Charging via USB is not usually recommended for mods with more than one battery, as for this to work correctly it would require balanced charging- monitoring of each individual cell, and charging that to its correct voltage and terminating when full.
Mods like the DNA200 offer balanced charging, while others do not. In a mod that does not offer this there is the possibility if the batteries are not at the same voltage, for the end charge to result in different voltages, which can be bad especially if one cell is overcharged.
I don't actually own the Alien in my collection, but do the H-Priv. I am pretty sure the instructions state it is just for upgrading and not charging, although the alien may be different.
This is one reason USB charging is not recommended, there are other reasons too such as the safety of the USB charging.
If you can, I would wait for your charger. I know what it is like to want to use a new mod though, so hope it arrives quickly
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EDIT: If the instructions do indeed say you can charge via USB, just doing it the one time I think would be OK if you really, really want to try your new mod out. Do make sure it is OK though and not just for upgrading, and make sure you are charging it somewhere you can keep an eye on it.
