Mobile devices

Fast charging is a common feature of most modern smartphones. In a few more years, it may become a standard feature for electric vehicles too. Yet, asking a consumer how long it would take to charge their device will most likely result in a confused answer. Even tech-savvy engineers will find it more challenging to provide a consistent figure for charge times. Why is it so?

There are several parameters that impact total charge time. Some of them may be obvious. For example, charging the smartphone with a small AC adapter will make less current available to charging, so it will take longer to charge. Similarly, browsing the web while charging will divert precious electrons away from the charging process. Again, it becomes slower.

But the less obvious parameters relate to definitions. That’s right, definitions of what it means to say the battery is “full.” This post will shed some light on two such definitions.

For the purpose of this explanation, I will use the standard charging methodology called CC-CV (constant current, constant voltage). The charging current is constant until the battery reaches its maximum specified voltage (in this example, it is 4.35 V) at which point the charging circuitry reverses the order and fixes the voltage while letting the current decay to near zero. When the charging current becomes “sufficiently small,” the battery is then considered fully charged.

Therefore, our first definition relates to the meaning of “sufficiently small” and therefore, the meaning of 100% full. There is a misconception that the battery is “full” when its terminal voltage reaches a maximum specified voltage (e.g., 4.35 V). That is not correct. The battery is considered full when its charging current decays to a value below a pre-defined threshold. This threshold is called the termination current and is calculated relative to the charge capacity of the battery.

Let’s say, as an example, a battery can hold a charge capacity of 1,000 mAh. Some companies consider this battery full when its charging current decays to a value of 1,000/20 = 50 mA. This is called C/20 termination. Other companies establish a different threshold of C/10, which means the charge is considered complete when the charging current decays to a value of 1,000/5 = 200 mAh.

The figure below shows the charging current (in green) for an actual lithium-ion battery with a capacity of 3,300 mAh. The charging current displayed on the right axis remains constant for the first 49 minutes (that’s the constant current portion), and then it begins to decay (that’s the constant voltage portion). Note that the axis for the charging current is on a logarithmic scale, so the decay is exponential (not linear). The current reaches a value of C/5 (660 mA) after 68 minutes, and a value of C/20 (150 mA) 28 minutes later, or 96 minutes after the start of charge.

Hence the first observation: Set a higher threshold for the termination current to make your total charge time significantly faster. Now note that this only makes the “total” charge time faster. It does not impact the charge time to 50% or 80% of full.

Our second definition relates to the time when the smartphone lights up green and says “charge full.” In other words, this relates to what the smartphone “displays”, i.e., what it chooses to tell you as a consumer vs. the real measured figure. And here, all device OEMs are complicit. They all, virtually without exception, choose to display 100% full at an earlier time before the termination current is reached. You must be scratching your head now and saying, “isn’t that a form of lying?” Well, it is a matter of perspective. To many consumers, the difference between 95% and 100% is sufficiently small to effectively consider the device full at either value. There some truth to that. But the reality remains that virtually all smartphones will say 100% in the upper right hand corner of the displays before the charging current reaches its termination value.

To see this effect, let’s examine the figure again but now point our eyes to the blue and red curves corresponding to the axis on the left hand side. The blue curve is the true and actual charge value of the battery during the charging process (as a percentage of total charge capacity). You will notice that the blue curve hits 100% when the termination current of C/20 is reached, i.e., about 96 minutes after start of charge. In contrast, the red curve is what the smartphone actually displays on the front screen. The red curve says “100% full” much earlier, at about 65 minutes. At that moment in time, the battery is only about 95% full, but the smartphone takes a little liberty in rounding the value up to 100%.  The difference between these two values is about 30 minutes.

Same device.  Same battery. Same charging current. Yet change the definition and feel that the charging is a lot faster. As a consumer, you now know that the value the smartphone displays is not really what it measures. If you really, really want to reach the true 100% full level, then keep your device connected to the charger for another 30-ish minutes. And don’t worry, you cannot “overcharge” your battery. Your device contains all the proper circuitry and intelligence for that.

A long time ago in a galaxy far, far away

smartphones used primitive energy sources called batteries 

that users could easily replace.

Then came the Apple iPhone and made it difficult to swap out the battery.

Batteries failed too often and even caught fire. Users got upset.

But the labels on the batteries stayed the same.

