My car
Yesterday the weather was bad. It was cold and it was snowing.
In the morning I had to go to the University, but my car broke down.
I did not know what had happened and I went by bus.
It turned out that the battery had discharged.
The last sentence could be - It turned out that the battery was flat.
In British English we almost always refer to discharged batteries as being 'flat'.
My car wouldn't start because the battery was flat.
I couldn't use my phone, the battery was flat.