Battery-Powered Medical Devices: Their Failure Modes and Mitigation Strategies

November 13, 2018
More and more products today are battery operated. As a result, rechargeable batteries have become ubiquitous in our daily lives and are present in a variety of products, including consumer electronics, vehicles, and medical devices. Understanding how these batteries are designed, manufactured, and used is essential to uncovering what happens when these batteries, and the devices they power, fail.

Lithium-ion is a type of rechargeable battery that provides numerous advantages over non-rechargeable batteries. This high energy-density chemistry has a mature custom manufacturing market, enabling the miniaturization of devices. In addition, its excellent cycle and calendar life extends the overall lifespan of the device. Such advantages are especially critical in medical implants, which are becoming increasingly sophisticated.

Active implantable medical devices (AIMDs) refer to devices implanted inside the body that require power to operate. Cardiac pacemakers, cardioverter-defibrillators (ICDs), drug delivery pumps, or neurostimulators are examples of AIMDs (Figure 1). Extending the lifespan of such devices with an appropriate rechargeable battery prevents frequent replacement surgeries and any subsequent complications that may arise, resulting in substantial cost savings. , Currently available rechargeable neuromodulation systems have an estimated lifespan of 9–25 years, compared to the typical lifespan of 2–5 years for a non-rechargeable battery.

Despite the numerous advantages of lithium-ion batteries, hazards associated with the failure of these batteries can be catastrophic, as was seen in the incidents caused by the Samsung Note 7 battery issues. The potential hazards include thermal events and release of heat, electrolyte leakage causing toxic exposure, and malfunction of the device due to battery capacity depletion. While these hazards can cause much harm to the user under any situation, the danger is amplified with medical implants, which reside inside the human body. Compared to consumer electronics, for example, premature battery depletion is considered serious in the medical device industry, an example being the recent recall of the St. Jude Medical ICDs.

With the increasing demand for AIMDs, whose market is estimated to reach approximately $27 billion by 2022, it has become paramount to have an in-depth understanding of the possible failure mechanisms for lithium-ion batteries and ultimately to have the ability to pinpoint the cause of failure to mitigate and resolve any issues encountered in the field. Battery failures in medical devices also need to consider the requirements specific to medical devices such as the operating environment (inside the body), need for sterilization, and interaction with various diagnostic equipment (e.g., MRIs).

In this article, we examine the possible causes of lithium-ion battery failures and discuss what measures can be taken during the product development cycle to prevent such failures, with special consideration to the highly regulated medical device environment.

What is in a Battery System?

A rechargeable battery system consists of the cell, charging circuitry, and battery management unit (BMU). It is crucial to conduct design reviews for each of these components early in the product development cycle to ensure the design incorporates safety features relevant to the application. The cell, typically a pouch containing the electrodes that store energy for use by the medical device, is the largest component of the battery system. This cell is paired with a BMU and charging circuit to control the current and voltage conditions between the battery, charger, and medical device. The BMU provides numerous protections to the cell, for example, preventing an external device short circuit from leading to cell thermal runaway and restricting current levels from exceeding over-current specification cutoffs. The charger circuit, whether it enables wired or wireless charging, defines the charging profile (charging current and voltage) such that it aligns with the cell specifications, for example, to prevent charging outside a cell’s rated temperature. It is especially important in multi-cell battery systems that the charger circuit actively monitors individual cell voltages to prevent cell over-charging.

There are numerous requirements and safety considerations in designing a battery system. For implantable medical devices, further considerations are necessary due to the need for sterilization, special environmental conditions, and the potential of injury to the patient. Environmental and electrical loading conditions play a significant role in the health of the cell. The conditions that provide the highest performance also result in the fastest degradation of internal components, potentially resulting in greater risk. The trade-off between performance and safety should be evaluated based on the level of risk acceptable for a device. The safety of a system can be improved by an intelligently designed battery management and charging circuit to reduce risks of cell failure.

Root Causes of Battery System Failures

The risks associated with lithium-ion batteries include cell thermal events, in which the cell vents and heats to dangerous levels for the patient; electrolyte leakage, in which toxic chemicals can leak from the cell enclosure; and capacity depletion, in which the battery cannot supply as much power as it was designed to deliver resulting in decreased device runtime. Multiple root causes can lead to such battery system failures.

Custom sized pouch-type cells provide much-desired space savings but come with increased risk due to complicated assembly. Defects introduced within the cell during the assembly process can lead to internal short-circuits or premature degradation. Such defects are often related to the relative positioning of critical cell components, such as cell tabs that connect the cell to the BMU or individual electrodes aligned inside of the cell, or to the quality and purity of materials. A contaminant particle, for example, could lead to a cell that cannot be charged and/or discharged, overheating, or thermal runaway. In addition to size and shape, cells can be customized for specific power consumption conditions, such as high current (called power cells) or long life (called energy cells), but such customization can result in cell imbalance that leads to the formation of short circuits with extended use. Customization comes with significant risk if choices made at the cell level are not well understood by the cell manufacturer and device designer.

