I have a Garmin 50LM. My battery is running low, so I plugged it into my Mac with the provided Garmin USB cord. The screen on the Garmin shows it is plugged into my computer and the Mac recognizes it. My problem is that it is not actually charging the Garmin battery. Every time I disconnect the device and turn it on, it shows the "low battery" warning, no matter how long I have it plugged in. Any suggestions?