I use a Switchmode DC-DC Jaycar Lappie supply that has variable output.
I also use an HP lappie and found an interesting quirk with it
It comes with a std AC-DC 18.5V @ 3.5A supply
( and 18.5V is what the manual specifies )
But they also sell a DC - DC adapter as well. This has 2 output voltages.
If you use a 14V supply ( IIRC ), the lappie will still run happily,
but it disconnects the internal battery charging circuit
which gives a much lower current draw.
Thus running the converter from battery to +14V will give better life to the supply battery, vs going up to 19V or so.
Andrew