Probably the issue -- i've had issues with external batteries and 3rd-Party chargers like you're having. I've had good luck with Belkin 3rd party chargers -- faster charging too it seems) both in my car and in my wall at home.
I have the one that has an oblong plug, you can rotate the prongs and fold them down, and it has two USB power slots on the charger, can charge my iPhone and iPad at the same time. I think it was $35 or something.