Am aware that USB outlets in modern cars are primarily for data connection and do not provide charging power at amperage requ'd by Tomtom. (I prefer Tomtom nav because it provides far more traffic info than Ford Sync3. Ford currently have problems with update system for maps and cannot say when it will be resolved!) So I have been plugging Tomtom device into cig lighter type socket, using Tomtom original USB outlet adapter rated at 1.2 amp output. But that seems to be insufficient to match current drain during a long journey - GO device dies with no warning. Have tried small Tomtom branded dual cig lighter adapter rated at 2.4 amp output on its red USB outlet (which I use on its own) - same problem. I have also tried the Tomtom bulky adapter (rated at 4.2 amp input but unrated for output) which can accept a cig lighter plug but has 2 USB outlets, one of them red - same problem. Do not think it is a battery problem in the GO device which charges readily from PC USB or phone charger connected to mains. Holding down the ON switch on the Tomtom takes me to an info screen which shows battery voltage charge. Left switched off & unused, it holds charge at or near 100% for over a week, and holds steady at 60% charge for same period - I have not tested for longer than a week.
Any ideas from experts out there? I am beginning to suspect that the cig lighter outlet on the car may have some timer which reduces or cuts its power to the cig lighter even when ignition is on and engine running. Next step might be to check thap possibility with, eg, a USB powered light which might indicate when it turns off but I do not have any such thing. I cannot think of any way of checking amperage continuously while GO device is operating and plugged into the car's cig lighter.