user37231: I encountered an odd problem with my Uno, it works completely fine through USB on the PC or when powered by a power bank, but as soon as I connect it on a 9v battery it just freaks out. I have a relay connected to it that is connected to a switch, works fine, but on the battery it keeps turning on and off super fast, like so fast that the relay doesn't even show it's doing it, I noticed because of the noise it makes. I even tried to slow it down with a delay but nothing changed.
Anyone have any idea why the battery would make this happen?

user51102: It's actually not just the relay that takes up a lot of current. The 9V-port on your arduino uno is actually not meant for a 9V block, because the arduino uno needs a higher current than your 9V block can deliver. You can put 6 1.5V battery cells (AA batteries) in series, they will deliver the current you'll need.

user521: Not enough current available from the battery, relays take quite a lot. As soon as the relay tries to switch the voltage drops and the Uno resets due to brownout (low voltage)

37231: <p>I encountered an odd problem with my Uno, it works completely fine through USB on the PC or when powered by a power bank, but as soon as I connect it on a 9v battery it just freaks out. I have a relay connected to it that is connected to a switch, works fine, but on the battery it keeps turning on and off super fast, like so fast that the relay doesn't even show it's doing it, I noticed because of the noise it makes. I even tried to slow it down with a delay but nothing changed.</p>

<p>Anyone have any idea why the battery would make this happen?</p>
 51102: <p>It's actually not just the relay that takes up a lot of current. The 9V-port on your arduino uno is actually not meant for a 9V block, because the arduino uno needs a higher current than your 9V block can deliver. You can put 6 1.5V battery cells (AA batteries) in series, they will deliver the current you'll need.</p>
 521: <p>Not enough current available from the battery, relays take quite a lot. As soon as the relay tries to switch the voltage drops and the Uno resets due to brownout (low voltage)</p>
