Electronics & systems

10Nov 2015

The lithium-ion battery market is large. For consumer devices, it will exceed $10B in 2015 corresponding to a total output capacity of 40 GWh. The promise of even greater markets in stationary energy storage and electric vehicles is attracting interest and investment. Goldman Sachs estimates that the energy storage market could reach a demand greater than 700 GWh in 2015, eclipsing the expected 175 GWh capacity demand for electric vehicles.

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14Aug 2015

Not yours, of course….the smartphone’s waist. We see a race among the smartphone makers to go thin. The iPhone 6 Plus is 6.9 mm thick and it is already been outflanked by some new devices coming from China. In particular, the Oppo R5 boasts a thickness of only 4.85 mm, and the Vivo X5 Max is an even thinner 4.75 mm. So what determines how thin one can go?

Naturally, the mechanics of the device are clearly one limiting factor…nobody wants their smartphone to “bend.” For the most part, manufacturers are now using hardened aluminum cases for added resistance to bending. With the exception of some early complaints about the iPhone 6 Plus, there have been no credible reports of additional bending failures. Another limiting factor is the touch screen. There have been some great innovation here, most of it related to fusing the touch glass with the display, thereby reducing the touchscreen thickness. For example, the AMOLED screen on the Vivo X5 Max is only 1.35 mm thick.

So that leaves the battery as the last frontier…why am I not surprised? The battery seems to consistently win the title of bottleneck, and this is the topic of today’s discussion. Why can’t we make batteries ultra thin?

The answer is actually “yes, we can.” Batteries can be made really thin, I mean thinner than you might imagine, sub 1 mm. But naturally, there are tradeoffs. The first tradeoff is that thinner batteries cannot boast the same energy density than their thicker counterparts — there is just too much “electrical overhead” (e.g., connectors, plates) that they become dominant when the battery is too thin. See this earlier post that shows the impact of thickness on energy density. For a smartphone device, somewhere around 3 mm is the lower limit of battery thickness. Some smartphone makers instead choose to go thick just to provide more battery capacity — the most recent example is the Moto X whose thickness is a whopping 11 mm, more than double Oppo’s thickness !!!! So the first tradeoff is battery capacity vs. stylishness. Judging from the market trends, stylishness seems to be winning for now.

There is also a second and very important tradeoff, and that relates to swelling. I described in a very early post what happens to the battery as it ages…it bloats, and consequently becomes unsafe. This “swelling” phenomenon, through which the battery physically grows, has two components. They are shown in the next chart.

This chart shows the actual and measured thickness of a 3-Ah cell used in the LG G2 smartphone. It is a polymer cell and is embedded (i.e., non-removable) inside the mobile device. The thickness is measured over 60 cycles of charging and discharging. One readily observes two separate trends, almost like a yoyo on an escalator:

  • One trend is a fast variation in thickness with a known periodicity of one cycle (this is the yoyo effect). The thickness varies by about 0.15 mm, or approximately 3% of the cell’s thickness but is a fully reversible effect. This is due to the physical expansion of the graphite anode. During charging, lithium ions intercalate (fancy language for “insert themselves”) inside the carbon-graphite material (also known as matrix) thereby pushing the carbon atoms aside and causing physical growth. During discharge, the opposite happens and the anode returns to a thinner state.
  • The second trend is a slow, semi-linear growth in thickness (this is the escalator effect). This is related to irreversible damage to the graphite anode — as the lithium ions go in and out of the anode, they leave just a tiny bit of damage that accumulates over time into this irreversible thickening of the anode (and consequently of the cell). As one can immediately observe, this second trend is significantly larger in magnitude than the first trend. For this cell made by LG Chem, the increase in thickness over 60 cycles is 0.15 mm, or 3% of the original thickness. Typically, over 500 cycles, this may reach 8 or even 10%.

As a result, manufacturers of smartphones need to make an allowance inside the device for the battery cell to grow in time — this allowance is somewhere between 10 and 15% of the cell’s thickness, or up to 0.7 mm; quite a significant number. Failing to provide this allowance risks placing large pressures on the touchscreen and cracking it.

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02Aug 2015

A statement of this devilish nature during the middle ages would have earned its author a burning at the stake. The mere notion of a “battery” would have probably been in the realm of druids and witches, and reviving anything dead would have been…well, enough said, it is the 21st century.

I will digress today a little and talk about your average primary (i.e., non-rechargeable) battery, the type that Energizer, Duracell, and their competitors sell billions of every year, all of which wind up in the trash bin or recycling centers — excepting the batteries that sit on one’s desk for ages as if somehow they will disappear on their own. How do these batteries die, and when they do, are they really dead? Let’s explore.

