* it reboots or shuts down.

Anyone have any ideas what could be causing this? Could it be that the battery is actually flat (seemed odd to me that it arrived fully charged!), and that the phone is not detecting this somehow?