The day before I got my used full package arduino uno. I tested it on USB plug. Everything worked fine. I wrote a piece of testing code and uploaded. After I have accomplished with the test I have unpluged arduino from PC and connected to battery and I see that it does not work. I have used a multi-tester to find out the voltage. It showed 4.7V. Now I wonder should I buy a new battery or there is another reason for this. Anybody, please suggest me what to do. Update The battery came with arduino set. It is written that it is a 9V battery. I am plugging it to the battery input in my arduino uno. Polarity is ok. The same program working on connection via usb, does not when connected to battery.
Did you make sure the battery is charged? Does it have enough voltage to power the board? Remember that Arduino has internal voltage regulator, so you need something little more than 5V in order to get it working okay.
(Comment by bpinhosilva re-purposed as an answer so this will stop showing up as unanswered).
@bpinhosilva you are right, the battery was not supplying enough power to make arduino work.
Please accept this so that the number of unanswered questions goes down. :)
simply you can weld a little electrolytic capacitor eg. 4 or 6 microfarads, 12 or more Volts, with the positive wire on the reset pin and the negative on the GND pin. in this way you can solve the restart without the USB connection. This is what I did Enjoy with Arduino. By Daniele