For the purpose of this discussion, let’s focus on Alkaline batteries. The typical ones, like AA or AAA, are nominally rated at 1.5 V. In other words, when they are fresh and unused, one would measure 1.5 V at the battery terminals. As the battery is inserted into a gadget and gets used, the voltage at the terminals drops, and fast it does. As the voltage drops, it reaches a point where it is no longer sufficient to power the electronics in the gadget. Often, these gadgets, such as toys, employ inexpensive electronics. This means that these electronics do not employ the most modern electronics circuitry. This is parlance for electronics that are not low-voltage and low-power. In other words, these inexpensive electronics draw more current than they need to, and they operate at higher voltages than they should — all in the spirit of saving costs. But these operating requirements place a bigger burden on the battery, the result of which the battery’s voltage drains rapidly and meets an early death.

It should be apparent to the reader that the end of the battery — its “death” —  is now defined as the time at which its terminal voltage is no longer able to power the electronics. This is somewhat subjective because that clearly depends on the quality and sophistication of the electronics in your gadget. Usually, many inexpensive electronics begin to stop operating somewhere between 1.2 V and 1.35 V. Very rarely, one may see electronics get lower in operating voltages but such gadgets would most likely be associated with higher price points, and could very well just use an embedded lithium-ion battery to project an image of a “good” product.

Looking at the Energizer E91 specification sheet, one immediately can observe that this battery has a life of less than 2 hours to hit 1.3 V, and 3 hours to hit 1.2 V (assuming a discharge current of 250 mA). At this point, the electronics begin to stop operating; the cheap display on your child’s toy begins to fade, and voila, you pronounce the battery dead and discard it.

But wait! Is it really true that the battery is dead? Again, it is a matter of definition. For a helpless parent trying to appease a screaming child, the battery is DEAD. But to some engineers and entrepreneurs, they will be quick to observe that this battery continues to hold a lot of charge and energy. Looking at the voltage chart above for the E91, the area under the red curve is the amount of “energy” that the battery holds. So it becomes immediately clear that if the battery is declared dead at 1.2 V, it continues to hold about 75% of its original energy. This is a lot!

So the magic question becomes how to access this extra energy well? and how to do so in a cost-effective and reliable manner? This is where I will put a plug for the company Batteroo Inc.. The team figured out an elegant solution to put a very thin reusable sleeve around the presumed dead battery with low-power electronics that will “boost and regulate” the raw terminal voltage of the battery back up to a higher voltage, say 1.5 V, sufficient now to operate a gadget. This has an effect of reviving this “dead” battery and substantially extending its life. I love clever and simple ideas! Batteroo’s challenge is now to fight off the battery vendors who will not be pleased with selling fewer batteries.

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30Jun 2015

That is 3,000 mAh….this is the battery capacity that consumers will see in most mid-tier to high-end mobile smartphones for the foreseeable future. Why? It’s simple, this is the capacity that gives consumers an honest full day of operation.

This begs a first question: what is an honest full day? no one really knows since usage varies considerably across the consumer base. But manufacturers are not able to tailor the battery to different consumer groups, therefore, an honest full day of operation ought to fulfill the demands of the largest cross section of consumers, including the spectrum from travelers to stay-home parents and teenagers who are glued to their favorite social network app. It would be fair to say that an honest full day ought to deliver at least 10 hours of talk time per day, preferably more, and at least 10 hours of screen usage time, including web browsing and app usage.

The two charts below examine talk time and web browsing time for a number of commonly available smartphones as measured by GSM Arena in their battery tests. For talk time, the relationship is immediately obvious. More battery capacity equals more talk time. Simple and easy.  Some smartphone makers are a little better than others, but overall, there is a simple relationship that says that about 3,000 mAh gives about 20 hours of 3G talk time. Now these are lab-based tests, so in real life, you would want to give yourself a little extra margin.  But I would say that 3,000 mAh is probably sufficient for most phone-talking needs, most likely lasting you several days if all you do is only using your smartphone to talk.


Now, talking on the phone does not need the screen to be turned on, but everything else, from simple messaging to browsing and app usage does. The screen is a major power hog as I explained in a previous blog. This is where the battery begins to get challenged. The next chart shows measured usage time for web browsing, a good proxy for having the display as well as the radios turned on.


The picture now gets a little more involved. Clearly a bigger battery equals more time, but also the choice of smartphone does matter. For example, Apple and Sony seem to do a better job managing the power budget than HTC and LG do. Nonetheless, the chart is also specific in saying that if you are gunning for about 10 hours or more of screen time per day on a device that has a 5-in display, then the battery capacity needs to be right around 3,000 mAh (or more).

