Modern pacemakers run on a sealed lithium-iodine battery that typically lasts around a decade without ever being recharged or replaced. The battery is built into the device’s titanium housing and implanted just under the skin near the collarbone. Unlike your phone or laptop, a pacemaker draws so little current that a single charge at the factory can keep it ticking for years. That straightforward answer, though, opens the door to a more interesting set of questions about why this particular chemistry works so well, what happens when the battery eventually fades, and whether future pacemakers might never need a battery at all.
The Battery That Changed Everything
Before lithium-iodine cells arrived in 1972, implantable pacemakers relied on zinc-mercury oxide batteries that were bulky, leaked, and lasted only a couple of years. Wilson Greatbatch and his team developed the lithium-iodine cell specifically for cardiac pacing, and it became the standard almost immediately.1PubMed Central. Trends in cardiac pacemaker batteries Over two million pacemakers have been implanted using this chemistry, and cells manufactured today store more than three times the energy density of the original 1972 design.2PubMed. The lithium/iodine battery: a historical perspective
The reaction is deceptively simple: lithium combines with iodine to form lithium iodide.3ECS Transactions. The Lithium/Iodine-Polyvinylpyridine Pacemaker Battery – 35 years of Successful Clinical Use There is no liquid electrolyte sloshing around. The lithium iodide that forms during the reaction is itself a solid electrolyte, which means the cell is entirely solid-state. That matters for two reasons. First, a solid-state battery cannot leak. Second, the internal resistance rises gradually and predictably as the cell discharges, giving doctors a reliable way to forecast when the battery is approaching end-of-life. You get years of warning rather than a sudden failure.
Why a Tiny Battery Lasts So Long
A pacemaker’s longevity is not just about battery size. It is mainly about how little electricity the device actually uses. A pacemaker spends most of its time doing almost nothing: monitoring the heart’s electrical signals and waiting for a beat that does not come on schedule. It only fires a pacing pulse when the heart needs one. In engineering terms, the device sits in a sleep state and wakes up briefly to deliver a tiny electrical stimulus, then goes back to sleep.
To put the numbers in perspective, the pacing pulse generator is the most power-hungry component, consuming on the order of tens of microwatts. The sensing electronics that monitor the heart use a fraction of a microwatt.4ScienceDirect. A low energy ASIC for triple-chamber cardiac pacemakers with contact resistance measurement A microwatt is a millionth of a watt. Your phone’s screen at full brightness draws roughly five million times more power than a pacemaker’s sensing circuit. The total average current drain in a modern pacemaker runs in the low-single-digit microamps, and engineers design the circuitry so that the device can operate for over a decade on a battery with about one amp-hour of capacity.
Newer leadless pacemakers, which are capsule-shaped devices implanted directly inside the heart via a catheter, face tighter constraints because they are much smaller. The latest generation of these devices has pushed battery capacity up modestly, from about 120 milliamp-hours to about 142 milliamp-hours, while simultaneously cutting current drain by roughly ten percent through smarter accelerometer signal processing that keeps the microprocessor in sleep mode longer.5PubMed Central. Device longevity of a leadless pacemaker family Five-year follow-up data from one large leadless pacemaker registry showed a median projected battery life of about seven years remaining at the five-year mark, tracking with an initial projection of over twelve years total.6European Heart Journal. Leadless pacemakers at 5-year follow-up: the Micra transcatheter pacing system post-approval registry That is impressive for a device smaller than a large vitamin capsule sitting inside a beating heart.
What Happens When the Battery Starts to Fade
Pacemaker batteries do not die without warning. As the lithium-iodine cell depletes, its internal resistance climbs in a predictable curve. The device monitors its own battery voltage and, once it crosses a pre-set threshold, triggers what is called the elective replacement indicator, or ERI. Think of it as a low-fuel light on a car’s dashboard. At this point, there is still enough charge left for the pacemaker to keep working for weeks or months, but it is time to schedule a replacement.
