If you search the forum this has been discussed in more detail before.. but the short of it is that lithium batteries can't be overcharged, or to be more precise, it's very dangerous (fire, explosions) to overcharge them even a little so the N800 (and all devices using lithium-ion batteries) has charging circuitry to turn off charging when the battery voltage reaches a certain level. In addition there's also a protection circuit in the battery itself that will disconnect if the voltage passes 4.3 volts (if this happens you'll need special equipment to re-connect it). But this won't happen unless the N800 charging circuitry fails.

In other words, you can't accidentally overcharge.. because the N800 charge circuit switches off the charging when it's full. And lithium batteries aren't trickle-charged, so it won't continue charging again until the voltage has lowered a bit.

It's been one report (that I'm aware of) on this forum about a puffed-up battery, this is almost certainly caused by a faulty battery and not because of overcharging in general (btw if this ever happens - don't punch a hole in the battery, it can burn. Very dangerous. Must be disposed of properly -- call the device vendor.)

Nokia, as other manufacturers, used Ni-MH batteries in the past (and before that, Ni-Cd). These batteries were very different: First, they needed a very long initial charge, secondly the charger was a simple timer-based thing and thus you could overcharge if plugged in without discharging, and thirdly there was what was called a "memory effect" (well it wasn't really that, but the result was the same) that made it essential to fully discharge the battery quite often in order to retain full capacity. Now, the problem with Nokia is that they have left instructions for the old Ni-MH batteries all over the place in manuals meant for new Lithium-Ion powered devices. This has created a lot of confusion, unfortunately.

With lithium-ion batteries it's like this:
- Give it a good charge now and then, 4-6 hours is ok. They go quickly to "apparently full", but that's when the real charging starts. It isn't full until the device tells you it's not charging anymore.
- Avoid discharging. This kills the lifetime of lithium batteries. The exception is in the beginning, you may discharge it a couple of times first (but even here you shouldn't floor it completely. This _will_ reduce the capacity and lifetime of the battery.)
- Don't be afraid of topping up the battery whenever you get the chance. This won't hurt the battery.
- Finally: _if_ something goes wrong with the battery or the battery protection circuits it is very dangerous.. although this doesn't happen often (because of the double protection: One circuit in the device, a second in the battery).

Read all about batteries at http://batteryuniversity.com/
which is a great site someone told me about a long time ago.