Whether you browse the web searching for a teardown of your favorite smartphone, or are sufficiently skilled to take a smartphone apart, you will always find a battery, a lithium-ion battery, with a whole bunch of markings on it.  Some of them are obvious to decipher, such as the name of the manufacturer. Other label marks may be puzzling such as a dog safety mark — yes, dogs seem to occasionally savor batteries. Then there are cryptic numbers that can mean very little to the average reader. The purpose of today’s post is to shed some light on what one can glean from the label of a lithium-ion battery.

The left photograph above is for the battery used in the iPhone 7 while the right photograph is for the Samsung S8 battery. The iPhone 7 battery has fewer markings than its Samsung S8 counterpart. That is typical of Apple’s batteries. It clearly shows the Apple logo but it does not say who manufactured the cell. Rumors abound on who manufactures Apple’s batteries in Asia, but Apple does not disclose this information on their battery labels. By contrast, the Samsung label clearly states that Samsung SDI manufactured this particular cell in Korea, and assembled it with its electronics in its factory in Vietnam.

Battery labels also state some required product certification marks depending on where the smartphone is sold. Both of these cells carry the PSE mark required by the Japanese Electrical Appliance and Material Safety Law. The Samsung S8 cell also carries the European CE mark as well as the Korean KC certification mark indicating compliance with the European and Korean product safety requirements. The iPhone battery carries the UL recognized component mark for the US market (which looks like a cRUus logo). These marks usually indicate that the product conforms with certain guidelines established by a regulatory body or government, but they do not guarantee the safety of the battery. Safety remains the responsibility of the smartphone manufacturer.

Both iPhone 7 and Samsung S8 battery labels also state some important electrical characteristics, in particular the battery’s capacity and its voltage. Battery capacity is stated in two units: maximum charge capacity measured in milli-amp-hours (mAh), and maximum energy stored in the battery measured in Watt-hours (Wh). The first is a measure of electrical charge (how many ions the battery can hold). The latter measures the total amount of energy. If you recall your high-school physics, energy is electrical charge multiplied by voltage. That is the third figure that one can read on the battery label.

For the iPhone 7, the maximum charge capacity is 1,960 mAh. For the Samsung S8, it is a nominal 3,000 mAh. In terms of maximum energy stored, the iPhone 7’s figure is 7.45 Wh which pales in front of the S8’s value of 11.55 Wh. So when we say that the Samsung S8 has a bigger battery than the iPhone 7, we mean that its  capacity is larger, not that it is physically bigger.

Now we get to the tricky conversation regarding voltage. First, we notice that the iPhone 7 battery reads only one value, 3.8 V. The Samsung S8 batteries reads two values: (i) a nominal voltage of 3.85 V and (ii) a charge voltage of 4.4 V. What do they mean?

Let’s start with the easy one. The charge voltage is the maximum voltage that the battery can be used in charging the cell. The Samsung S8 cell is rated to a maximum of 4.4 V. It does not mean that the charging is at 4.4 V. It only means that it can go as high as 4.4 V. We know that Samsung derates the cell to 4.35 V instead of 4.4 V to mitigate concerns about safety.

The nominal voltage needs a lot more explaining.  For that, we will need to examine the next graph showing the battery’s voltage and its dependence on state of charge (the measure of how full it is).

When a typical lithium-ion battery is empty (at zero percent), the voltage across its two terminals is low, about 2.9 V. As the battery is charged, its voltage will rise to its maximum charge voltage.  The “average” voltage throughout this charging process is called “nominal voltage.” It turns out that if the maximum voltage is 4.4 V, the corresponding nominal voltage is 3.85 V. But if the maximum voltage is only 4.35 V, then the nominal voltage is 3.80 V. So it becomes easy to figure out that the iPhone 7 has a maximum voltage of 4.35 V even though it is not stated on its battery label.

You have now become an expert in reading battery labels. But whatever you do, always remember to stay safe and keep your battery away from metal objects.

T’is the season of new smartphone releases. The Samsung S8 is here and the drums are beating loud ahead of the much anticipated Apple iPhone 8 (or Edition, or whatever they will call it).

These devices and their makers clearly tout their performance features: faster processors, better camera, pretty displays, more memory….etc. But for this year and possibly for many years to come, the #1 feature is look and feel, otherwise known as industrial design, or just plain ID.

Industrial design includes how the device feels in the hand and eliminating or at least reducing the bezel to make the display reach out to the edges. It also includes thickness and profile, often some type of a rounded design that is comfortable in the palm. Invisible to the consumer are the havoc that these aesthetic features wreak on the battery. For example, thin smartphones mean thinner batteries; I mean really thin (less than 3 mm). Round profiles can mean non-planar batteries to maximize space utilization inside the smartphone. Are these batteries difficult and expensive to make? Absolutely. Given that the battery consumes between ½ to ⅔ of the overall space inside the smartphone, pushing the industrial design means serious business as far as the battery is concerned. Today’s post shows how your choice of a smartphone as a consumer impacts the battery and its underlying design.