The replacement procedure involves opening the pocket of tissue under the skin where the pacemaker generator sits, disconnecting the old unit from the leads (the thin wires that run into the heart), and snapping in a new generator. The leads themselves usually stay in place, which simplifies things. Still, generator replacements carry real risks. Data from a large multicenter registry found that roughly four percent of patients undergoing a straightforward pacemaker generator replacement experienced a major complication, and the risk climbed substantially when doctors also had to add or revise leads.7PubMed. Complication rates associated with pacemaker or implantable cardioverter-defibrillator generator replacements and upgrade procedures: results from the REPLACE registry Infection is a particular concern: patients who developed bleeding in the first thirty days after the procedure had nearly triple the risk of developing an infection in the following year compared to those who did not bleed.8PubMed Central. Incidence of Bleeding-Related Complications During Primary Implantation and Replacement of Cardiac Implantable Electronic Devices
One important caution for patients and clinicians: when a pacemaker is near its ERI point, any activity that increases the device’s power draw can temporarily drop the battery voltage below the pacing threshold and cause a dangerous pause in heart rhythm. A case report documented a nearly fourteen-second episode of ventricular standstill in a pacemaker-dependent patient whose device was at ERI when testing maneuvers increased the current drain.9PubMed. Impact of increasing current drain in a pacemaker operating during activation of the elective replacement indicator The lesson is straightforward: once the low-battery signal appears, avoid unnecessary testing that taxes the device and schedule the replacement promptly.
Could Future Pacemakers Charge Themselves from Heartbeats?
The idea of harvesting energy from the heart’s own motion to power a pacemaker has moved from theoretical physics into animal studies over the past decade. Several research teams have built tiny generators that convert the mechanical squeeze of each heartbeat into electrical energy, using one of two main approaches.
The first relies on piezoelectric materials, which produce a small voltage when they are bent or compressed. Researchers have demonstrated flexible piezoelectric films that conform to the surface of the heart, lung, or diaphragm and convert the rhythmic motion into electricity. In animal models with organ sizes approaching human scale, these harvesters generated enough power to run a pacemaker.10PubMed Central. Conformal piezoelectric energy harvesting and storage from motions of the heart, lung, and diaphragm A separate study using piezoceramic cantilevers sized to fit inside a leadless pacemaker’s battery compartment found that the in vivo heart motion could produce about 1.1 volts, though the power level was modest and depended heavily on the geometry of the harvester.11PubMed Central. Conceptual Piezoelectric-Based Energy Harvester from In Vivo Heartbeats’ Cyclic Kinetic Motion for Leadless Intracardiac Pacemakers
The second approach uses triboelectric nanogenerators, which produce electricity when two different materials repeatedly come into contact and separate. Several groups have built capsule-shaped devices that sit inside the heart and generate power from the sloshing and vibration of each beat. One team implanted a coin-battery-sized triboelectric generator in an animal and showed it could charge a lithium-ion battery and power a functioning cardiac pacemaker, using body motion and gravity to drive the internal moving parts.12Nature Communications. Self-rechargeable cardiac pacemaker system with triboelectric nanogenerators Another group built a battery-free intracardiac pacemaker that delivered stable pacing in a pig model for three weeks using only harvested cardiac energy, producing an open-circuit voltage of about 6 volts in vivo.13PubMed Central. A self-powered intracardiac pacemaker in swine model A more recent iteration achieved even higher volumetric power density and demonstrated reliable heart-rate control in a pig powered entirely by heartbeat energy.14PubMed Central. Self-sustaining leadless intracardiac pacemaker powered by triboelectric nanogenerator
There is also a charmingly low-tech variant: one research group repurposed the clockwork mechanism from an automatic wristwatch, which is designed to harvest kinetic energy from wrist movement, and adapted it to capture energy from cardiac wall motion. The optimized device demonstrated sensitivity to the full range of tested heart motions.15PubMed. Energy harvesting from the beating heart by a mass imbalance oscillation generator It is a reminder that the underlying concept is not exotic: self-winding watches have been turning motion into stored energy for over a century.
