The Importance of Battery Technology in Wearables

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Even though low-power design is resulting in ever more capable microcontrollers consuming less and less power, higher resolution screens in larger sizes and “always-on” applications such as GNSS keep raising the bar, especially on wearable devices with their limited battery capacity. Sadly, there’s no Moore’s Law for batteries: the annual rate of improvement in battery capacity is only around 6 to 8 percent, so longer battery life & shorter charging time have long been at the top of user wish lists. Current batteries are also bulky, slow to charge and prone to overheating, but several new developments are aiming to, ahem,  “jump-start” this lamentable state of affairs. Sorry.

Battery chemistry based around lithium has gradually become the leading choice in rechargeable since its commercial introduction in 1991. There are a number of reasons for its supremacy: lithium is highly electropositive, resulting in a higher cell voltage (3.6 V vs.1.2 – 1.5 V for Ni-based batteries); it’s the lightest metal, giving a high energy density (1470 Wh/kg) and high capacity (3.82 Ah/g); it’s relatively easy to manufacture; and it can be recharged thousands of times. Over the years, various pretenders to the throne have come and gone, but promising results in the lab are just the beginning; commercial products must also operate under a wide range of environmental conditions, be economic to manufacture, minimize the use of scarce natural resources, and be easy to dispose of. Oh, and preferably not burst into flames when punctured or overheated, as has happened to lithium batteries on several well-publicized occasions.  Solid-state batteries solve this problem by replacing the flammable liquid electrolyte of traditional lithium batteries with a solid material of high ionic conductivity; they’re particularly suited for wearables because they can be made to fit into curved watch faces or flexible wristbands. A battery is made by layering a high capacity cathode, a lithium metal anode and a solid electrolyte. Gaps in the electrolyte can lead to short circuits; this has been a barrier to high-volume production, but major equipment manufacturers such as Applied Materials are now shipping equipment that they claim solves the problem.

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