Depends on what your Sat Nav is. If you use one with an external (by that I don't mean outside of the car, just on a cable) receiver, then these have internal batteries. First use could take a while to 'connect' and if you don't use it too often the battery discharges and you have to start again, these can take a while to acquire a signal from satellites. Once acquired, it will store last location details. Other things you want to check are drivers, setup to make sure you have selected the right type of receiver, and so on. If its internal (built into the GPS device) you may have the same issue in terms of initial acquisition time, but there have been a lot of reports of the TomTom all in one systems being flaky....