If you want to charge with only the usb power and have the device on, then that is not recommended. Third-party sync cables try to allow this but may not really charge your device or cause damage to your usb port.
The best way is to use the official dell sync cable (
http://accessories.us.dell.com/sna/P...&page=external) that includes a socket to plug in the ac adapter, so power comes from the usb port and the ac adapter.
I have experimented using the dell sync cable with a usb-to-powerplug cable (
http://semsons.com/usbpocaudc.html) so I would be charging my device with two usb ports and not need to bring the ac adapter. It looks good so far, but the safest way to use this is to plug them into an external usb hub instead of directly to a computer. This works out for me since my gps and phone can also charge via usb so just bringing a small usb hub will save me from bring an ac adapter for each device.
