Common (nominally "12 Volt") automotive electrical systems vary in actual voltage from about 10 V (during starting draw in cold weather) to around 14 V (maximum charging voltage). Any car charge is already up-converting the voltage to the 14 V to 19+ V needed to charge a laptop, so should have some level of internal regulation. This regulation generally need not be especially precise, because the charging circuits in the computer also have regulators, including the "smart" circuitry needed to manage lithium-ion batteries.
Therefore, it's very unlikely that a car charger can damaged your computer's motherboard; the only likely failure modes are "dead" (no voltage out) and "straight through" (battery voltage appears on the connector rather than correctly converted voltage) -- and the latter will be a lower voltage than designed, so will do no damage.
Now, if you drive an electric or some hybrid cars (Leaf, Prius or others that don't have a separate 12 V battery for accessories) it's very slightly possible that a fault in the car's systems could connect much higher voltage to the outlets in the car -- so, in a Prius, you could get full motive battery voltage (96 V, as I recall) at the nominal 12 V outlet. This is very unlikely (such a failure would also destroy all 12 V accessories in the car, so it protected against), and further would instantly destroy the charger's circuits, so it's doubly unlikely it would harm the laptop.