Today I purchased a Energizer XP2000

Initially I had some trouble getting it to charge the n900, the trick here was the XP2000 has a check/on button, it appears to only activate charging for a brief time if nothing is connected.

If I connect the n900 without pressing this button, I get nothing, if I press the button after connecting, I get nothing.

I have to press the button and connect the microusb to the n900 within a 1-2 second window, but after doing so it seems to charge normally.

Perhaps a similar process is needed to get the n900 to pick up on whatever charger you may be using?