So there you have it….anything less than 3,000 mAh will leave consumers unhappy with their battery performance. Anything much more than 3,000 mAh will leave the manufacturer with a more expensive battery that will not likely earn this manufacturer any additional sales. So it seems that 3,000 mAh will be the right figure for a little while.

Now let’s find the approximate charge times for such a battery. Such a battery has an equivalent energy of about 11.5 Wh. So a standard 5-Watt AC adapter will charge this battery at nearly 0.4C (=5/11.5) for which the charge time is an agonizing 3+ hours (see my earlier post on charge times). New AC adapters capable of charging at 12-18 Watts will accelerate the charge times. In other words,  such larger batteries will undoubtedly go hand-in-hand with fast charging…and that’s what consumers will want to see in their mobile smartphones soon: a full-day battery that can be charge in the fastest possible time. Expect such new crops of smartphones to emerge in 2016.

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18May 2015

Fact: I am able to ride a significantly longer distance on my bicycle than I can drive in my electric vehicle….it can be frustrating to see my electric vehicle run out of juice! So I began to wonder on a recent cycling trip to what extent my Ford Focus Electric vehicle was less  energy efficient than my bicycle? or perhaps was my electric vehicle not well designed and the battery appropriately sized? It was time to dig a little deeper.

Curiosity meant that I had to take the thought one step further: how would these two transportation modes compare with nature-provided bipedalism, and ultimately, with the modern gasoline-powered automobile. This is most likely a purely academic exercise in the sense that none of these transportation modes is meant to replace the other, at least not today, but is meant with the hope that we can learn from one method to improve our design and engineering methodologies.

So let’s start by measuring an estimate of energy consumption for each of these transportation modes. First, I know from the dashboard of my electric vehicle that I have averaged 235 Watt-hours per mile (Wh/mi) over the past 20,000 miles of driving.  I also know from my bicycle instrumentation that I am averaging approximately 50 kCal per mile — granted, that is at a faster pace than most casual riders but still represents a good starting number. We also know that a medium-sized gasoline-powered sedan has an average fuel economy of approximately 25 miles to the gallon (mpg). And lastly, a wide range of sports publications estimate that a running human burns about 100 kCal per mile.

Next, we need to harmonize these units for a useful comparison. I will spare you the math and give you the conversion factors. I assumed here the EPA’s equivalent figure of 33,700 Wh in each gallon of gasoline. The factors that matter are:

1 kCal/mi = 1.162 Wh/mi = 0.00003449 Gal/mi
Summarizing into one table, we get:
I took the liberty of adding electric bikes and the Tesla Model S to the mix as well as adding an approximate gross weight for each mode, noting that the weight of the bikes do not include the weight of the rider. So what are the results telling us?

First, something we already knew or suspected: An electric car is about 4 to 5x more energy efficient than a sedan with a gasoline-engine. This is primarily due to the fact that an electric motor is more than 90% efficient, whereas a gasoline engine is near the 20% mark…in other words, 80% of the energy in a gasoline engine is lost to heat, and only 20% is used for movement.  Go EVs!

My bicycle is nearly 4x more efficient that my Ford Focus Electric. That is a little odd because the human body is not a very efficient machine. So why is the bicycle powertrain more efficient that this ultra-efficient electric motor? The answer is weight! A bicycle with a rider weighs 1/20th the weight of an electric car.  The Tesla Model S is considerably heavier than my EV and consequently consumes more energy. The next time you wonder why the Toyota Prius has better fuel economy than a regular sedan, think weight. Geez, we kind of knew that, didn’t we?

But now, we start making observations that are less intuitive. An e-bike is more efficient than a bike, which in itself is more efficient than a human running, yet all three are sufficiently close in weight. The human body is mechanically not very efficient, especially when compared to the power train of a bicycle. The rolling motion of a bicycle lends itself to lesser friction than walking and running, and is thus more efficient. An e-bike replaces the rider with an electric motor that is more efficient than his or her leg muscles — though not as healthy!

So what does it all mean? First, electric-powered transportation is the way of the future — as long as we are not getting the electricity from dirty coal-fired power stations. Second, shed the weight, and that is the weight of your vehicle, and your own weight if you like to ride. Third, a banana provides a human being with about 105 kCal, or equivalently, 120 Wh of energy. That can power a human rider on a bike for 2.5 miles. No power-generator can turn a banana into sufficient electricity for any useful purpose. In other words, while electricity looks green and clean, by the time we consider its cost of generation both in dollar terms and impact on the environment, it probably cannot compete with a healthy banana. Besides, eating a banana beats all forms of fast-charging….Be safe in all your travels!

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