Charging laptops in cars. Is it safe?

I have a macbook and a power inverter. I have done this before, and it works fine. However, I have never really been certain about how damaging it is to the batteries involved both laptop and vehicle.

My understanding is that if the car is running, the voltage is higher and you run less risk of damage from dipping too low. But you increase the chance of damage from voltage spikes. If you charge while the car is off, you get the opposite. So if what you are charging doesn’t draw a lot of power, clearly vehicle off is the smart thing to do. But I am going on a long trip, so the point of my question is.

How safe is it to charge a macbook with a 60watt, 16.5v, 3.5a power supply in a car that is running?