Safety and reliability are the key concerns when determining the right power source for a medical device. Lithium-ion (Li-ion) batteries are often considered for their higher energy density, lighter weight, longer cycle life, superior capacity retention, and ability to withstand a broad range of ambient temperatures. However, with an increasing number of potentially dangerous incidents — including fires and explosions — from Li-ion batteries, quality assurance is more important than ever for error and risk prevention, particularly for medical devices that use these power sources. Having a holistic quality approach across the battery product lifecycle is essential for achieving safe, high-quality, and environmentally friendly products.
Quality and your safety are the main requirements of Li-ion battery packs. For the development and manufacture of Li-ion battery packs, many factors must be considered from a quality assurance perspective in order to ensure basic requirements.
Battery pack definition. Quality assurance should already be a part of the creation and definition of the requirements proposal for the Li-ion battery pack to be developed. Therefore, the markets, customer needs, and application requirements must be considered. All applicable standards and regulations must also be included in the proposal. In addition, it is important to consider future requirements, recognizing which standards and regulations might change or be added during the development.
Supplier/manufacturer’s qualifications. The qualification of suppliers and manufacturers runs parallel to the development process. The selection and qualification is driven by the standard requirements and the previously defined requirements document. At this stage, fundamental quality management standards, as described in ISO 9001, ISO 13485, TS 16949, and ISO 14001, must be adhered to by suppliers and manufacturers.
The correct choice of suppliers to battery manufacturers and OEMs is crucial. Good supplier management includes supplier selection, supplier qualification, supplier development, and supplier assessment. This starts with an appropriate supplier selection process. Once a supplier has been chosen, it is necessary to assess, develop, and improve this supplier over time according to the requirements (see Figure 1).
For components and assemblies that do not conform to a standard, the manufacturing processes must be viewed in more detail and, where required, these processes must be validated and/or the results must be verified. Outsourced processes are also included in this assessment. The flowchart in Figure 2 provides the steps to follow and questions to ask to help identify when processes need to be validated.
The statistical process control, which is the recording of process key figures and the actions for correction and improvement derived from this work, should be used as an aid for controlling the manufacturing process.
Product development is based on the requirements catalog and runs parallel to the supplier development process noted above. Based on the requirements catalog, the battery is developed and tested and then verified by an internal department or external independent body. The requirements catalog can include everything from essential operational requirements and related functional requirements to features and benefits of the device. This qualification approach will determine whether the battery has met the criteria according to the V-model, a verification and validation model that reviews the implementation process over time from project definition through test and integration.
The battery is subjected to different tests and testing standards (e.g., electrical safety, temperature, shock, and vibration tests) in various environmental conditions (humidity, temperature, pressure). Charging and discharging cycles are performed with various current flows. The battery is then checked in the overall application system, making sure it meets all requirements.
Temperature and the value of charging/discharging currents has shown to be a special stress factor for the lifecycle and charge cycles of Li-ion batteries. Each battery carries its own risk, which must be assessed. There is a risk on the battery-cell level, and there is also the risk that the Li-ion battery poses in the medical device application. The use of process failure modes, effects, and criticality analysis (P-FMECA) is a common method of risk assessment, risk analysis, and risk/error. This method can be used to determine the cell-level risks as well as those that may be encountered once integrated into the medical device.
The materials used for the batteries must also be qualified during development. The battery supplier or an independent test laboratory must provide proof that the materials meet the requirements. The requirements catalog and the FMECA determine what those requirements must be.
Furthermore, the material flow — the flow and stock of materials between processes through the system — is also subject to quality assurance. Material flow analysis software is an example of one tool that has proven suitable for this task, which starts with the qualification of the material by means of initial sample inspection reports within the framework of the battery development.
Manufacturing. In the battery manufacturing process, the incoming material is subjected to an incoming materials test. It may also have been inspected at the manufacturer when it was shipped. For an incoming materials test, random samples are taken according to the defined acceptance quality limit (AQL) and tested according to predetermined test specifications.
In battery manufacture, a relevant quality control (QC) inspection is performed after each production step. It is important to distinguish between 100 percent QC testing and random sample testing, which consists of in-process quality control steps (IPQC). For IPQC steps, random sample quantities are tested at regular predetermined intervals, defined by quantity or time, in order to monitor the production process.
All assembled battery packs should undergo a 100 percent materials outgoing control (OQC). This multistep testing method provides for different quality gates through which a Li-ion battery pack must pass, thus ensuring the manufacturing and product quality.
Traceability. Each individual battery cell has its own serial number, which, in combination with the serial number of the assembled printed circuit board (PCB) in the memory chip of the pack, can also be linked to an ID number on the outside of the pack. This linking of the different numbers creates a unique device identifier (UDI). Additionally, the date code must be printed on the battery pack label. This marking allows for the identification of the assembly groups used.