Backup, arduino

is it possable to make a arduino battery backup that only works when there is no usb power??

Yes.

But not without added electronic components, I think. Someone will correct me where I’m wrong, but the Arduino boards currently treat the two power supply inputs as being fairly interchangeable. I think it uses the battery even when there’s USB power available. A “backup” should only supply current from the battery when the primary external source is failing. There are some good LiPo batteries available, but I would think there’s a good need for a reliable but thrifty automatic power selecting circuit.

erm…

wut about a 9-volt???

Battery alone will not do it. Like Halley said, you’ll need additional components. If you just put a battery in parallel with another power supply, it will be drained.

You might search Google for examples of “battery backup circuit”.