The answer is actually quite simple. The battery isn't replaced, the entire pacer is always replaced. Only the leads remain. There was a rechargeable pacer in the 70's. Researcher even developed a plutonium powered model developed that would last 30 years, but the idea of carrying plutonium in your chest didn't go over well. Rechargeable seems like a good idea. But if you're putting a pacemaker in to keep someone alive, you don't want that to rely on their ability to recharge it. So the interest in rechargeables has really dropped off quite a bit as the battery technology and circuit technology is allowing devices to last more than 10 years. Like any other electronic technology, pacemakers continue to advance in their abilities. A device implanted 8 years ago cannot compare to its replacement today.