question.... worn out battery

[hola]

mi pila de telefono estaba gastado my telephone batter was worn out/low

would this be the correct way to say it in spanish?

[Alfonso]

I would say:

La batería del teléfono estaba gastada.
Tenía la batería del teléfono gastada.
In the second sentence you convey an idea of possession, as in your original phrase, without using the possessive pronoun, which is not as much used in Spanish as you do in English.

[Tomisimo]

You can also use baja for low

the battery is low = la batería está baja