It wants 19 volts, give it 19 volts, that's what it's expecting at the power input socket. Buy a 12-19 volt adapter. In Oz they cost about AUD$50.00 and we're usually ripped off with electronics so you may well do better.
My lappy needs a 19 volt power brick, but the battery is 13.2 volts. 12 volts from a car battery is within 10% of the nominal laptop battery voltage and would probably be ok if applied to the right terminals, especially as it's on the low side, rather than the high. In theory I think you could identify the battery output terminals and apply 12 volts to the matching terminals on the lappy, but really, why bother when DC adapters are available and cheap?
AA