When designing a battery-operated medical device, selecting the battery cannot be postponed. The industrial design, human factors and mechanical engineering team members will be blocked from progressing with their designs until they know the size of the batteries and how they’ll be integrated into the device.
Being forced to select a battery so early in the design process can be uncomfortable because of all the ambiguity and unknowns. It’s important to quickly distill the product requirements and use cases to enable high-level decisions about electronic architecture and component selection that will inform the battery analysis.
Being forced to select a battery so early in the design process can be uncomfortable
USE CASE INFORMATION
To get started, the following use case information is needed:
- Does the device operate continuously or intermittently? If the device operates intermittently, how many times is it used per day/month?
- How long does the device need to operate (minutes/hours/days)?
- What are the worst-case use case assumptions that would impact battery life? (e.g. extended low-power idle before start of operation, operation interruption/restarts, max number of retries (if fault detected)
- Is the device intended to be disposable or durable? If the device is durable, how many months/years does it need to last?
- Does the device need to be sealed to withstand washing and immersion? If the device isn’t sealed, can field-replacement (or service center replacement) of the batteries be considered a design option?
ELECTRONIC ARCHITECTURE
Next create an electronic block architecture that meets the product requirements and begin making component selections for such items as microcontrollers, wireless radio modules, sensors and power regulators (linear, switching). Be sure to consider what operating voltages are needed and account for any buck or boost regulators needed for voltage conversion and their efficiencies. In a typical design there will be a few key components that will dominate power consumption, but on very low power designs don’t forget to account for the energy required to keep an LED turned on continuously or the stealth energy consumed by pull-up and pull-down resistors. Sum up the currents for each component considering both average and peak values. The peak current requirements are important for making some go/no-go decisions on battery types.
BATTERY OPTIONS
Combine the use case information with the total currents calculated for the electronic architecture to calculate the required battery energy in milliampere-hours (mAh) and peak battery current for the different use case operating modes. Use this information to create a list of possible batteries to evaluate. Keep an open mind to the wide variety of potential battery solutions instead on focusing on only one solution. Some possible battery chemistry types to consider include:
- Primary (Non-Rechargeable): Alkaline, Lithium Manganese Dioxide, Silver Oxide
- Secondary (Rechargeable): Lithium-Ion, Lithium-Polymer, Lithium Iron Phosphate (LiFePO4), Pre-Charged Nickel-Metal Hydride (NiMH)
For each battery chemistry, there will be different cell voltages and available cells sizes each with its own cell capacity (mAh). Evaluate cells in series to achieve a higher battery voltage to make trade offs between required boost regulators and additional cells. Also evaluate cells in parallel to provide higher discharge currents and to sum battery cell capacity to meet the required energy for the application. When hand assembling cells in parallel, careful attention must be paid to balancing the open-circuit voltages and matching cell impedances before assembling. Cells that are significantly out-of-balance can incur excessive and damaging currents when they are assembled in parallel. Battery pack designs needing both series and parallel cell arrangement are best left to custom battery vendors because of the complexities of assembly, safety, monitoring, cell balancing and agency approvals.
Common gotchas to watch out for are high internal resistance of certain batteries that limits their peak current. Rechargeable lithium and nickel-based cells used in medical devices must have IEC 62133 certification for the device to pass IEC 60601-1 safety testing, Selecting secondary cells that already have IEC 62133 certification simplifies compliance and avoids the costly ($10K- $20K) and time consuming (3-6 month) process of self-certifying the battery design.
For each battery option make a comparison of cell size, cost in quantity, availability, lead time, number of cells in series or parallel, initial charge and final discharge voltages, peak current, self-discharge rate, replacement schedule (primary cells), charge/discharge cycle life (secondary cells), energy capacity design margin, potential safety issues, safety certifications, shipping restrictions, and risk of mixing cells during field replacement (if applicable).
Create a list of pros and cons for each battery and eliminate those with issues such as not enough energy capacity design margin, too large, no safety certifications, too short a lifespan. Present the recommended remaining viable batteries to the team for review. When there’s more than one acceptable battery solution, it’s important to collaborate with the whole team on 3D space studies of the design concepts incorporating each battery solution to determine which will be most satisfactory for all the stakeholders.
Tensentric is a team of highly experienced engineers developing a wide range of medical devices and in vitro diagnostic systems. Tensentric has completed over 300 development projects for clients in the medical device and IVD space since the company’s inception in 2009 and is ISO 13485:2016 certified for design and manufacturing.