Car Chargers are infamous for not providing an adequate charge. Most of them barely give 500-750 mA of current and that is not enough to properly charge today's bigger batteries. Those chargers were barely good for the old Flip Phones, not to mention today's smart phones 3 to 10 times bigger batteries.
I suggest that your car charger is inadequate for the job at hand.
I went to an O'Reilly's auto supply store and they have some very good ones that are spec'd out at 3A and 5A
yes, they cost more than $0.99 cents, but you get what you pay for, and for me, being on the road a LOT with my motorcycle, I had the same problem as you with my Samsung Note 4, it would NOT charge up while I was riding, it just barely stayed at, or even slightly less than what it had.
Then I spent the $16.99 or $19.99 and bought a very good, high spec charger, that has LEDs on it indicating the state of charge rate. Now, the Note 4 will recharge in about 2 hours flat, or even quicker if I turn it off.
My ultimate solution was to get an Anker brand 26800 power pack, that puppy will charge anything in about 1 hour flat. and it is good for many recharges depending on what is being recharged. The Note 4 can be recharged about 5 or 6 times.