Kathrin -
You might also want to look into the Digipower DPS-9000 external battery pack, particularly since you say that you have the smaller battery in your Z.
I originally bought mine for my Olympus digital camera. But noticed a web page where the author used his with a 5x00 Zaurus. The DP-5v step down connector cable (included with the DPS-9000 that I bought) is used for Fuji cameras, which can\'t take the full 7.2v of the power pack, and works like a charm with my C860.
The DPS-9000 is 1800 mAh lithium ion, memory free, tripod mountable and has a set of 5 leds as a great \"fuel gague\". I have a belt pouch for mine. The AC charger is universal & a 12v DC charger was included with my unit.
Of course, if you are near an AC outlet, its easier to just plug in a wallwart but when out & about, the external pack greatly extends the operating time of a Z. Saved my bacon a couple times! <G>
Bob W
Miami FL