As YoDude stated before, there seem to be more than one effects at work.

One of them has to do with deep-discharging the battery. As far as I can remember, the N800 manual tells to charge the battery at least a certain amount of time before powering up - this also applies if the battery is dead.

My N800 had a phase where it went dead pretty regularly - caused by applications draining the battery completely in a few hours of standby (the main suspect is the canola configuration web server, but I didn't verify it enough to go to court). I had the impression that sometimes the tablet didn't completely shut down when I tried to switch it off (which I did to save battery power in the first place) and wore out the battery anyway.

Repeated attempts to switch on without recharging before (one or two were usually enough) drained the battery even more. Attaching the AC adapter and booting after about one minute resulted in the tablet starting up, realising the power was too low and going down again, draining the battery even further in the process.

After leaving the tablet on AC for a longer time (I didn't stop the time, shortest I remember to work was about 15 Minutes), it let me power it on again, but disconnecting the power cord too early resulted in an only partially charged battery again.

I didn't experience an empty battery in the last few weeks after eliminating some programs I suspected to be quite power hungry and disabling virtual memory on the sd card (which was also reported to possibly drain the batteries somewhere). Usage time in the process went up above the 3 hours or less of real usage I got before - but I continued to take the charger with me to work so it gets charged pretty often. I switch it off only occasionally now - in fact it reboots more often than that because of some cutting edge software I insisted to tryout.

Some of the magic described in the thread before may even work in this case because it takes some time to do it and lets the battery charge in the meantime.

cheers