Why wont Garmin nuvi work while plugged in?

[Dave R]

I've had my Garmin for a couple years now and it works fine, but it has a very short battery life (even after a full night's charge the battery only lasts for like maybe 30 minutes). Because of this short battery life, I had recently bought a car charger that plugs into that charge port in hopes of being able to use it while it's plugged in (similar to how you can use your cell phone while it's charging). However, whenever I plug my Garmin into it, the screen just shows the image of the computer, as if I've plugged it into my computer for updates; it won't let me use it (and now that I think about it, as long as that image shows, it may not even charge while it's connected) simultaneously. For the record, the charger I bought isn't Garmin specific, it's a $15 car charger that works with cell phones that has the male end that my Garmin requires. If anyone has any idea what the problem may be I'd appreciate it, thank you.