I like the Lind DC to DC power adapters designed for autos. I use an automotive sytle relay latched by accessory power to cut fused 12Vs to the adapter. This then turns off the power to the DC to DC power adapter when you turn off the accessory power.
The laptop will still be powered by its' internal battery (but won't drain the car battery).
check out http://www.lindelectronics.com
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the best way to use a car adaptor with a laptop & no shut down controller is to simply wire the car adaptor to only have power with ign, then set the laptop to react with a shutdown or standby/ hybernate etc when on battery power for what ever length of time you choose. I've been meaning to write-up a comprehensive power guide for laptop installs for a while now, just haven't had the time