First, and above all, every consumer wants his or her smartphone to last at least a full day. Now the definition of a “full day” is subjective, but there is broad consensus that it translates to a battery capacity of at least 3,000 mAh, preferably near 3,500 mAh for the top of the line smartphones. Indeed, if we examine the average capacity in smartphones over the past 5 years, we see that it has grown at about 8% annually. A battery in a 2017 smartphone contains about 40 – 50% more capacity (mAh) than it did in 2012.

The smartphones are also getting thinner, so lesser volume available for the battery. The chart below shows the thickness of iPhones (in orange) and Samsung Galaxy line (in blue) over the past few years. The trend is clear!

Capacity is increasing. Volume is decreasing. That’s more energy in a smaller volume. In other words, the energy density is rising rapidly thus creating serious headaches because of various implications to safety and quality as well as cost.

If you are a battery vendor and need to increase energy density, what can you do? First, you can pack more material inside the battery to store more of the lithium ions. Second, you can increase the voltage. If you recall from your high-school physics, electrical energy is the product of electrical charge × voltage. More voltage translates to more energy. If we look at the maximum voltage of batteries that have been shipping commercially in the past few years, we immediately notice that the voltage has risen from 4.20 V to 4.40 V for one individual cell. We even see prototypes today at 4.45 V and above. The chart below shows that going from 4.20 V to 4.40 V provides an additional 20% in energy, or the equivalent of four battery generations.

The challenge is that at these elevated cell voltages there is a heightened risk of lithium plating. Operating at 4.40 V is far from obvious or trivial. The margin of error is extremely small at these voltage levels. Manufacturing defects or design fluctuations are sufficient to cause the formation of lithium metal plating thus risking a potential battery fire.

So when you choose your next smartphone, be it a Samsung, Apple or any other brand, keep in mind how your choice as a consumer drives the OEM and in turn it drives the battery technology. The smartphone and its battery are ultimately the responsibility of the OEM, but an informed consumer will make the right and safe choice.

We, that’s all of us on this planet, buy every year 1.6 billion smartphones. It works out to one new smartphone every year for every four living human beings on this planet. Cumulatively, we own and use 4 billion smartphones around the world. Every region of the world, rich or poor, is buying smartphones. Many developing nations in the Middle East, Africa, and Asia are growing their smartphone subscriptions at a fast rate. Ericsson reports that by 2021, there will be 6.3 billion smartphone subscriptions, that’s nearly every man, woman and child around the world. Impressive!

Of course, each and every one of these smartphones has a battery in it. Your first reaction is: “that’s a lot of batteries.” Yes, that is true. Sadly, many of these batteries go to landfills after they are exhausted. The easiest way to gauge the size of the market for batteries is to calculate the entire energy supplied by all of them. Of course, that is a large number. It is measured in billions of watt-hours, abbreviated as GWh. As a reference mark, the battery in a top of the line Tesla S is 100 kWh. One GWh = 1 million kWh = 10,000 Tesla S.

In 2016, the battery factories around the world manufactured about 50 GWh worth of batteries for consumer devices. That drives an industry and a market worth in excess of $10 billions annually. Forecasts indicate that the consumer market will use about 65 to 70 GWh worth of batteries in 2020. Our appetite for more batteries is insatiable and the numbers show it.

Now let’s look at batteries in electrified vehicles, including both hybrid plug-in cars and pure electric cars (xEVs). This is a relatively new market. The Tesla S first came in 2012. The Nissan Leaf came a little earlier in 2011. Many states in the US or countries around the world haven’t yet experienced or experimented with such vehicles. In 2016, all of these vehicles accounted for a mere 0.9% of all car sales. In total, they amounted to less than 1 m vehicles in 2016.

However, in battery lingo, these cars accounted for an increasingly large number of GWh. The year 2016 was the first year that the battery capacity used in xEVs equalled that of all consumer devices, about 50 GWh. By 2020, xEVs will account for ⅔ of all battery production in the world. No wonder Elon Musk and the major car makers pay a lot of attention to their supply chain, including building these Gigafactories.

State-of-the-art lithium-ion batteries, whether used in smartphones or electric vehicles, all rely on the same fundamental cell structure: two opposing electrodes with an intermediate insulating separator layer, with lithium ions shuffling between the two electrodes.

