Its unwise to push a generic power supply to its full rated current as the cheaper ones may have over-optimistic ratings. ==> for a laptop with a nominal 45W mains PSU, get at least a 60W car charger.
Some laptops 'handshake' with the power supply to determine its output current and if there is insufficient current either do not charge the battery while switched on, or throttle back their performance to stay within the power available. Without spending some time researching your laptop, I cant tell if it has sensing contacts on its power in jack. If it does, you need either an OEM PSU or a model-specific aftermarket replacement, as the generic PSUs with interchangeable connectors frequently don't communicate the available power to the laptop properly. If the jack only has two contacts (+19V & Gnd), a generic PSU will work OK.
Another thing to watch out for is that car chargers almost invariably have a common ground from power in to out, and due to the large current they draw, there is likely to be a considerable ground offset with respect to the vehicle chassis and other parts of the wiring loom. This can cause serious damage if you connect to other vehicle powered electronics that doesn't have isolated interfaces.
The final thing to watch out for is the condition of the car socket. If its ever been used for a cigarette lighter, the contacts are likely to be badly oxidised and unreliable, which can cause the charger power plug to overheat and be damaged.