My sat nav battery only lasts about half an hour unless plugged into the
fag lighter socket. I was told that the display screen is what consumes
the power.

The sat nav has a touch screen, but you can touch it with anything - the
end of a pencil for example.

The smart phone (Samsung Galaxy Ace) can be used as a sat nav for hours
without appreciably draining the battery and also has a touch screen,
but you have to use a finger or special stylus - the end of a pencil
don't not work.

Can someone explain the differences in technology?

--
Dave - The Medway Handyman www.medwayhandyman.co.uk