Warranty is always based on acceptable and normal usage.
If you "abuse" it, warranty can be voided.
Example, it's normal for you to plug a charger in the ciggie socket and use it to charge a handphone.
Since it consumes very little current, no one will void warranty.
However, if you plug in a 3 port extension with very thin wires and you overload the extension and it catches fire, then warranty is voided.
The same as if you plug in a 12v heater to cook maggi mee in the ciggie socket. If it doesn't blow a fuse, but melts wire and start a fire, then warranty can be voided.
Ciggie socket is normally protected by a fuse of 10A or some newer car 15A, it means the power is limited, to protect internal wiring from overheating and going up in flames.