None of these heartbeat-harvesting devices has reached human clinical use yet. The engineering challenges are real: the heart’s motion is periodic but not smooth, the forces are small, and any device implanted inside the heart must survive millions of cycles without shedding fragments or interfering with blood flow. But the pace of the research is accelerating, and the idea of a pacemaker that never needs a battery replacement is no longer science fiction.
Wireless Charging, Solar Cells, and Glucose Fuel Cells
Heartbeat harvesting is not the only alternative energy strategy under investigation. Researchers are also exploring ways to deliver power through the skin or to tap the body’s own chemistry.
Wireless power transfer uses magnetic fields to send energy from an external transmitter through the skin to a receiving coil inside the implant. The concept is well proven in consumer electronics (think wireless phone chargers), but scaling it down for a pacemaker while keeping tissue heating within safe limits introduces complications. Studies have modeled and simulated wireless charging systems for cardiac pacemakers, focusing on finding an operating frequency that maximizes charging efficiency without causing excessive heating in surrounding tissue.16PubMed. Safety Enhancement by Optimizing Frequency of Implantable Cardiac Pacemaker Wireless Charging System Thermal safety research on similar transcutaneous energy systems for other implants has shown that surface temperatures stay within a degree or two of normal body temperature when delivering up to 10 watts continuously, which is far more power than a pacemaker would ever need.17PubMed. Thermal evaluation of a hermetic transcutaneous energy transfer system to power mechanical circulatory support devices in destination therapy The challenge is less about safety in a controlled lab setting and more about the practical realities: patients would need to wear or hold a charging pad against their chest periodically, the coils need to align reasonably well through skin and tissue, and the system must tolerate variation in implant depth and patient body composition. No wirelessly rechargeable pacemaker has reached clinical use yet.16PubMed. Safety Enhancement by Optimizing Frequency of Implantable Cardiac Pacemaker Wireless Charging System
Subcutaneous solar cells are another creative approach. Researchers placed small photovoltaic cells under the skin of human volunteers and measured their power output over an extended period. The cells captured enough light passing through the skin to produce a mean power output of about 67 microwatts, which the researchers noted is sufficient to power a cardiac pacemaker.18PubMed Central. Energy Harvesting by Subcutaneous Solar Cells: A Long-Term Study on Achievable Energy Output The obvious limitation is variability: output depends on ambient light, clothing, skin thickness, and pigmentation. A device relying on subcutaneous solar would almost certainly need a backup battery for nighttime and low-light conditions.
Glucose biofuel cells take a completely different tack. Instead of harvesting mechanical or light energy, they generate electricity from the glucose and oxygen already dissolved in your blood and interstitial fluid. Enzymes or inorganic catalysts at two electrodes oxidize glucose on one side and reduce oxygen on the other, producing a small voltage and current. One team implanted such a cell in a rat’s abdominal cavity and measured a power output of about 39 microwatts.19Scientific Reports. Single Glucose Biofuel Cells Implanted in Rats Power Electronic Devices Another group demonstrated an enzyme-free version using metallic nanoparticle catalysts operating in human serum at normal glucose concentrations, producing enough power to activate a pacemaker through a voltage-boosting circuit.20Electroanalysis. Pacemaker Activated by an Abiotic Biofuel Cell Operated in Human Serum Solution The appeal is obvious: if the fuel cell works as long as the patient’s blood contains glucose and oxygen, the power supply is essentially unlimited. The hurdles include long-term stability of the catalysts inside the body, biofouling of the electrode surfaces by proteins and cells, and scaling the power output reliably to meet pacemaker demands.21PubMed Central. Glucose-based biofuel cells and their applications in medical implants: A review
MRI Scans and Backup Power Modes
One situation that reveals how pacemakers manage their power is exposure to strong magnetic fields during an MRI scan. A large prospective study of over 1,500 patients with pacemakers or defibrillators who underwent MRI found that in about 0.4 percent of scans, the device temporarily reset to a backup pacing mode.22PubMed Central. Safety of Magnetic Resonance Imaging in Patients with Cardiac Devices The backup mode is a stripped-down, low-power state that ignores the heart’s own electrical signals and paces at a fixed rate. In eight of the nine cases, the reset was temporary and the device returned to normal afterward. In the remaining case, a pacemaker that already had less than a month of battery life left reset and could not be reprogrammed, prompting a device replacement.
