I had been searching the 'Net for some way to charge my Nuvi while using my PC and laptop. I found some articles that may have worked for the author, but not practical for me to emulate.
Two methods I found were:
- Rewiring the USB pins so that they can transmit 5v DC power to the Garmin device.
(I don't want to go this approach as I'm not good at soldering pins).
](./images/smilies/eusa_wall.gif "Brick wall")
- Booting up your Nuvi, waiting for the maps to load and waiting for the precise right time to plug/unplug the USB cable.
(I found this frustrating to get the timing right, because it means unnecessary wear and tear to the mini-USB port on my Nuvi). 
One day I found out the easiest method is to simply logically "eject" your Garmin drives from the computer. I assume that you have the supplied Garmin USB cable that can also charge the device. If you don't have one, you can also use ANY standard USB cable (e.g. for a digital camera) that can power the unit.
Here's how:1. Plug in your Nuvi into your computer/laptop. Windows will initiate the usual connectivity with your GPSr. Right now, you're in the mode where you can transfer data/to from Mapsource or browse the contents of your Nuvi (and/or SD card inserted).
2. Locate your system tray and look for the icon that says "Safely Remove Hardware". In my example below, it's the first icon from the left.
ScreenShot014.jpg
3. Left-click on the icon and you'll see whatever attached external storage devices attached to your computer. In this example I only have my Nuvi attached to my laptop.
ScreenShot015.jpg
4. Select the correct drive(s) in your list and click on the drives that you want to disconnect. If you have other USB devices like external hard disks, thumb drives they'll show up as other drive letters.
Be sure to eject only your Nuvi device.5. That's it! Your Nuvi will automatically reboot and go into the navigation mode, so you use it while charging it. To go into the charge-only mode, simply switch off your Nuvi.
6. To go back to the normal connectivity with the computer, turn off/on your Nuvi. Windows will revert to the normal USB storage device mode, enabling you to transfer data to/from Mapsource.
For Windows Vista/Linux or other operating systems, look for the similar command(s) to logically disconnect your USB device. This example is from a Win XP environment.
Hope this helps!
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But if you turn off the unit so that it goes into the charging mode, you can't use the Nuvi while charging it at the same time, can you not? 