Hi people! I am going to a 4 day festival where there is no power to charge USB devices. I have looked and there are so many portable chargers for phones and tablets that will last for a few days. The problem I have is that not many seem to state if the unit will axtaully charge Ecig batteries and I am now confused with output and the amh or other tech stuff... Can anyone advise the best portable charger that will charge ecig batteries and last for four days??? Help!!!