Once a cell design has been chosen, the charging circuitry and BMU are then designed specifically to support the safe charge and discharge of the cell under nominal conditions and protect the cell under stress conditions. A lithium-ion cell can have a range of operating conditions for voltage, current, and temperature. Operating at the extremes of these ranges can yield high performance but also increases the risk of failure due to premature aging and decreased run-time. Decreased run-time as the device ages is one of the most noticeable effects of mismatched cell capability and device power consumption. This type of failure affects the patient on a daily basis and will decrease trust in the medical device and treatment. Frequently charging an old phone may be acceptable in the consumer device space, but a medical device that supports vital functions will be held to a higher standard. Decreased runtime can also lead to a failure of therapy delivery, which can be life-threatening for critical devices such as ICDs, ultimately causing premature surgical battery replacement.

A poorly designed BMU and charging circuit can result in the cell being operated outside of its specifications, including charge/discharge with excessive currents, over-charge or over-discharge to excessively high or low voltages, respectively, or operation in over/under temperature conditions. For example, if a patient with an implantable neurostimulator has not charged the device when advised to do so, it is imperative that stimulation be automatically suspended by the protection circuitry to prevent over-discharge of the cell, as periodic over-discharge can lead to cell swelling and electrolyte leakage or introduce internal short circuits.

Wireless charging, ideal for implantable medical devices due to the contactless nature of the technology, is sweeping the consumer electronics market. However, safety considerations should be taken into account to ensure that an implanted medical device is not susceptible to unintended charging arising from cross-talk of other nearby devices. Protection features should be incorporated into the charging circuit to guarantee that the cell is always charged within specifications even if a third-party wireless charger is used by the patient.

A successful battery system will incorporate a high-quality, well-designed cell with a BMU and charging circuitry that appropriately limit the conditions experienced by the cell. Battery system failures can be understood by careful inspection and testing, and in the event that issues are found, careful analysis can reveal methods by which these issues may be mitigated.

Mitigation Strategies

Mitigation strategies to avoid battery system failures include, for example, designing for multiple levels of over-current protection to protect against short circuits or conducting an audit of the cell manufacturing process to ensure quality control. In addition, failures should undergo a root cause analysis, and corrective actions should be implemented in the product development cycle to improve the overall safety and function of the system.

After a battery system has been designed, testing should be conducted to validate and verify that the safety protection mechanisms outlined in the cell and circuitry design have been effectively implemented. The testing process strains the system by exposing it to worst-case operating conditions to demonstrate the system’s ability to handle such conditions. A design review alone is inadequate to comprehensively evaluate system safety because of the complexity and interplay between the device, circuitry, and cell. For example, a condition under which cell over-current protection activates may not be foreseen in the design. A device’s response to mechanical abuse conditions, such as vibration and shock during transport, cannot be assessed in a paper design review and must be evaluated by physical testing.

Testing standards for batteries and devices containing them provide a good starting point for evaluation. Three common standards referenced for lithium-ion batteries are UN 38.3 (Transport of Dangerous Goods), UL 1642 (Standard for Safety for Lithium Batteries), and IEC 62133 (Safety Requirements for Portable Sealed Secondary Cells). These standards contain a series of electrical, mechanical, and environmental tests designed to stress the cells individually and when installed in devices. While useful as a baseline, the conditions in these tests may not reflect conditions experienced by a specific device during use. In addition, the standards do not evaluate batteries for construction quality, such as purity of materials, assembly of components, and electrochemical fitness. Cell construction quality plays a significant role in many thermal events and while guidelines for best practices exist, such as IEEE 1725 (IEEE Standard for Rechargeable Batteries), they are not mandatory pass/fail tests.

A typical evaluation might begin with a non-destructive assessment such as X-ray or computed tomography (CT) scanning. This allows for visualization of the internal components and detection of potential contaminant particles. Further analysis might include destructive techniques to verify the composition of materials and correct assembly of the cell. Visiting and auditing the cell manufacturer and device assembler presents another opportunity to assure that cells are manufactured and handled using best practices. Finally, devices should be tested under conditions that represent worst-case scenarios including foreseeable misuse. Continued engagement with manufacturing partners to maintain consistency and manufacturing quality is integral to the success of a medical device.

Future Considerations

It is impossible to design a medical device with no functional risk whatsoever. Medical device manufacturers need to be aware of the risks and employ design features or procedures to mitigate these risks such that the benefit of the medical device outweighs the risk. The U.S. Food & Drug Administration requires all medical devices have design controls and risk assessments that consider user needs and incorporate a design review process. The safety features discussed in this article should be carefully considered at both the component level and the system level when designing a medical device powered by a battery.

In the face of an adverse event due to battery failure, it is important to identify the failure mode and the root cause of the failure. A failure investigation will require comprehensive testing and review, calling for experts with backgrounds in electrical circuit design, cell chemistry and manufacturing, as well as the product development cycle of the device. While medical devices require special considerations and a high level of scrutiny, such expertise can be extended to any device with lithium-ion batteries, including consumer electronics and vehicles. With the increased pace of technological innovation, the boundary between consumer electronics and medical devices has become more ambiguous, and battery failure incidents should be approached in a multi-disciplinary manner.