Most usb chargers don't work with the 505. It is designed only to pull a charge on the USB cable if it's attached to the computer, and even then not all computer USB ports work. The going theory is that the device requires slightly higher voltage through USB than standard wall chargers provide.
To be sure that your 505 is charging, look at the LED on the top of the device, it will glow red when the device is charging, and flash amber when the memory is being accessed.
If you want to use a wall wart USB charger with your 505, buy a PSP usb charging cable, like this one:
http://www.amazon.com/gp/product/B000HCVL36
It has an AC adapter plug on one end, and a standard USB/A plug on the other. I'm charging my 505 using a Belkin iPod wall wart usb charger and the cable linked above right now.