Here's a nutty idea. I read somewhere that the car charger actually charges the batteries slightly faster than the regular power supply one. Don't know yet if that's true, but I figured that it would be nice if you could actually be charging two batteries at once anyway, so you could get flying again faster. So I looked around and found this well-powered, very inexpensive AC to 12VDC adaptor. It works great. The next time I can quantify that two batteries are at 20% after flying, I'm going to time if it really is faster, but like I said, charging two simultaneously is nice.
https://www.amazon.com/gp/product/B0143J9900/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B0143J9900/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1