The positive electrode during charging, usually called the cathode, consists of a multi-metal oxide alloy material. Lithium-cobalt-oxide, or LCO, is by far the most common for consumer electronic applications. NCM, short for lithium nickel-cobalt-manganese oxide, also known as NMC, is gradually replacing other materials in energy storage and electric vehicle applications. LCO and NCM have a great property of storing lithium ions within their material matrix. Think of a porous swiss cheese: the lithium ions insert themselves between the atomic layers.

In contrast, the anode, or negative electrode during charging, is almost universally made of carbon graphite. Carbon historically was and continues to be the material of choice. It has a large capacity to store lithium ions within its crystalline matrix, much like the metal oxide cathode.

So how do manufacturers increase energy density? In some respects, the math is simple. In practice, it gets tricky.

Energy density equals total energy stored divided by volume. The total stored energy is dictated by the amount of active material, i.e., the available amount of metal oxide alloy as well as graphite that can physically store the lithium ions (i.e., the electric charge). So battery manufacturers resort to all types of design tricks to reduce the volume of inactive material, for example, reducing the thickness of the separator and metal connectors. Of course, there are limits with safety topping the list. To a large extent, this is what battery manufacturers did for the past 20 years — amounting largely to about a 5% increase annually in energy density.

But once this extra volume of inactive material is reduced to its bare minimum, increasing energy density gets tricky and challenging. This is the difficult wall that the battery industry is facing now. So what is next?

There are two potential paths forward:

1.  Find a way to pack more ions (i.e., more electric charge) within the electrodes. This is the topic of much research to develop new materials capable of such feat. But any such breakthrough is still several years away from commercial deployment, leaving the second option to….

2.  Increase the voltage. Since energy equals charge multiplied by voltage, increasing the voltage also raises the amount of energy (remember that energy and charge are related but are not commutable). This is the object of today’s post.

The battery industry raised the voltage a few years back from a maximum of 4.2 V to the present-day value of 4.35 V. This was responsible for adding approximately 4 to 5% to the energy density. A new crop of batteries is now beginning to operate at 4.4 V, adding an additional 4 to 5% to the energy density. But that does not come without some serious challenges. What are they?

First, there is the electrolyte. It is a gel-like solvent that imbibes the inside of the battery. Short of a better analogy, if ions are like fish, then the electrolyte is like water. It is the medium within which the lithium ions can travel between the two electrodes. As the voltage rises, it subjects the electrolyte to increasingly higher electric fields causing its early degradation and breakdown. So we are now seeing a new generation of electrolytes that can in principle withstand the higher voltage — albeit, we see in our lab testing that some of these electrolyte formulations are responsible for worse cycle life performance. This is a first example of the compromises that battery designers are battling.

Second, there is the structural integrity of the cathode. Let’s take LCO as an example. If we peer a little closer into the cathode material (see the figure below), we find a crystal structure with layers made of cobalt and oxygen atoms. When the battery is fully discharged, the lithium ions occupy the vacant space between these ordered layers. In fact, there is a proportion of lithium ions to cobalt and oxygen atoms: there is one lithium ion for every one cobalt and two oxygen atoms.

courtesy of visualization for electronic and structural analysis (VESTA)

As the battery is charged, the lithium ions leave the cathode to the anode vacating some of the space between the ordered layers of the LCO cathode. But not all the lithium ions can leave; if too many of them leave, then the crystal structure of the cathode collapses and the material changes its properties. This is not good. So only about half of the lithium ions are “permitted” to leave during charging. This “permission” is determined by, you guessed it, the voltage. Right about 4.5 V, the LCO crystal structure begins to deteriorate, so one can easily see that at 4.4 V, the battery is already getting too close to the cliff.

Lastly, there is lithium plating. High energy-density cells push the limit of the design and tolerances in order to reduce the amount of material that is not participating in the storage. One of the unintended consequences is an “imbalance” between the amount of cathode and anode materials. This creates an “excess” of lithium ions that then deposit as lithium metal, hence plating.

These three challenges illustrate the increasing difficulties that battery manufacturing must overcome to continue pushing the limits of energy density. As they make progress, however, compromises become the norm. Cycle life is often shortened. Long gone are the days of 1,000+ cycles without intelligent adaptive controls. Fast charging becomes questionable. In some cases, safety may be in doubt. And the underlying R&D effort costs a lot of money with expenses that are stretching the financial limits of battery manufacturers without the promise of immediate financial returns in a market that is demanding performance at a the lowest possible price.

It is great to be a battery scientist with plenty of great problems to work on…but then again, may be not.