I'm on hollydays and using a travelling charger with my notebooks.
It has also a usb port.

I wanted to charge my n900. But it didn't work.

Then I added a second mobile phone to the power charger (with a "Y" cable.)

Now it charges even if I disconnect the second phone again.

This works all the time.

What is going on? Can I reproduce this without a second phone?

Please take a look at this thread, it's likely the Dedicated Charging Port on your charger doesn't support the USB Battery Charging Spec, but that your Y-cable fixes that by shorting the data pins on each port.

So does a CA-146 charger adaptor (from Nokia), and so does the Proporta retractable cable.

You can also sacrifice generic micro-usb cable and short data ping on it. Or, like me, open charger, and short it's data pins - they're not used anyway, and this way You fix it to follow the specs

We should add to these threads that charge.sh (follow the link in Estel's sig) should be able to act as a software workaround to the problem - not yet personally tested by me, but if it can charge when in USB hostmode, then it can similarly bypass the specifications for a dumb charger.

So if anyone desperately wants to use an external USB charger without having a pure charge cable or a soldering iron at hand... there's your answer.