Apple’s Watch and the wearable battery life challenge

The Apple Watch exemplifies the current state of wearable technology. Unveiled last month, Apple’s first product “designed to be worn” embodies much of what’s going on in the wearable industry — from the exciting, such as useful but cool and secure functionality, personalization, appealing styles, and comfortable yet sleek form factors, to the most challenging aspect… battery life. Despite all of the advances in other areas, the Apple Watch is expected to still require daily recharging when it comes to market in early 2015.

There are three basic ways to address battery life for a given device: choose a more advanced battery technology, improve charging, or increase efficiency. Let’s have a look at each approach as it relates to the Apple Watch.

Apple has not yet told us how its forthcoming watch will be powered, or how often it will need to be re-charged.

Apple has not specified how the still evolving Apple Watch will be powered, but most likely it will use the currently leading battery technology for mobile devices, Lithium (Li)-ion batteries with cobalt-oxide electrodes. Li-ion battery technology was developed in the 1970s, with commercialization of the cobalt-oxide electrode material in 1991.

Though this battery technology dwarfs its rivals in performance, it is nearing its theoretical limits. Newer battery concepts are in development, such as Li-ion batteries with tin nano-crystal electrodes that are more effective at absorbing and releasing Li-ions than cobalt oxide electrodes, potentially doubling the energy capacity. However, these and other next-generation battery chemistries with even greater potential are a ways from being commercially available.

With a battery life of just a day or so, the Apple Watch needs a convenient charging routine to become a viable product. Inductive charging is the solution, with a small charging pad/connector that uses magnets to easily snap to the back of the Apple Watch.

Once the watch and pad adhere to each other, energy is transferred wirelessly by magnetic induction from the transmitting coil in the charging pad to the receiving coil in the watch. No matter how simplified the process is though, a daily recharging requirement really puts a dent in the user experience.

On the efficiency front, the Apple Watch has a lot of opportunities to whittle down the power use because it is loaded with power hungry features. Besides being a very accurate watch that works with the iPhone to check its timekeeping against the definitive global time standard, the device includes a color touchscreen, phone, messaging, email, Bluetooth connectivity for headphones, secure payment capability, and an array of sensors for fitness tracking.

A custom main processor is at the heart of the Apple Watch, with a design surely optimized and every operation surely scrutinized for low power consumption. Apple further elevates efficiency by employing an always-on motion co-processor to collect and keep all motion data on the device while the main processor sleeps.

After all this though, we are still talking about battery life for wearables in terms of days. Real breakthroughs will only come by taking on the battery life challenge at its most fundamental level, the IC architecture. One promising possibility is the use of subthreshold design techniques. 

Mike Salas has over 23 years of semiconductor experience. Prior to joining Ambiq Micro, he was the vice president and general manager of the microcontroller division at Silicon Labs. Before Silicon Labs, he was the founding CEO of Layer N Networks, which was acquired by Thales e-Security. He has also held senior management roles at PMC-Sierra and Texas Instruments. He holds a B.S. in Electrical Engineering from the University of Texas at Austin. This article was previously published on EETimes' Planet Analog website. A recent technical whitepaper by Ambiq Micro goes into the subject in depth. “Subthreshold Design – A Revolutionary Approach to Eliminating Power” can be found here.

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