Amps aren't "drawn". The 500 mA limit is at the source (as is the case everywhere), regardless. However, the charging strategy should change to adjust - First, there's the multi-stage charging of the battery (Li-Pos are mighty temperamental). Second, there's the charge minimum - if you don't provide as needed the battery will discharge instead of charging. But the phone thinks it's charging so there's a real possibility to over-discharge it.

And finally, to charge from an USB at 500 mA you have to negotiate the draw with the USB hub. That is, you only get about 100 or so unless you identify yourself and report you need more. Letting N900 try and charge at 100 (via bad cable, for example) will NOT charge, because draw is larger than charge, destroying the battery (eventually).

The wall wart (dumb charger) has shorted wires, signaling the N900 that it is a wall wart. As a result, the N900 starts a charge is it is satisfied with the conditions. That is, if you use a battery pack that can't deliver it will cut the charge short and give you the finger (as reported several times). The fact that it distinguishes them means it has different strategies (will only charge if shorted and satisfied, or if has a hub on the other side even if it can't deliver).

Additionally, N900 is a very sensitive device, as it is quite complex. Not delivering, poor delivery, noise, etc can all do bad, bad things to it, starting with hanging and end of charge condition, ending with root FS corruption and nuking the battery. Your solution to adapt varying power to it directly is likely to have repercussions.

IMO, you need at the very least two stabilization stages to pull it off, or a self-correcting DC-to-DC, both of which require higher voltages and some assembly. In fact, without a way to guarantee that the charging system will have the needed input voltage and plenty of power, any smart Li-based charger will give up mid-charge, at the very least eating up charge cycles.

Appreciate the ingenuity, like the idea, not a good real life solution. My 2 cents.

As a stopgap, use batteries but if you carry all those with you it's easier to get a travel charger - it's lighter and more compact.

APC sells 4-AAA pocket chargers with USB. They will NOT charge the N900. Will charge other Nokias and older iPhones.