“It's running out of juice.”
You're on a long car trip with a friend. You've been watching the directions on your phone, but now the battery power is getting low. You tell your friend.
It's running out of juice.
When an electronic device's battery is low, you can say that it has "run out of juice". "Juice" is a slang term for electrical power.
You can also say things like:
It's almost out of juice.
It doesn't have enough juice.
Other phrases for talking about a low battery are:
My phone is dying.
My battery is almost out.
When the amount of some kind of supply becomes low, you can talk about it like this:
The printer is running out of ink.
Or you can say that you are "running out of" something:
We're running out of toilet paper.