That single case illustrates an important point: a pacemaker’s power-management system is designed with layers of fallback. Even when the primary programming is disrupted, the backup mode keeps the heart pacing. But backup modes use fixed settings that are not optimized for the patient, so they are a safety net rather than a long-term solution. And for patients whose batteries are already near the end of their life, even the minor extra demand of recovering from a reset can tip things over the edge.
What Happens to Pacemakers After They Are Removed
When a pacemaker is replaced because of battery depletion, or when a patient with a pacemaker dies, the device is typically removed. In wealthier countries, these explanted devices are usually discarded as medical waste. Yet many of them still have years of battery life remaining, especially when the patient died of an unrelated cause. This has prompted organized efforts to collect, sterilize, and donate used pacemakers to patients in low- and middle-income countries who cannot afford new devices. One proof-of-concept program works with funeral homes and crematories to recover explanted devices, evaluate them at an academic center, and resterilize them before shipping them abroad.23PubMed Central. Pacemaker recycling: A notion whose time has come
The ethics and logistics of pacemaker reuse are still debated. Regulatory frameworks in many countries do not clearly address the reimplantation of used devices. There are questions about liability, about how accurately remaining battery life can be estimated, and about whether older device models perform adequately compared to current technology. But the core reality is hard to argue with: a functioning device with years of charge left is being incinerated in one country while a patient in another country is dying of a treatable arrhythmia because they cannot afford a new one. The battery longevity that makes pacemakers so reliable in their first owner is the same feature that makes reuse technically feasible.
Why Rechargeable Batteries Are Not Already Standard
Given that rechargeable lithium-ion batteries power everything from phones to electric cars, it might seem strange that pacemakers still use non-rechargeable cells. The reasons are practical rather than theoretical. A rechargeable pacemaker would require the patient to periodically charge the device through the skin, adding a compliance burden to someone who may be elderly or cognitively impaired. If the patient forgets or is unable to charge, the consequences could be life-threatening. The non-rechargeable lithium-iodine cell avoids this entirely: it works from the moment it is implanted and demands nothing from the patient until it eventually needs replacement years later.
There is also a reliability argument. Rechargeable batteries degrade with each charge cycle, gradually losing capacity. A lithium-ion cell that starts at 100 percent capacity might retain only 80 percent after a few hundred cycles. For a phone, that means shorter battery life after a year or two. For a pacemaker, it means the projected lifespan becomes less predictable, and the patient needs more frequent monitoring to track how the battery is aging. The lithium-iodine cell’s predictable discharge curve is itself a safety feature: doctors can forecast the replacement date years in advance with reasonable accuracy. That said, rechargeable systems are already standard in spinal-cord stimulators and some neurostimulators, where the power demands are much higher and a non-rechargeable battery would need replacement every year or two. If pacemaker power demands ever increase substantially, perhaps due to more sophisticated sensing or multi-chamber pacing, the calculus could shift.
For now, though, the sealed lithium-iodine cell remains one of the most successful engineered solutions in modern medicine: a battery designed half a century ago, refined incrementally, that quietly does its job inside your chest for a decade at a time. The next generation of pacemakers might draw their power from the very heartbeat they regulate, but that transition, when it comes, will have to clear a high bar set by a battery chemistry whose track record is hard to beat.