That powerpack looks like it would be a good solution. Was it wasy to build? Is it used to just charge, or does it act like a high-capacity battery without charging the internal one on the Zaurus?
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It was pretty easy. I took 2x4 battery adapters from Radio shack, bolted them to a slab of plexi glass to hold them together, and wired them in parallel to a jack.
Hooking the jack up to the case was the hardest part, and that was because I didn't want to cut any holes in the case itself. I probably took an hour or so to build it.
It acts like the external charger would - charging the battery simultaneously with use. You can tell when the pack starts getting low because the charging light on the Z goes out. I don't remember how long it will power the Z afterwords - you still get quite some time out of it.
The only down side is that when both the pack and the internal battery are exhausted the Z just turns off - it doesn't suspend because it still thinks it has power because of the plug being in. (AFAIK, anyway). Not a big deal as you can see when the internal battery goes down.
I was using it for my entire trip to Japan (~14 hours use), and it was still able to charge the battery by the time I'd made it to where I was staying. I left the Z on overnight just to exhaust the batteries. I got an honest 16hrs out of it last year when I took a bus trip using 1800mAh batteries. You could probably get away using a 4 battery version for daily school use pretty easily. 8 batteries (~4.6 Amp-hours official rating - about the size of the battery on my P3-866 laptop) seems to be overkill.
I didn't have to use it on the way back because I'd bought my 860 while I was there, and my flight back was only ~6 1/2 hrs. It'd still be good for going war-walking with the 860, I suppose
The other thing to remember is that you have to use rechargeables - 4x1.2 = 4.8 volts. The 4x1.5=6volts of alkaline batteries is a *very* bad Idea for the Z.
Instead of building your own, check out eBay for a power extender intended for a Compaq Ipaq. The Compaq 3600 series uses the exact same power connector and voltage as the Zaurus. You can usually find a cheap, unregulated 4 battery adapter for a few dollars.