I also own an external battery pack. When I bought it charging the n900 worked fine.
Lately I wanted to use it again, but it didn't work anymore. Tried it several times. I assumed it could be due to 1.3.
Maybe some settings did change. I didn't find the time to reflash to 1.2 to see if my assumption is true.

Does anyone know what this sound means?
When my n900 is off and I plug in the external battery, the n900 executes its minimal startup: showing the usb sign,screen lightly lit, led starts flashing orange. This goes on for a little while, then n900 shuts down again, with a sound like blonk (hard to describe)
I already heard this sound on other occasions when something in the startup/shutdown did seem to work as it should.

Any idea?

EDIT: Just found that the solution from this thread http://talk.maemo.org/showthread.php?t=38230 did the job.