Modern pacemakers run on a small lithium-iodine battery sealed inside the device’s metal casing. The entire device uses only about 10 to 20 microwatts of power, roughly a million times less than a standard light bulb, which is why a battery smaller than a coin can keep a heart pacing for years.
How Lithium-Iodine Batteries Work
Nearly every pacemaker implanted today uses a lithium-iodine primary cell, a chemistry adopted in the late 1970s that proved so reliable it has dominated the field ever since. Inside the battery, a lithium metal anode sits in direct contact with an iodine cathode. When lithium reacts with iodine, it forms a thin layer of lithium iodide between them. That layer acts as the battery’s electrolyte, the material that lets charged particles flow and generate current.
This design has two key advantages. First, the solid electrolyte means there’s no liquid that could leak. Second, the self-discharge rate is remarkably low: less than 10% over ten years. That means the battery loses almost none of its charge just sitting there, so nearly all its stored energy goes toward actual pacing.
Why So Little Power Is Needed
A pacemaker’s job is to send tiny electrical pulses to heart muscle, and sensing the heart’s own rhythm between those pulses takes even less energy. The total draw of 10 to 20 microwatts allows manufacturers to use a battery roughly the size of a stack of two or three quarters and still get years of service from it. More complex pacemakers that pace multiple chambers or include wireless communication features drain the battery faster, but the fundamental power demand remains extremely small by the standards of any other electronic device.
How Long the Battery Lasts
Most pacemakers last 7 to 14 years before the battery needs replacing. That wide range depends on how often the device actually fires. If your heart only needs occasional pacing, the battery stretches toward the longer end. If you depend on the pacemaker for every heartbeat or use a device that paces both ventricles, it drains faster.
Leadless pacemakers, which are tiny capsules implanted directly inside the heart, use the same basic battery chemistry in a much smaller package. The first-generation Medtronic Micra device had a projected median battery life of about 12 years. After five years of real-world follow-up, that estimate held steady, with the median remaining projection at about 6.8 additional years. A second-generation version is expected to last around 17 years.
How Battery Depletion Is Tracked
You won’t feel your pacemaker’s battery running low. Instead, the device monitors its own voltage internally and flags specific milestones during routine checkups, which a technician reads using an external programmer held over the device.
The first flag is called the Recommended Replacement Time, sometimes labeled ERI (Elective Replacement Indicator). When the battery voltage drops to this threshold, the device is still working normally, but it’s time to schedule a replacement. If the voltage drops further to End of Service, the pacemaker may no longer pace, sense, or deliver therapy reliably, and replacement becomes urgent. Routine monitoring, typically every 6 to 12 months and more frequently as the battery ages, catches depletion well before it reaches that critical stage.
What Replacement Involves
When the battery reaches its replacement threshold, surgeons swap out the pulse generator, which is the sealed metal case containing the battery and electronics. The thin wires (leads) that connect the generator to your heart are tested during the procedure. If they’re functioning normally, they stay in place and simply get plugged into the new generator. The surgery is an outpatient procedure, similar to the original implant but faster, and most people go home within a couple of hours. Lead replacement is only needed in rare cases where a wire has fractured or malfunctioned.
Leadless pacemakers present a different situation. Because the entire device sits inside the heart, a depleted unit is typically deactivated and left in place while a new one is implanted alongside it.
Nuclear Pacemakers: A Legacy Technology
Before lithium-iodine cells became standard, some patients received pacemakers powered by plutonium-238. First implanted in France in 1970, these devices used the heat from radioactive decay to generate electricity through a thermoelectric converter. Because plutonium-238 has a half-life of 87.7 years, the generators were designed to remain active for decades, far outlasting any battery available at the time.
Nuclear pacemakers solved the longevity problem but introduced others: regulatory complexity, disposal concerns, and the need for patients to carry documentation about the radioactive material. Once lithium-iodine batteries proved they could reliably last a decade or more, the nuclear approach was abandoned. A small number of patients still carry functioning plutonium-powered devices implanted in the 1970s and 1980s.
Harvesting Energy From the Heart Itself
Researchers are working on pacemakers that would never need a battery replacement because they generate electricity from the heart’s own motion. One recent approach uses a piezoelectric cube, a tiny structure that produces voltage when it vibrates, paired with a free-moving sphere inside it. Each heartbeat shifts the sphere, which strikes the walls of the cube and sets off vibrations that generate current.
In testing on a living pig, one of these harvesters produced a root-mean-square power of about 7 microwatts per heartbeat, which falls within the 10 to 20 microwatt range that a pacemaker needs. The entire harvesting unit measured just 0.15 cubic centimeters and weighed under a gram, making it comparable in size to the battery compartment of existing leadless pacemakers. The peak voltage reached 3.3 to 8.8 volts depending on heart rate, comfortably above the threshold needed for pacing pulses. These devices remain in the prototype stage, but the core engineering challenge of extracting enough energy from a heartbeat has, at least in animal models, been solved.