The internal N800 battery is pretty studly, but there are contexts in which I don't want to have to swap out the battery for a spare. Have to reboot the machine to do that and if you're trying to run a program for a long time or continuously, that's not acceptable. Plus you have to notice that the battery has run low and replace it.
So, I have made myself an external battery holder. It's an "External Li-ion Battery Pack". Input is 5V, 500ma. Output is 5V, 6.8Wh. By way of comparison, the internal BP-5L battery is 1.5Wh, so this battery has four times the power. Cost me $26 bucks postpaid from CaBattery. Comes with a power adapter with a standard-size Nokia coax connector, a USB to coax connector (which can also be used to charge the battery from any USB port), and a set of coax to (whatever) adapters including an N-series coax.
I scavenged an old USB cable connector and got an N-series coaxial power adapter from a local electronics store. Cut the plastic off both of them and greatly shortened the cables, being careful to check and double-check the polarity (center positive for the coax). Both cables were marked red (postive) and black (negative) so it was no trouble.
I have a supply of polycaprolactone (capa for short) which I purchased from Shape-Lock also sold as Polymorph or Friendly Plastic. I covered the N800 and external battery with aluminum foil because the capa will stick to plastic. The cable needed to be covered with plastic, so being careful not to short out the cable to the aluminum foil, I opened two holes for either end of the cable and plugged it in.
The capa is soft like modeling clay and hardens like nylon when cool. Thin pieces cool off pretty quickly, so you don't get much work time. I wrapped the capa sheet around the back of the N800, covering the external battery and wrapping around to the front. I purposefully made the right-hand wraps thicker to hold the cable and connectors in place. The left-hand wraps are thinner, so I could bend them to fetch the N800 from its embrace.
As it turns out, I didn't use enough capa, so quickly heated up another batch and made the corner fingers and covered the battery a little better. It will stick to itself if both surfaces are reasonably warm.
I still need to cut some holes for the external battery charger and power monitor. Also need a hole to fetch out the stylus and connect the headset. Capa cuts pretty easily as long as you don't heat it up. It softens then and melts rather than cuts.
I've run it on the battery mostly idling but wifi-connected for nearly two days before the external battery ran out and I noticed the internal battery start to lose its charge. I'm sure I could have gotten a full two days out of it. I expect to get ten hours of solid use out of the combination.
UPDATE 8/20/2008: Note that the N800 and N810's power supply must be within certain limits. See Nokia's charging interface specification.