You can charge the N800 with any regulated 5V DC source (although it can accept slightly less, but more important also a bit more. I've heard people charging it with 6V for example, although personally I'm wary of such experiments because it all depends on how robust the N800 charger control circuit is. Which I don't know). The charger that comes with it can deliver 890mA, which charges it reasonably quickly. The smaller Nokia chargers which deliver less than half can also be used, but the charging takes much longer (and can barely keep up if you run the N800 hard at the same time).