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?

This hasn't happened again since I had the screen out a second time. Maybe I had just managed to make a poor connection somewhere...

Nope, spoke too soon: just took the phone out of my pocket and it was switched off again - this time it didn't ask for time & date though. Battery shows 58% charge. What's going on here?

It's a PolarCell, it's worse than a 5 year old original battery. I bought a brand new original Nokia battery for Lumia 800 and it was just a little bit better than my previous over 3 year old battery.
I say ditch the PolarCell, it's total crap and look for an original one.

Ok, third time lucky; I opened the phone again and took a look at the battery connector under the microscope. I'm pretty sure I found the problem: the PolarCell battery actually has the wrong connector for the N9, or the plastic housing for the connector has been incorrectly moulded - it had a protrusion on the left-hand side which didn't quite fit in the connector on the N9, causing the connector to be pushed sideways to the right by about 0.3mm when inserted. This in turn caused the three thin terminals in the N9's connector to bend slightly, particularly the left-most one (positive). Furthermore, the plastic around the positive terminal on the battery's connector was poorly moulded as well, and a small piece of extraneous plastic was present which caused one half of the terminal to be held back inside the housing. This was on the right hand side when the connector was inserted, so the two effects may have worked together to cause a very unreliable connection (the left-most terminal pin got bent away from the good half of the positive terminal on the battery's connector). I took a scalpel blade to it under the scope and carefully trimmed the protrusion and freed the inoperative half of the positive terminal, reassembled and all seems good; no amount of shaking, or pushing/flexing the screen or case, causes the phone to reboot or shut down. Hopefully that's the last of it.

I thought PolarCell was supposed to be one of the better aftermarket batteries? I'm not sure I'd want to buy an original battery even if it's unused, since these will have been sitting on a shelf for 3-4-5 years by now...

I understand your concerns although even old original will be better than any aftermarket battery for a not popular 5 year old phone. What I would also do is check your previous battery with Nokia Energy Profiler. You can "measure" the capacity by charging your phone fully, disconnecting from charger and start running the profiler. In the upper right corner it shows how much Ah has already been drained.
My battery after 3 years had 1200mAh and the new original I bought had 1400mAh - even though it was older than mine.

I was going to suggest checking the connection as soon as I read your first post, but it seems you're gotten it figured it now! I recently replaced my battery too and had the exact same issue, reboots and shutdowns. I suspected it was an issue with the connection because the plug kept wanting to pop out of its socket during the install. Took it apart and that was exactly it. The battery I bought was a genuine Nokia, and get this: the manufacturing date was 2015! I couldn't believe it. I'm on my N9 as I write this, but when I get to a PC next I'll share the link in case anyone wants to buy a reliable new battery!

@kapu2 -- hard to belive but yes, please share link.

I bought "new" original battery's for N9 last year, but manufacturing year is 2012 :/ Still, they are pretty OK, usually last whole day.

FWIW, I just got back after ~12h out and about, during which I used my N9 with the PolarCell battery. Happy to say I had no issues with spontaneous reboots or shut-downs, so that does seem to have been fixed by the connector adjustments. The battery was fully charged when I left, and I spent some time browsing the web, checking and replying to emails, one ~10min call, a couple of texts and quite a few snapshots with the camera. Battery was at 77% when I got back - with the old battery (original, 2011) it would have been down to almost nothing. So far so good!