My phone does this weird thing that I can't really explain with words and it won't charge correctly when I plug it into my boyfriend's radio's usb port.
Here's a video showing what happens. http://imgur.com/gallery/gkZ8tWR
I've tried googling this and going into my settings to try and make it work but I haven't found a solution. I also haven't found anyone else who has had this issue. My boyfriend had a newer android phone with a larger battery than mine and it doesn't have any issues charging in the same usb port.
The model of the radio is JVC KW-R920BTS. I have a radio with a usb port in my own car and a text box does pop up every time I plug my phone into it, but it doesn't impact my charging at all.
Some USB music ports in cars don't provide enough power to actually charge a phone. They are there only to stream music from your phone to the car radio. It looks like this port may be one of those.
Like I said, my boyfriend's phone has a way bigger battery than mine and it charges his phone with no issues. It's an Asus phone. I haven't seen this exact same issue from my attempt of searching but I've seen other issues similar with other Samsung phones so I'm wondering if there is a setting I could change in my phone since his phone works fine.
Your phone is slow charging as indicated by the white charging animation. The USB in your car radio isn't powerful enough. You could use a car adapter that has fast charging capability. Aukey or Anker are good choices.