What about thinking about it this way?

If it was charging a li-ion battery (which is what? 97% efficient at taking a charge? It's one of the most efficient charging chemistries I know that much anyway..(wikipedia says 99.8% charge/discharge efficient)) at 1C. We'd all have full batteries in just over an hour. It always takes my battery from flat a good couple of hours plus (2.5? guessing..) to be fully charged? That wouldn't be happening if it was charging at 1C, 0.5C yes.

>> added info:

Since writing this post I did a little experiment. When it's happy my phone idles at 8- 9- and 12-13mA through the powermeter.sh script.

Using a Fluke 111 to measure the current through the AC10u charger I am getting 642-648mA. With the current I am getting around the same. Taking in the 12mA standby current there is about 20-30mA difference between the two readings. That is, between what the current reading from the powermeter.sh script says and what the Fluke 111 offers up. Incidentally the built in current monitor appears more finely grained and is reading those 20-30mA more than the Fluke meter. An aside, I altered the powermeter.sh script slightly to reflect the bq27x00_battery kernel appears to reading out correctly in micro amps now (uA).

The current measured by the bq27x00_battery module does not change more than 4mA or so when the Fluke meter is present. So the current shunt is very low resistance.

>>> MY CHARGER IS FAkE

I just tried the other AC10u charger we have here and it provides a more healthy 830mA at the State of Charge the battery was in. The current was tapering down so indeed the original OEM Nokia chargers could possibly provide a full 1A when the battery is fully flat

Cellphoneforever (a notorious ebay seller):
So-called NEW (actually refurbs) N900s with the following:
- Send the phone back with no tracking when the phone needs repair (then they say it got lost in the post)
- Keyboard is fake (non-OEM) The keypad is wears out.
- The battery fake
- And now, found a fake charger! There is very little to distinguish between the fake charger and the real one other than the lead is slightly more heavy duty and slightly lighter in colour and more "rubbery"

Not exactly. Li-Ion's are not accepting same current due whole charging process. Basically, nominal "C" value of charge (for example, 1C) is only happening during ~half of charging process. Then, battery is "reporting" ~4.2 V, and charger starts to gradually decrease current, keeping battery at no more than 4.2 V (less in case of N900, as it's charging and using power at the same time), during so-called "saturation" stage. BTW, the higher initial current, the faster "saturation" charge happens. This sole fact defeat purpose of charging LiIon's to full with more than ~0.8C, as time saving is negligible (due to longer saturation phase).

OTOH, there is method called "fast charging". Basic principle of it, is to charge only as long, as time/power ratio is optimal (i.e., only to about 70%-80% of maximal capacity). This way, we benefit from using higher initial current, thus charge time decrease dramatically. Of course, drawback here, is that we don't use full potential of battery, but it have added benefit of extending overall battery life (Li-Ions lifetime benefit from keeping it between 20%-80% of State of Charge [SoC] - this is how they're used in "heavy duty" environments, like electric trains, where cost of replacing huge battery's arrays is higher, than benefits from charging them to 100%).
---

As for Your discovery about fake charger - it explains mysterious 650 mA max. How lame, 50% of declared max current

/Estel

That's the only reason I'm guessing it's fake, because of the amount of current compared to the other charger. Otherwise the charger looks as good as the original--very minor cosmetic differences only between the two.
However, the genuine AC10u charger isn't a stellar performer either. I have a no-name charger that outperforms both in terms of output. So the genuine AC10u isn't current limited by the SoC of the battery.

Not entirely correct.
First up, the CC/CV (constant current / constant voltage) charging process switches to CV somewhere at 80% capacity, not 50% --edit-- I got doubtful about this and I'm currently looking for relevant information --end edit--
Secondly, the N900 definitely does charge its LiIon cell to 4.2V; the negligible power it's using to keep itself running in the meantime is irrelevant.

Coulda been worse.

Of course, it was also a "mental shortcut" from my side - when it switched to CV, depends on current during CC phase. Higher current in CC (as % of battery C) - the sooner CV will happen. So, charging standard bl-5jwith 1250 mA it happens near 50% SoC, but charged with 950 or 800 mA, CV will jump in around 80% of SoC.

As for N900 LiIon being charged to 4.2, I can't confirm it. Every measurement I took - both from in-device (bq27200) and using external components - shows little less than 4.2V as absolutely max. IIRC, it was something like 4150mV-4170mV at most.

Genuine N900 charger was charging at full 950 mA during CC, after all (just tested with one borrowed from a friend) - so it's limited by chip. Haven't tried to charge with 1250 mA - it isn't my charger, and I don't want to fry it during experiment

/Estel

I thought I'd update this. I've been using the N900 ever since I fixed its port, and it's still running fine. The port did come unglued at some point, but after regluing it on with two-part epoxy instead of the melt-glue I'd used originally it's been behaving.

As foreseen, it's been useful several times to use thumbdrives on the fly.

I now wish more phones came with a full-sized USB port.