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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?

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    Use the battery in a smoke detector, but not for an arduino project. It is too weak for a relay. – Jot Mar 26 at 20:03
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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)

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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.

  • Really? I'm able to run my Arduino-based power control box just fine from a 9V battery, and it has a LCD display, piezo buzzer, 2 temp/humidity sensors, LEDs, a motion sensor, and various other components. (It drives power relays, but those are only powered when it's plugged into mains power.) Granted, the LCD backlight starts to dim when the 9V battery starts to run out of gas, and eventually the voltage droops to the point where it doesn't work any more, but it will run the whole project for a couple of hours. – Duncan C Mar 27 at 0:13
  • Doesn't a fresh 9V have about 500mAh capacity? Should be enough for an hour if the Arduino draws 1/2 amp, which is enough for quite a few low power accessories on top of the Arduino itself. – Duncan C Mar 27 at 0:13
  • @DuncanC the problem is that if someone grabs just any 9v battery, it might not be fresh and it might not be a good brand. The main problem is the maximum current that a 9v battery can deliver. A 9v battery with lithium is 1200mAh, that would be better (for a few hours). – Jot Mar 27 at 3:47

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