I know it can reboot when it crashes, but I've noticed when it's got a good battery on it (80%) sometimes my tablet turns off overnight.
Anyone else have this happen occasionally?

Either something is running away and draining the battery overnight, or a dodgy process is crashing and perhaps causing a kernel panic.

Mind you whenever mine has done that, it has rebooted rather than just poweroff, but very occasionally it has gobbled battery and then switched off.

I experienced the same dead battery overnight condition. I power down my wireless access point with my PC so there's no WiFi for the N810 to connect to unless it finds a signal from another router. One morning I awoke to the sound of my N810 loosing a WiFi signal and the battery was very low. I guessed that perhaps my N810 was on the edge of another signal which came and went causing the N810 to search for a new signal repeatedly. My solution was to put my N810 off line at night and the problem never reoccurred.

i wonder if it could be related to the mce bug that causes crashes with worn batteries.

The thing is though, that I can turn it on and the battery still has about 2 bars left on it. (Advanced Power). I've noticed my Nokia Phone does this too, but it's alot more frequent (like once every 2 weeks) (this does it once a month or so)
But yeah, the battery on mine is dying, but I want a new one to replace it as this one will have bad life anyways, but you can't buy and store a battery :/... or can you?

not with rechargeable types at least. At best you can, with the lithium ion kind, charge them somewhere around 50% and stash them in a chilled (tho not freezing) and dry place. But i think they will at best last a year that way.

Cause someday Nokia will stop making the battery for the devices this uses and I don't think I'll be able to buy another one.

even if nokia stops, other will contine to i think. At least for some time.

I've read up on some others batteries, and Nokia's seem to be... 1500? mAh compared to 1200, and I read a thread on the MUGEN battery that it doesn't work very well. I need a standard battery cause I have a boxwave horizontal leather case for it that's really nice.

The mugen battery in my n800 is working fine..

As the batteries get older though, the charge meter becomes increasingly inaccurate. With my previous battery, 3 bars was 5 minutes left to empty/batterylow warning...