Dumb question about sat nav units

I've got a Garmin Nuvi and it lasts about 4 hours once fully charged. Can't remember how long the TomTom lasted for

Is that how people generally power them anyway? ie just plug into car socket only when you need to?

As was (rudely I thought) pointed out to me, not long enough to go anywhere that you'd actually need to use it:op

I had a Navman that would last about 4 hours. I now have a Tom Tom and that doesn't last very long at all so always have that one plugged ihnto the car when I set out on my journey

If i need a bit of extra, I use a Power Monkey, but then mine's not a dedicated vehicle sat-nav.

Garmin Nuvi too, which lasts about 4 hours

We usually have ours plugged in to the cigarette lighter and charging when it's on in case we need to use it on foot (or find where we've parked the car!)