question.... está - es

[hola]

mi bateria esta baja or mi bateria es baja? (my battery is low)

when exactly is the word "es" used? it seems as though "esta" is used everywhere and for everything. is there a rule or a sign to let you know exactly when to use the word "es"?

im pretty sure in the example sentence that the correct word to use is "esta" but i dont see why "es" cant be used. i know you cant speak spanish like it is english but the word "es" only means "is". what harm would it do to the sentence to use the word "es" instead of "esta". i dont know why someone cant say mi bateria es baja.

and i believe if you want to say my battery WAS low you would say mi bateria estaba baja. but when is the word "es" used? when can it be used?