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October 17, 2014


Electronics & Systems
Mobile Devices
The Basics
Nadim Maluf

You own the newest smartphone model; you love its screen; it has a very fast processor; you are a power user and you are glued to your device….but you are unhappy with the battery. You ask “Why can’t I get a battery with larger capacity?”, but no one seems to listen or give you a satisfactory answer.

The simple reason is embedded in two words: Energy Density. It’s not large enough and it’s not growing fast enough.

What is energy density? As the name implies, it is the amount of electrical energy, measured in Watt-hours, contained or stored in a given volume, usually in liters. The most common unit of energy for batteries is Watt-hours per liter, abbreviated as Wh/l. If you don’t like this unit, then you can convert it to other units such as Joules per cubic meter or even btu per pony if you so desire!

Why does energy density matter? If you desire a larger battery capacity without growing the size of your smartphone, then you need to pack more energy into the same battery dimensions, which means your energy density should grow.

A state-of-the-art lithium-ion battery has an energy density of approximately 600 Wh/l; that’s the energy density of a typical 3,000 mAh battery in your smartphone. To grow the battery capacity to 4,000 mAh without getting a physically larger smartphone, the energy density will need to grow to 800 Wh/l. That’s where the problems begin to pile.

Historically, energy density in lithium-ion batteries has grown very little. From 1995 to 2014, energy density grew at a slow annual pace of about 6%. This is not unusual. Innovations in new materials require time and a lot of capital. Yet, the performance of a smartphone and the ensuing insatiable demand for more battery has grown at a far faster pace. For example, an iPhone 6 introduced in 2014 is about 50X faster than the first iPhone introduced in 2007, yet energy density has grown by only 1.3X over that same period.

Energy density has increased by a mere 1.3X since the introduction of the first iPhone in 2007.

So, when should we expect to reach an energy density capable of delivering 4,000 mAh in a standard 5-in smartphone ? At the rate we have been going, not before 2020 !

But the situation may be far more dire. You see, the present material system of lithium-ion batteries uses carbon-based electrodes. This system has already reached its energy density limit, somewhere near 600 to 650 Wh/l. There are promising new materials that can potentially provide fuel for future growth, but the development and manufacturing challenges remain quite significant. Any delay in the development of these new materials could further delay the potential introduction of batteries with high capacities.

Until then, learn to live with capacities near 3,000 mAh, or use a physically larger mobile device that is able to sport a physically larger battery. Neither is an attractive solution…so now you understand why the battery is headed to become the #1 consumer problem in mobile devices.

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