Medical devices equipped with battery packs are required to provide optimal safety and performance when in use. Whether these devices are used for monitoring, diagnostic, or surgical applications, battery packs can pose serious risks to patients and medical staff if they malfunction, overheat, or are inefficient.
Manufacturers face a major obstacle when trying to find the perfect balance between device safety and performance. Oftentimes, they seek to increase battery performance for better product reliability. However, this factor can lead to lower safety results by selecting batteries with unstable cell chemistries, higher operational temperatures, or a lack of biocompatibility.
Battery pack used in medical room setting.
When determining the appropriate level of device performance to ensure quality medical care, take into consideration the application, regulatory requirements, cell balancing, electrical protection, and thermal limitations.
Regulatory Requirements
The design and manufacturing of medical devices and battery packs fall under strict regulatory oversight. The FDA, UL, DOT, IATA, ISO, and IEC organizations provide requirements and guidelines for the safety and performance of battery cells, battery management system (BMS) units, and wiring assemblies. Safety requirements cover the commercial and residential use of medical devices, the biological safety of batteries, and quality management systems.
- IEC 60601-1: Safety and Performance Standards within Clinical Environments
- IEC 62133: Overcharging, Thermal Runaway, and Short-Circuiting Standards
- ISO 13485: Quality Control and Documentation
- UL 2054: Battery Enclosure Standards
Determining the requirements for your specific application involves evaluating industry standards for similar devices already out on the market. Then the devices and battery packs must go through the appropriate testing and certifications through specified organizations. You must also have stringent policies with your approved vendor list that may be subject to scrutiny by the FDA.
Companies may also work with manufacturers who provide several different options that adhere to guidelines established by the IEC and ISO for a smoother and cost-effective certification process.
Cell and Chemistry Selection
Medical device applications have varying power and longevity needs based on usage. However, all devices need to have stable power output, robustness, and a predictable runtime. Where the device will be used also plays a significant role in the type of cell and chemistry selected during the design phase.
For example, patients who will be using the devices in their homes may have varying voltage and current available for charging the battery pack. This factor may significantly impact the safety and operation of the device. The battery pack must charge effectively in an acceptable amount of time.
Size and portability also need to be considered. Lighter batteries may limit operability and have reduced runtimes because of lower capacity. Heavier batteries, while offering longer cycles, may become too bulky for the device to be used or stored. The number of cells added to increase capacity may also have an impact on the device's weight and available space.
When selecting a cell, consider the mechanical and electrical characteristics of the chemistry. Characteristics such as energy density, thermal management, and chemical stability should be evaluated. Common chemistries used in medical applications include the following:
- Lithium: Lithium-based chemistries, including lithium-ion and lithium-iron phosphate (LiFePO4), offer high energy density and stable power output up to 5,000 cycles. They also have lower self-discharge rates when in storage. This chemistry offers a lightweight size and can be developed with slim profiles, making it ideal for portable devices and wearables. A major downside to lithium batteries is the need for thermal management, which requires BMS units to meet regulatory compliance.
- Nickel: Nickel-based chemistries (NiCad and NiMH) provide higher energy density and long cycle life up to 3,000 cycles. They also handle fluctuating environmental temperatures well, making them more suitable for applications that may be used in extreme temperature settings. Drawbacks for these batteries in medical settings include their lower energy density and voltage limitations. Another factor is that nickel batteries may experience memory effects. Memory effect is when the cells are not fully discharged during use, as they lose their capacity and cannot reach a full recharge state.
- Silver-Zinc: Silver-zinc batteries have high capacity and high energy density at smaller sizes. The batteries are also more eco-friendly when compared to nickel-based cells. However, silver-zinc has a lower cell cycle than other cell chemistries and requires specialized charging systems.
When deciding on cell chemistry, electrical protection is critical and mandatory through regulatory organizations. Standards such as the IEC 62133, UL 1642, and ISO 13485 are considered for safety verification and pre-market access. Evaluating the FDA regulations as well as the EU Medical Device regulations allows you to determine the type of material, design, and manufacturing protocols used for the specific medical device and battery pack.
For a lithium battery pack, a BMS unit is required. The BMS should offer battery health monitoring, track performance, and collect data regarding the state of charge (SoC) and state of health (SoH) of the battery. It also should offer safety controls to handle overcharging, deep discharging, and overheating of the cells. A BMS can also be programmed with cell balancing based on the device. High-performance and critical devices gain benefits from active cell balancing to redistribute energy between cells for longer life cycles and lower operating temperatures. Lower-performance medical devices can have passive cell balancing to dissipate excess energy from cells for cost-effectiveness.
The firmware, diagnostics, and fault handling devices are placed within the BMS for the battery pack. These aspects consist of on-board microcontrollers and communication protocols. The BMS becomes the "smart hub" of the battery pack by gathering the health of the battery, comparing this data to established programmed parameters, and implementing safety controls when needed.
Summary
Heart monitors, hearing aids, and other medical devices that require portability and efficient operation rely on battery packs to provide sufficient energy. These battery packs are heavily regulated nationally and internationally to ensure they meet safety and performance standards.
Key Takeaways
- Medical battery packs must balance safety, runtime, portability, and performance while meeting strict regulatory requirements for patient and device protection.
- Standards such as IEC 60601-1, IEC 62133, ISO 13485, and UL 2054 play a major role in determining battery design, testing, documentation, and certification requirements.
- Battery chemistry selection directly impacts energy density, cycle life, thermal stability, charging behavior, and device portability, making application requirements critical during development.
- Lithium-based batteries are commonly used in portable medical devices because of their high energy density and lightweight design, but they require robust thermal management and BMS protection systems.
- A properly designed battery management system (BMS) acts as the intelligence center of the battery pack by monitoring battery health, managing cell balancing, tracking SoC and SoH, and activating safety protections when abnormal conditions occur.
