how to get N9 to charge from a universal wall or car charger
If you plug in a Nokia N9 to a 2.1 Amp wall or car charger, it says "not enough power from the USB in order to charge the device". The reason is that those chargers typically do not implement the charging standard exactly, and N9 requires it. If N9 does not get the signal "take-it-all", it assumes that the supply can not deliver.
Not enough power
Thus, keeping just that simple 2 x 2.1 Amp charger with you while travelling,
or having in your car a dual-output car charger is no good.
And look, now we are charging ...
Mark this cable clearly that it is meant just for N9 charging. It will not carry any data because the data wires have been cut. Do not connect it to your laptop, because N9 may try to pull too much current, and especially it will make a USB hub mad.
Now you can keep this cable with you when travelling, and have just one USB charger (wall or car) with multiple outputs, and charge all your devices - the tablet, the N9, the 3G-WLAN modem, whatever.