iPhone Battery discharging while using Car Play

On a recent 6 hour trip, my iphone almost went dead while using Googlemap thru Car Play. I've made similar trips (and longer) before but never had this happen. Luckily, I was able to reach my auxiliary charger, disconnect GPS (which I hated to do b/c I had no idea where I was) and charge it up enough to last a little while longer. Then had to charge it up again when traffic slowed down and I could fiddle with it. I called Apple afterwards and they tested my iphone, said it still has 87% battery left and they don't recommend replacing until 80%.

I find it odd that my battery continued to die while plugged into the car yet it charged up with the external charger while still using the GPS. I called the dealer and they never heard of this before and said I could drop the car off for several hours for them to check out the port, etc. Has anyone else experienced this? The dealer also suggested it could be the cord but it is an Apple cord and is less than 2 years old. Do they go bad that quickly? Any ideas/suggestions? I just don't have time right now to sit at the dealers for several hours and wait. Hoping there might be some tests I can try at home first.