QUOTE(jananan @ Jun 29 2010, 07:13 AM)
like Andy214 said, you might need to short the usb pins D+ and D- responsible for the data, pin 2 & 3 that is, the two in the middle. according the articles on the internet, the n900 would detect it as a charger and take advantage of the external power pack if it can supply more than 500mw.
Lol. Even if I have a 10000mah power pack, that does not change the the fact that most battery packs ONLY supply 500ma/5v through the USB port anyway, as per the specs outlined by the USB consortium.
As for the rest, it's a bit tl:dr (and I might be wrong about this) but here goes...
The mah does not determine charging speed, it's just an estimation of capacity at a given output IINM, which for most battery packs is 5v.
I'm gonna talk in MA now, and IINM, 1ma = miliampere so please convert accordingly.
Okay, how fast DOES the external battery pack charge an N900. From a full dead battery --> full battery (assuming the N900 is turned off), it should take 2-4 hours IINM. But that doesn't really matter, because the MAIN use of the battery pack in my usage scenario is to EXTEND N900 use time, which it does pretty well.
Okay the battery pack 2000mah, with simple calculations that should give the N900 2x the battery life. In practical use, power is also lost through the USB connection (it's not terribly efficient) in the form of heat and such, and that takes some of the power away.
Also, we must take into account how much power the N900 eats up and how much the battery pack can supply (which in this case is 500ma/5v). I'm assuming the N900 uses less then 500ma per hour (and looking at the battery and run time I'd estimate it to be in the vacinity of 200-350ma under full 3G load. 4 hours and 1300mah = 1300/4 = 325ma per hour).
That means that the battery pack is topping off an excess 100-200ma per hour (500ma supplied, +-300ma being used) and this extra power goes to charge the N900 internal battery. Also, the USB battery pack has limited capacity, thus under full load with the internal battery al 40%, the N900 will most likely drain the pack long BEFORE it can charge the N900 battery to full.
Take into consideration the AC charger supplies 1000ma/5v, that's an extra 600 frigging ma over worst case scenario use (300ma), which is why the AC charger charges the N900's internal battery so quickly. The car charger is also 500ma/5v but it's backed by your car engine = unlimited supply; in other words it's slow but will top up the N900 battery to full given enough time.
Thus charging up the N900 battery with a USB battery pack while under heavy load will be extremely slow, but it DOES extend running time of the device by quite a while, although less then the raw capacity would lead you to believe.
If you want it to charge faster, find a 1000ma/5v usb battery pack (google for Axim compatible USB battery pack) or a iPad compatible battery pack.
AND DON'T FORGET a dumb charger wire with the data pins shorted out (otherwise you're getting no more then 100ma, IF you get any power at all to begin with).
Or sometthing. My electro-speak is rusty lol.
This post has been edited by Eiraku: Jun 29 2010, 10:10 AM