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Improving battery life
Dealing with the power management
problems of new technology
By Yogesh Ramadass,
Design Engineer, Texas Instruments
The wearable electronics market is seeing a tremendous growth with a variety of products ranging from smart watches to 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. Wearable medical devices,
in particular, necessitate unique power management
designs and require specific 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 like smart watches that have displays, sensors,
and always-on radios consuming a lot of power. These devices typically have a battery with a
few hundred mAh capacity onboard. 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 means constant recharges. 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,
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 worn outdoors. This would extend the
battery life by seven percent for constant outdoor use. However, when used indoors, the harvested power falls off drastically, making solar harvesting impractical. Considering the device’s
high-power consumption, 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 sensors within the device. The different smart watches available in the market today play
with the features to improve battery life. Once the features are set, the other key technique to
increase battery life is increasing the system’s power train efficiency. Given 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 a dedicated
DC/DC converter bringing with it associated losses.
Improving the efficiency of these converters provides a direct increase in the battery lifetime.