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October 13, 2014


Electric Vehicles
Electronics & Systems
Mobile Devices
Nadim Maluf

I was greatly entertained to read about a newly reported discovery from Nanyang Technology University claiming a new battery that can charge to 70% in a mere 2 minutes, yet last 20 years, or about 10,000 cycles. There were recent reports in the media for similar claims from StoreDot; that battery was charged at an even faster rate of only 30 seconds. Every so often, we hear similar reports about breakthroughs in batteries, yet reality tells us otherwise.

First, let’s do some simple math. Let’s look at the required power to charge a battery in a very fast time window. If we assume a lithium-ion battery with a very modest capacity of 2,000 mAh typical of 2013-vintage smartphones, and require to charge it in 1 minute, we find that the charging power requirement is in excess of 450 Watts. In other words, we require a massive power supply, not your standard off the shelf AC wall adapter, to provide the required charging power into this mobile device. Considering that most charging systems have at best an efficiency of about 90%, this translates to nearly 45 Watts of heat dissipated into your mobile device. Let me translate that into more practical terms: Your smartphone including its battery will just vaporize if you dare put 45 Watts of heat into it!!! And we haven’t even looked into the chemistry yet.

Let’s put another arrow into this dying beast. A typical lithium ion polymer battery in a standard smartphone carries a current density in the range of 1 to 3 milliamps per square centimeter (abbreviated as mA/cm2). In other words, if one were to hypothetically slice through the electrodes while being charged, one you find that each cm2 of cross-sectional area carries about 1 to 3 mA. Why is this meaningful? Batteries and its materials have upper limits of how much current they can realistically carry.  At higher current densities, lots of bad things happen. Bad reactions accelerate; materials decompose; local heating damages the internal structure; safety concerns are enormous…etc. For a lithium ion battery including its thin electrodes and its electrolyte, the limit is about 5 milliamps for each cm2. Even in the most optimistic scenario where materials capabilities improve by 10X in the next decade, we are still looking at a best case limit of 50 mA/cm2.

So what is the current density to charge a 2,000 mAh in 1 minute?  About 200 mA/cm2! In other words, the inner materials of a typical battery will be fried almost instantaneously at such current densities. Additionally, at such currents, even the hint of an electrical resistance in the electrical path is enough to eliminate any notion of meaningful charging.

I am certain that one of my esteemed readers will suggest reducing the capacity of the battery to a few mAh instead of 2,000 mAh and thereby reduce the power and current. Sure, one can do so but a few mAh battery does not offer much useful power to begin with. Another reader may suggest making the areas larger to reduce these densities. Sure, again one can do that, but that you would eliminate the volume advantage offered by high-energy density batteries; in other words, portability and mobility will be lost.

Of course, this is not to say that the science behind these claims is bogus. There is plenty of great material research being conducted at the nano and molecular level aimed at improving the rate at which these novel materials can accept charge. But going from such basic material research to making enormous claims about the battery and its charging speed at the system level is stretching scientific credibility by a loooooong mile!

Naturally, the big question is “who cares about charging a battery in 1 minute?” Well, I cannot think of many, if any, applications that require that kind of speed, at least not yet. Charging in 15 minutes would most likely be plenty fast to most applications, and guess what, it is very possible without having to change the material paradigm of present-day batteries.

The moral of today’s post: Beware of big and utopian claims about batteries. They most likely are not true or at least are many years away from realization.

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