don't have a n900, but the n9 is also behaving strange when charging from the pc.
when i plug it, it will say „insufficient energy”, but will charge none the less. Slower but that is expectable.
I suppose this behavior is caused by the charging module, as you know the lithium batteries charging is quite finiky. Maybe a new batt will change the charging behavior, but you could try to discharge completely the battery to see if that will make a difference. (discharging completely is not recommended too often, but will reset some parameters in the charging module).
Hope this helps.