The battery is an integral part of the film pack. When you put a fresh pack in, you get a fresh battery. A dead battery means the rest of the photo pack is unusable.
Original photo packs are no longer manufactured and all remaining unused Polaroid stock will be long out of date with flat batteries and stale chemicals. If kept stored cold, the chemicals may be fresh but the batteries will be flat as cold kills batteries even faster. Such photo packs cost a lot, are not guaranteed to work, and you only have the vendor's word that they have been cold stored. They're only usable in cameras modified to accept an external 4.5v dc power supply.
You can buy new and fresh photo packs from The Impossible Project
, though. Just be aware that it's not the same as the original material and is really intended for artistic use. It's also not light stable and fades quickly, so scan any images you wish to keep, but is designed to be far more manipulable than the original material.
Click here for the company website where you can learn all about their product and how best to use it. You can buy directly or the site shows you where a few retailers are (not many worldwide).