Galaxy S9 Weird Car Charging Issue

[SMR282018]

I bought a Anker car charger and a power inverter, my S9 won't fast charge with the car charger but it will if I use the wall plug with the power inverter, anyone know why this could be?

 

[Deepcuts]

Have you tested the power inverter with another device to be sure it is working while in car?

later edit: read your post again and now I see only fast charging is not working but regular charging it is?
If that is the case my guess is that the power inverter cannot draw enough current in car.
Don't take my word for it. I am not an electrician

 

[Rukbat]

The charger probably isn't a fast charger. (But the one you run off the inverter is.)