I pottered down to Halfords assuming that I could get a battery operated one but was told that only hand held models had this facility. Apparently all of them have an internal back up battery but this only lasts an hour or so and takes a few hours to charge.
This is the sort of mis-information that gets Halfrauds a bad name - it's totally incorrect. Just take a look at the Nuvi range on www.garmin.com/uk
and you'll see that many of the in-car models have a battery life of 4 or 5 hours. I bought one for precisely that reason, though I did later fit a cigarette lighter socket ( for phone, battery conditioner etc).
No connection with Garmin etc.