I'm going on a long flight soon, and as my N900 is music player, and there are no power points in economy, I bought a 20,000 mAh power bank. It's quite snazzy, and works well.

Weirdly though, not with my N900. I plug it in, the power bank detects it, the N900 doesn't start charging, so the power bank switches itself off again. I've tried with normal USB cables and charging ones with no data lines. Same behaviour...the N900 just doesn't seem to draw any current, so the bank switches off.

Any thoughts/ideas?

Quick reply...
It's a complicated matter. Does N900 charge itself from PC, through USB cable? If yes, then does N900 fail to draw current from the bank when N900 is connected to the bank by the same USB cable? If yes, then can you compare {power given by the bank to N900 before the bank switches off} with {power given by PC to N900 while charging}? Maybe, N900 doesn't recognise the bank as charger? Because N900 has to differentiate between low current of PC-charging, and higher current of adapter-charging, and the bank apparently resembles neither of them.
Best wishes. Bumping the thread...
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Per aspera ad astra...

Having tried my 2nd N900 (which was off) which worked, I swapped them over and my N900 still didn't charge. I restarted it, and it charged when off. It then booted and continued to charge. I then unplugged it, waited for the bank to deactivate, then plugged it back in and it now charges.

BME glitches I suspect. Incidentally, it was plugged into a wall charger overnight and my morning alarms were mixed in with low battery beeps. It does this occasionally, really quite annoying. Cheers for the help though!

PS, the bank has charge two ports, a 2.1A one and a 1A one, very snazzy

You may have to short the data pins of your cable. Search for N900 and dumb charger

I have one which has 4 ports. The N900 only charges when plugged in to the 2.1A and 1.0A ports.

edit: I also carry a spare battery on the longer trips just in case.