With my first TomTom I could plug it into a USB port on my computer and after a period of time (a couple of hours) it would be fully charged. With my GO 5100 I plug it into the compueter and leave it for an hour or two but it dosn't charge. If, after doing this, I turn on the GPS without having it connected to either the computer or the cigarette lighter in my car I immediately get a message telling me that the power is low. After a minute or so it dies. Am I missing a step?