The wearable electronics market is seeing a tremendous growth with a variety of products ranging from smart watches and medical patches to fitness monitors being introduced by multiple companies. The dimensional constraints of these devices give special meaning to the ever-present concerns of battery life. This article looks at power management needs of wearable medical electronics, and presents techniques to improve the time between recharges.
Consumer wearable electronics
Today’s wearable devices can be classified into two major types, based on their average power consumption. The first category includes devices such as smart watches that have displays, sensors, and always on radios that consume a lot of power. These devices typically have a battery with a few hundred mAh capacity on board. For example, the recently announced Samsung Galaxy Gear comes with a 315 mAh rechargeable battery and needs to be recharged almost daily . The high-power consumption of the display, processor, and radio makes the time between recharges quite small. Given a daily recharge rate, the device’s average current consumption can be deduced to be ~13 mA.
To extend battery life of wearable electronic devices, two things must be considered. First is to augment the energy available from the battery with harvested power from ambient light or body heat. For a 2 cm2 solar cell incorporated as part of the wearable device, it is possible to get an average of 1 mA of charge current into the battery when the device is worn outdoors. This would extend the battery life by seven percent for constant outdoor use. However, while used indoors, the harvested power falls off drastically, making use of solar harvesting impractical. Considering the device’s high-power consumption, using the energy harvested from body heat is not significant enough to cause a difference in battery life.
The second way to improve battery life is to decrease the power consumed by the display, radio and the multitude of sensors within the device. The different smart watches available in the market today play with the features they provide to improve battery life. Once the features are set, the other key knob to increase battery life is to increase the system’s power train efficiency. Given that the different load circuits within the system (such as the microcontroller, radio, sensors, analog front-ends) are driven from a voltage supply that is different from the battery voltage, each of these requires its dedicated DC/DC converter bringing with it associated losses.
Improving the efficiency of these converters provides a direct increase in the battery life time. Switch-mode inductor-based DC/DC converters are the preferred choice owing to their superior efficiency compared to linear regulators. However, because of their cost and area penalties, it is not possible to use a dedicated inductor-based switching regulator for each rail. Approaches such as multirail DC/DC converters with inductor-sharing need to be explored. Switched-capacitor DC/DC converters with fully integrated capacitors also should be considered as alternatives to linear regulators to improve efficiency and thus battery life.
Healthtech wearable electronics
The other class of wearable devices more applicable to the medical industry includes patches and fitness straps worn to monitor and report vital bodily signals. For aesthetic and convenience reasons, these patches need to be extremely thin, which limits the amount of energy storage capacity on board. In these devices the patch periodically monitors the vital signs and radios these stats to a central hub.
Since the device needs to sense and transmit information a few tens of milliseconds every few seconds, aggressive duty cycling is employed within these systems. This brings down the overall power consumed to less than 100 µW, thereby achieving longer battery life time with smaller batteries. Battery lifetime can be further improved, or in some cases extended indefinitely, by using energy harvesting. Consider the earlier example of embedding a 2 cm2 solar cell on the device. In this case, even when used indoors, the harvested power of a few tens of µW is significant enough to have a major impact on the battery life. Wearing the device outdoors brings a 50-hour improvement in battery life for every one hour the device is exposed to bright sunlight.
Even while the average power available from the harvester may be higher than the average device power, it may not be viable to completely replace the battery in certain applications. This is due to the limited amount of secondary energy storage allowable. In these types of systems, a small primary battery can be used to support the application during periods of dark time. The majority of the power is drawn from the harvester as the primary battery supplements the system during extended periods of low harvester input.
This type of a system is shown in Figure 1 where an energy management IC, such as the bq25505 , is used to extract the energy from the attached harvester and charge the secondary storage element. Only the essential pins of the IC are shown to aid the description. The IC automatically determines if there is enough energy in the rechargeable storage to power the end system from the harvested path by bringing the VB_SEC_ON pin low. During periods of low-energy harvesting input, the system shifts to the primary battery path by bringing the VB_PRI_ON signal low. The IC’s ability to autonomously transition between the primary battery and the harvesting source enables a smooth operation of the wearable device with enhanced battery life time.
A solar or thermal harvesting element can be connected to the input of the IC to charge the secondary storage, as long as the voltage output by the harvester is greater than 120 mV. The system constraints and the availability of harvested power dictate the sizes of the primary and secondary storage. Using this scheme allows the user to have a tiny primary battery, a small solar cell, and reasonable secondary storage capacity (using a supercapacitor or thin-film rechargeable storage) to meet the energy needs of the wearable device. In certain applications where the harvested power is available for extended duration, the secondary storage can be made quite small and the primary battery can be eliminated altogether. The flexibility of the architecture shown in Figure 1 allows for a compact solution with extended lifetime of the wearable patch or fitness monitor.
Download a datasheet for the bq25505: www.ti.com/bq25505-ca.
About the Author
Yogesh Ramadass is an analog/mixed signal circuit design engineer at Texas Instruments. He is currently involved in the design of high voltage drivers and controllers for GaN FETs. Previously Yogesh worked in the Battery Charge Management group designing TI’s next generation energy harvesting chips, wired chargers, and low-power DC/DC converters. He received his Master of Science and Ph.D. degrees in electrical engineering from the Massachusetts Institute of Technology. You can reach Yogesh at: firstname.lastname@example.org.