Hi RasmusÅkerholm ,
You can charge using the USB-C port on your Surface Book 2 but only when Surface Connect power source is not in use. The USB C port can provide output power, and draw power for charging. Although we always recommend using the Surface Connect charger that came with the device. We know there are times people may not have their charger with them and a USB-C charger can be used.
Regarding your question about using a car charger, we have a vehicle charger that is recommended on your Surface device. This charger can quickly power your Surface device and it inserts effortlessly into your car's accessory power outlet to make powering up easy. Currently, it is only available on Microsoft Store US.
I hope this helps but should you need further assistance, feel free to reply back.