When engineers evaluate a battery, it’s natural to focus on day-one performance. Higher capacity, strong power output, and impressive energy density can make a battery look like the obvious choice.
But for implantable medical devices, the most important performance benchmark isn’t the first day. It’s the last.
A battery may spend years powering a cardiac monitor, neurostimulator, or drug delivery device inside a patient’s body. During that time, clinicians and patients depend on it to perform reliably and predictably. If battery performance declines sooner than expected, the result may be more than a maintenance issue. It can mean device replacement and an additional surgical procedure.
That’s why battery performance at the end of its life is often just as important as its performance at the beginning.
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Looking Beyond Day One
Consumer electronics, almost exclusively powered by Lithium-Ion, are typically judged by their initial performance. If a smartphone battery degrades, users can simply recharge it more often or replace the device.
Unfortunately, implantable devices don’t have that luxury.
Many implantable systems are designed to operate for years, and in some cases more than a decade. Throughout that time, the battery must continue delivering the power needed to support therapy, monitoring, or diagnostics.
The key question is not, “How much energy does this battery provide when it’s new?”
It’s, “How much usable energy will remain years from now?”
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Longevity Means Different Things for Different Battery Chemistries
Not all implantable batteries age in the same way.
For rechargeable lithium-ion batteries, longevity is typically measured by how well the battery maintains its capacity over time and through repeated charge-discharge cycles. As the battery ages, usable capacity gradually decreases, due to increases in internal resistance for the first 500 cycles and then consumption of active material, Lithium. Engineers evaluate factors such as cycle life, capacity retention, and calendar aging to determine whether the battery can continue supporting the device throughout its intended lifespan.
For primary lithium batteries, the focus shifts to long-term energy retention. Since these batteries are not recharged, longevity is largely determined by self-discharge characteristics and the ability to maintain predictable performance over many years. Low self-discharge rates are critical to ensuring the battery retains sufficient energy to power the device throughout its service life.
While the mechanisms are different, the objective remains the same: predictable, reliable performance for as long as the implant is expected to operate.
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Why End-of-Life Performance Matters
For implantable devices, reliability is directly connected to patient outcomes.
The battery must provide sufficient power not only after implantation, but years later when the device is still expected to function exactly as intended. Whether supporting neuromodulation, cardiac monitoring, or drug delivery, consistent power delivery helps ensure therapy remains uninterrupted.
As device manufacturers continue pushing for longer service lives, battery technology must evolve alongside those expectations. This requires careful battery design, chemistry selection, and extensive testing focused on long-term performance.
Simply put, a battery’s true value is measured by how well it performs over time, not how impressive it looks on a specification sheet.
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Testing for Long-Term Reliability
Predicting battery performance over ten years requires more than standard testing.
Because rechargeable and primary batteries age differently, they must be evaluated using different long-term performance metrics.
For rechargeable lithium-ion batteries, engineers often evaluate:
- Capacity retention
- Cycle life
- Calendar aging
- Material consumption – loss of Lithium
- Internal resistance growth – most occurs in the first 500 cycles
For primary lithium batteries, key performance indicators typically include:
- Self-discharge rate
- Self-discharge rate for primary Lithium cells depends on temperature, time and discharge protocol.
- One advantage of medical implantable applications is the temperature is constant at 37°C which removes on variable.
- Accelerated aging performance
- A significant advantage of primary Lithium cells is that accelerated aging, at 72°C is equivalent to 11.3 x over 37°C.
- Long-term energy retention – what does this mean?
- End-of-Service voltage is predictable
- Overall reliability under expected operating conditions
These measurements provide a more complete understanding of how a battery will perform throughout the life of an implantable device, not just when it is new.
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The Right Solution for the Right Application
No two implantable devices have identical power requirements.
Some applications benefit from rechargeable lithium-ion technology, while others are better served by primary battery chemistries designed for long service life. Factors such as energy demand, device size, therapy requirements, recharge strategy, and target longevity all influence the optimal solution.
For many developers, an existing battery platform may provide the fastest path to market. Off-the-shelf cells can help accelerate development timelines while reducing engineering complexity. In other cases, device requirements demand a custom solution optimized for size, capacity, form factor, or performance characteristics.
The ability to choose between proven standard battery platforms and custom-designed solutions gives device manufacturers greater flexibility as they move from concept through commercialization.
Selecting the right battery is ultimately about balancing performance, safety, reliability, and product lifecycle requirements while ensuring the device can deliver therapy consistently throughout its intended lifespan.
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Ready to Evaluate Your Implantable Battery Strategy?
At Resonetics Medical Power, we help device manufacturers develop power solutions designed for long-term reliability and predictable performance. Whether you’re looking for a proven off-the-shelf battery to accelerate development or a custom battery designed around your application’s unique requirements, our team can help identify the right solution.
From chemistry selection and performance optimization to scalability and manufacturing support, we partner with customers throughout the product development journey.
