A look inside the world of batteries

16Aug 2017

School started this week for most of us so it is time to resume the posts. Today’s post continues with insight into the subtleties of the lithium-ion battery. It is surprising how a simple device, with only two contacts, can be so intriguing and complex.

As summer nears to an end, several smartphone makers ready their newest and greatest devices for launch. Samsung announces their Note 8 on 23 August. LG is announcing their flagship V30 a week later. And we are not forgetting Apple as they ready their newest iPhones in September.

All of these new devices will come with amazingly beautiful and large displays, top-of-the-line processors and of course, batteries to power them. At an expected price point in excess of $700, consumers are keeping their smartphones for two or even three years. So will their batteries last that long?

We will examine here one of the parameters that impact the longevity of the battery…and give you some tidbits on what you can do to keep your battery fresh for longer than average. Today’s post is on voltage. Voltage is the alt-nature to state-of-charge (SOC). This is very much the principle of operation of the fuel gauge — how you get to read at the top of your screen the percentage of remaining battery life.

When I say voltage, I mean the maximum voltage that the battery will see. It also determines the maximum available capacity in mAh. Look at the label of a battery and you will observe a maximum voltage during charge and maximum capacity for that battery. Most state-of-the-art batteries operate at a maximum voltage around 4.35 V or 4.4 V. This is also the voltage that corresponds to a 100% battery reading.

If you choose to charge your smartphone to a lesser percentage, say to only 90%, then the battery voltage stops at a lower value. For a battery that is rated 4.35 V, 100% corresponds to 4.35 V. At 95%, the voltage is 4.30 V. And at 90%, the voltage is 4.25 V. These are small differences in voltage values, but significant differences in capacity.

Let’s take a particular example with a battery having a maximum capacity of 3,100 mAh at 4.35 V. Therefore, at 4.25 V, the maximum available capacity becomes a little over 2,800 mAh.

You are now wondering: why would anyone want to do that?

The answer is: Battery longevity. If you don’t have the best battery, or your smartphone manufacturer is not putting the best battery management intelligence on your device, then you ought to be very concerned whether your battery will last you more than one year. Battery issues after 6 months or one year are a significant cause for warranty returns.

Let’s back it up with some measured data.

The following chart shows the maximum available capacity for a battery rated at 3,100 mAh at 4.35 V. At this voltage, this battery will only last about 400 cycles, or about a year. You will complain about the loss of use much before that.  The brown line shows that your battery has lost 250 mAh of capacity after 6 months….that’s about 2 hours of use time. Ouch!

 

Now, let’s look at the case where the smartphone is charged to only 95%. That is a maximum available capacity of 3,000 mAh instead of 3,100 mAh. Now follow the dark green curve in the chart. It fades at a much slower rate than the brown line. In fact, it crosses over the brown line at about 300 cycles, or about 10 months. In other words, after 10 months, it offers more capacity. This illustrates the tradeoff between voltage and longevity.

A smartphone maker who has implemented advanced intelligence on their battery (like Qnovo’s) will not suffer from this ailment. But if you suspect that your device does not have such intelligence, then you will do yourself a big favor by charging your battery to a maximum of 95% or even lower if you can.

23Jun 2017

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.

04May 2017

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.

21Apr 2017

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.

Capacity

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.

volts

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.

31Mar 2017

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.

 

Screen Shot 2017-03-31 at 9.48.15 AM

 

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.

 

GWh

 

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.