That will be much easier than messing about with a TV power supply. Everyone should be using these things to power their laptops as opposed to bulky and much more inefficient AC inverters.
Take your voltmeter to the end of your laptops power cable and verify that the voltage is 19v and that the outside of the plug
is ground (it almost always is but occasionally it is not).
Once you are sure of everything you can either order a cheapo chinese replacement charger
for your laptop model and chop its plug
off and wire to it. Or just chop your current
chargers plug and wire to it if you don't intend to ever use it again on AC line power.
One more thing, often times these cables
are coaxial so you might need a razor blade to shave off enough plastic insulation
so you can have enough room to pull the ground shielding back and get to the inner conductor.