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 neuromodulation device, cardiac monitor, or drug delivery device inside a patient’s body. During that time, clinicians and patients depend on it to perform reliably and predictably. Unlike consumer electronics, where a degrading battery can often be recharged more frequently or replaced, an implantable device may require an additional surgical procedure when its battery reaches the end of its useful life. 

That makes battery longevity a critical consideration. A battery must not only deliver the required performance when new, but continue providing the energy and power needed to support therapy, monitoring, or diagnostics throughout its intended service life. 

If battery performance declines sooner than expected, the consequences may extend far beyond an engineering concern. It can affect patient health, disrupt therapy, and lead to an additional surgical procedure for device replacement. 

The key question isn’t simply, “How much energy does this battery provide when it’s new?” 

It’s, “How much usable energy will remain years from now?” 

Understanding that requires looking beyond capacity and day-one specifications to how the battery performs over time.

 

Longevity Means Different Things for Different Battery Chemistries 

Not all implantable batteries age in the same way. 

For rechargeable implantable batteries utilizing Lithium-Ion chemistry, 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 declines due to a combination of increasing internal resistance and the gradual loss of active Lithium and other electrochemical aging mechanisms. 

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 a deeper look at how cycling conditions affect long-term battery performance, download our whitepaper, Lithium Ion Cycling Charge and Discharge. 

 

For primary Lithium batteries used in implantable applications, which are not designed to be recharged, the focus shifts from cycle life 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. 

 

 

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 over time  
  • Cycle life under expected operating conditions  
  • Calendar aging effects  
  • Loss of active lithium and other aging mechanisms  
  • Changes in internal resistance, especially 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 one 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 over extended service intervals 
  • Predictable end-of-service voltage characteristics 
  • 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. 

 

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 Lithium batteries 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 development. Commercially available, off-the-shelf cells can help accelerate prototyping, reduce engineering complexity, and avoid the extended lead times often associated with fully custom development programs. As discussed in this article, “Implantable Medical Device Batteries 101: How to Choose the Right Power Solution“, off-the-shelf battery platforms can be a valuable option for early-stage development and concept validation while longer-term battery requirements are being refined. 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. 

Ultimately, the right battery is not necessarily the one with the highest initial capacity. It’s the one that can consistently and safely meet device requirements throughout its expected service life. Simply put, a battery’s true value is measured by its performance over time, not how impressive it looks on a specification sheet. 

 

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 support immediate development work 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 safety operations, scalability, and manufacturing support, we partner with customers throughout the product development journey. 

Ready to evaluate the right battery strategy for your implantable medical device? Contact the Resonetics Medical Power team to discuss your application and explore off-the-shelf and custom battery solutions. 

 

Speak with a Medical Device Battery Expert Now!