Dell Axim X50 Upgrade Owner's Manual page 59

You can charge the battery using a 5V/500 mA USB port when the device is
turned off.
NOTE:
Using an external USB hub with 5V/100 mA ports to charge the device
battery may cause a temporary voltage drop in the hub, and other USB devices
connected to the ports on the hub may not work normally until the device
is removed.
NOTE:
Charging the battery through a USB port takes more time than direct
AC adapter charging.
The device charger stops the charging process after seven hours. If you need
more than seven hours to charge the device battery, disconnect and
reconnect the USB cable to restart the charging process.
USB connector on the computer
Operating the Axim
59