Having lived off a car battery for power needs a few times, this isn't really an issue when you're traveling by car. Anybody who is going to use the N810 with GPS for car navigation just needs a Nokia car charger or a Car Inverter (the smartest thing one can buy to avoid $$ dedicated charger cords). The battery life just isn't going to be there to run the GPS for any length of time due to high CPU cycles in mapping mode and running the screen at high/full brightness. Just like any standalone GPS, it's going to have to be plugged in. Done with the car trip. Take the tablet with you and it should be all charged up.

But, yes, if you have two devices each running off their battery, you can get double the battery life, and might get a better GPS experience ... but you're also going to carry another device.