USB power question

I recently got one of those things that you plug into the cigerette lighter and plug the M250 into it and you can listen by tuning in the correct radio station.

The device has a 1/8 inch plug that goes into the headphone out on the M250. It also has a USB power socket.

I know that my M250 will not charge the battery, but if I plug the USB cable into my M250 and to the cigarette lighter/usb power port, will it run my M250 on the car’s power and not run down the battery or will it do nothing and still run the battery down? 

if you try to plug it in to a USB Charger, the device will just go to the connected screen.  you won’t be able to access the menu nor play music.