I'm seeing that the supplied AC-10 (US) sometimes activates charging, and sometimes doesn't.

Is this kind of non-deterministic charging behavior being seen by others?

Yeah, I have that issue too. But USB charging always works o.O

Happens on any external battery save for one. Of course the regular charging methods such as wall and usb from computer, still work.

I have a feeling that the N900 lies and doesn't actually take exactly 5 volts but something more like 4.95v and is very sensitive. But what do I know, I'm not an electrical engineer. :P

Here is what the manual says

If a battery is being used for the first time or if the battery has not been used for a prolonged period, it may be necessary to connect the charger, then disconnect and reconnect it to begin charging the battery. If the battery is completely discharged, it may take several minutes before the charging indicator appears on the display or before any calls can be made.

This is not a "fully-discharged" problem, but one with the phone reasonably charged and generally running.

It is the phone failing to properly recognize that a charger has been attached. It happens with the supplied AC-10 regularly and there are inconsistencies with the way it treats other chargers, past the "short the pins" game (which, in my opinion, is just bad designon Nokia's part if their phone is not compatible with the large number of iPhone/iPod and other USB power supplies out there).

The failure to charge has been confirned through "hal-device bme" so this is not a question of some annunciator being missed or a state where the charging light fails to illuminate. This is a bug. Plain and simple.

I just got the DC-11 and I realized that it has no charger and the AC-10 charger for the N900 has only a micro USB output but the DC-11 needs a 2.0mm input (like the one from the AC-4 or AC-8). Is there an adapter for that (micro USB female to 2.0mm) or how do you charge your DC-11?

BTW, there was a bugreport about N900 only sometimes recognizing charger, which was fixed, and might be coming in a future firmware.

This wont make nonstandard microusb chargers work, though.

Same problem here, the charger (DC-11) wont chrage if the N900 battery level is below 50%..

Piece of crap, pay ~30€ and get some aluminium brick... I hope nokia fixes this.

Even if you first turn the DC-11 on and then connect it to the N900?

Yep, if the battery charge in N900 is below 50%, the DC-11 does not charge it here.

(And same if i unlock the device before connecting the DC-11)