Using backup battery for project

Hi.
i would like to use my UNO project in a car and I bought the 12v Cigarette Lighter Socket with 5V usb output .
I also have li-on 1cell battery, charger for the battery and up converter to 5V.

So the uno should run with the 5V usb from the 12V Cigarette Lighter Socket and when the power is disconnected it should switch to the battery. I need some sort of a way to detect the usb power is down and trigger the battery
any thoughts?
Thanks.

Can your up-converter handle an input between about 3V and 4.3V? If so, then a simple solution is to feed the up-converter from the battery via a Schottky diode, and from the USB power supply via a silicon diode. Then you can always power the Arduino from the output of the up-converter.

I've worked out an alternative solution that uses the USB power directly when it is available, but it is more complicated than I would like.

Thank you very much for your help.
My up converter can handle 2.5V-5V to 5V , so Yes.

Does this diagram correct? if so can you please provide me with specific diodes (up to 500ma)?
Thank you very much for your help!

Yes, that diagram looks OK to me. I would use 1N400x (x = 1 to 7) for the silicon diode, and 1N5817 for the Schottky one (both rated at 1A). I'm assuming your Li-Ion battery is internally protected aganist over-discharge.

Hi. Great
the battery includes built-in protection against over voltage, over current, and minimum voltage.
I really appreciate you help, thank you.