"I need a battery."
Translation:Necesito una batería.
la pila is used too. "Almuerzos que te recargan las pilas" [Lunches that recharge your batteries]
It's also worth knowing that la batería is also a drum kit/drums (as in musical instrument)
And, in central Mexico, if you need only one AAA battery you can walk into a small mom & pop tienda and they'll break a package and sell you just one. Same for band-aids and aspirin, all sorts of little things. Try doing that at Target or WalMArt. ;)