battery is getting low

Absolutely yes.

This sounds like a generic type question, WB; "mobile phone" is not specific in this sense.

When a mobile phone is running out of power, is it idiomatic English to say this?:

Thank you JTT. You're always so perceptive~

"Cellphone" is the commonest term in the USA I think, and here we talk of our "mobile".

"Mobile phone" is not much heard. But well understood, of course; just a bit longer.

All these terms are interchangeable.

Thank you, Mctag~~