Best bang for the buck battery for the n810 and n800 are made by Cameron Sino IMO. About $9 each, including shipping, they seem to have exactly the same life as the nokia stock battery.
I have 3 of them, 2 for my n810, one for the n800.
http://cgi.ebay.com.au/ws/eBayISAPI....m=200392285709

My battery took a heavy hit today. Unplugged for about 2 hours, no open apps and bam low battery froma full charge...

On the contrary, almost all threads about Mugen batteries say they work fine.
I also replaced the original one with a new Mugen one, and it works fine, giving the advertised capacity. The only small problem they have is the design of the battery contacts, which are a bit loose, but it is very easily solvable with a small piece of cardboard in the bottom of the battery to keep it tight against the contacts.

Hmmm... does GPS suck serious power? I left it on w/ navit by accident, in about 20 mins my tablet was dead. Had a girls # on notes not saved... autosave is a lifesaver

I've a Bluetooth GPS receiver with an 1100mAh battery that lasts for many hours so my guess would be the GPS isn't your issue.

In our case, yes, the gps sucks serious power. Of course not enough to kill the battery in 20 min, but in about 5 hours.

Mine took 2 bars off and it was dead in 15 mins. Although, the 'not calibrated' part may have had an effect.

Maybe it's just that your battery is getting old. I think that this kind of flakiness is pretty common for an old battery.

Maybe I should try the cardboard approach. I tried putting duct tape over the bottom and it hasn't seemed to be enough to avoid glitches.

This issue has been a major irritant to me. When my N810 is on, it runs great. But if I reboot the system (e.g., b/c the desktop freezes), then it restarts for a couple seconds and then shuts off. Similarly, if I power the device down and then turn it on again it restarts for a couple seconds and then abruptly powers off. In order to boot again, I need to plug it into the power supply, take the battery out, replace it, push it into the connectors as hard as I can, and power on. Once it has successfully powered on, I can replace the battery cover panel. But if I have to start up away from my power cord... forget it. Sounds bizarre, but the behavior is very consistent. I can give a more exhaustively detailed account if that would be helpful. It's possible that there is some interaction with the boot menu as I am dual booting.

I am forced to reboot often since that damned second row of icons from the tray locks open and prevents access to apps in diablo5.

I have a mugen battery for my N810 and one for my N900 and they are working fine.