Criminal Sentence 44: Your or You're?

My battery died earlier today, conveniently allowing me to go to an auto parts store and see this:

"Bring in you're old battery and we'll give you $3." (Or something to that effect.)

That should be "your." You can test if the contraction ("you're") is right by pretending you don't have a contraction and spelling it out:

"Bring in you are old battery and we'll give you $3."

As you can see, this does not make sense. It would make a cool insult, though: you old battery.