Wearable Devices: Battery Selection & Power Consumption Management

Wearable Devices: Battery Selection & Power Consumption Management

Thoughtful battery planning is crucial to the success of a medical device that both meets the device’s technical requirements and, particularly important for a wearable device, delights the user. Choosing the appropriate battery and implementing proper power management are two of the primary challenges when designing a wearable medical device to ensure performance is met in every use case.

Battery Selection

The path taken to select the right battery requires close communication and debate between all members of the Key Tech team on topics like device features, user interactions, regulatory requirements, and system BOM cost. The critical steps to arrive at the final battery selection are:

  1. Define critical device requirements. The client and design team must define the critical device requirements before the battery selection process can proceed. How will the device be positioned on the user? Will a door be included for user access to replace the battery or must the system accommodate an internally rechargeable battery? What kind of sensors need to be integrated? What data processing must be conducted on-board, versus offline in the cloud? What wireless communication protocol makes the most sense for the amount of data being transferred? How will the device be shipped and does the transportation method prohibit certain battery chemistries?
  2. Develop system architecture and select critical components. One of the most critical computer engineering architecture selections is the processor. This processor must perform various active functions, but it also needs to support low-power modes. During these modes, the processor may still need to monitor the device state or respond to external events but must do so while drawing as little power as possible. Electrical engineers must choose from many options for sensors, actuators, and mechanisms, but the need to consider low-power modes compounds these challenges.
  3. Sketch rough device form. Industrial designers must consider who will be using the device. Form factor requirements to accommodate an adult male are very different than that for a small child. In addition, the mechanical engineering team must take into consideration enclosure materials and fabrication methods compatible with any cleaning protocols, user touchpoints, or device weight considerations to design a device that can be fabricated in a cost-effective fashion. Identifying the proper battery position in the device can be a very challenging exercise.
  4. Estimate power consumption. Milli, micro, nano, and pico amps – every coulomb matters! It is essential to define what operational and supporting circuitry needs to be enabled during active and low-power modes, which are often divided into multiple sub-categories of operation. Peak power and average power requirements are both important inputs to define as well.
  5. Select the battery. Finally, to battery selection! What chemistry is appropriate? Is a custom battery pack required to meet the form factor constraints? Does it have the required certifications? Does the battery meet the system BOM cost requirements? How does it connect to the other electronics? Does it have sufficient capacity to prevent user annoyance, or worse, failure of essential performance or loss of crucial patient data? There is no “perfect” battery but understanding the needs of a particular application is key to selecting the right battery for a given product.

Throughout this process, it is common to run into roadblocks and return to a previous step to challenge design assumptions and requirements before forging ahead to arrive at the most elegant solution to meet the device and user needs.

Power Consumption Management

The design process does not end when hardware selection is finalized. Power management must be considered as a high-level strategy at the beginning of the development of a wearable medical device. Developing an efficient processing scheme is crucial to minimizing power consumption and extending battery life. Most modern microcontrollers are designed with low-power applications in mind. The right solution will always be product-specific, but below are several important techniques to implement:

  1. Minimize high-power mode. Microcontrollers support a wide range of power modes, with each mode enabling a subset of the microcontroller’s functionality. It is important to have a deep understanding of the power options for your microcontroller when designing your software. A thoughtful software architecture will minimize the time spent in high-power modes while still meeting the product requirements.
  2. Configure microcontroller peripherals creatively. Processor sleep time and power consumption can be reduced through the creative use of microcontroller peripherals. For example, data can be collected, stored, and transmitted without waking up the processing core if peripherals are configured properly.
  3. Minimize component “on” time. Software must be designed to aggressively power down PCB components and microcontroller peripherals when they are not needed. Nuances of putting each component into a power down or sleep state must be understood and verified that they are appropriate for the application.

It is clear that when developing wearable devices, battery selection and power consumption management is key to overall device success. The engineering and design team’s challenge is to carefully balance the variables in each device to meet all of the requirements with finite power available based on the battery selected.

Reach Out!

Are you currently developing a battery-powered wearable medical device or planning to kick off a new project? Key Tech can help!  Please TalkToUs to see how we can support your product development imperatives.

Abbie Shoemaker

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