The Basics

22Sep 2015

Pause for a second and wonder why electric vehicles have frustratingly limited driving ranges? or why your smartphone lasts only for a few limited hours instead of an entire month? Yet, a good ol’ combustion-engine car can go for hundreds of miles without a problem. This is the manifestation of energy density. Let’s talk about it in more detail and hopefully give the reader a bit of more intuitive sense on the importance of this metric.

Energy density, as the title implies, is a measure of how much energy is stored in a certain volume. A battery or a gas tank has a certain limited volume, therefore it is important to have a metric that relates to how much energy can be stored in that volume. Obviously, energy is what powers our smartphones or vehicles, therefore energy density is a metric that describes how far one can drive  or use a device given the limited amount of energy stored in the “tank.”

The following table compares a select number of energy-storing materials or mechanisms, all the way from the traditional lead-acid battery (the type that you will find under your hood) to much more sophisticated energy sources such as nuclear fission. So what is this table telling us?

EnergyDensity3

The first nine rows are all batteries, or devices that can store electrical energy. Batteries are either primary (i.e., non-rechargeable) or secondary (fancy term for rechargeable). The last three rows are widely used energy sources in our society today and are used here for comparison purposes: Ethanol and gasoline are examples of carbon-based fuels, and the last row, well, we all know what nuclear power can do, both the good and the evil.

The first observation: Even the best battery, the absolute best, has 10X lower energy density that carbon-based fuels. That means the same tank (or equivalently sized battery) will let you drive 10X more miles using carbon-based fuels. I cheated a little here — electric systems are more efficient than carbon-fuel systems, so the difference is more like 3X rather than 10X, but that will be left to another discussion.

The second observation: The difference between the best battery and worst battery (in terms of energy density) is substantial, also about a factor of 10X. Lead-acid batteries, discovered over a 150 years ago, don’t provide a lot of energy density. NiCd batteries also leave a lot to desire. Do you remember the bulky batteries in the early cell phones back in the 1990s? Or just google the GM EV1, the first electric vehicle from GM that used lead acid batteries.

But…there is always a but: While NiCd have for the most part disappeared, lead acids are incredibly inexpensive, and they survive. Until the day comes when the price point of alternative batteries drops radically, lead acids will continue to be the king of batteries in applications where energy density is not critical — i.e., where it is ok to occupy a larger volume, for example backup systems for cell phone towers.

The third observation: Energy density increased by a factor of 10X over 150 years! That’s not terribly promising unless the future brings forth some serious breakthroughs in materials. Is there anything on the horizon? There is a lot of promising good material research, but when one takes into account cost, cycle life, and other constraints such as manufacturing and capital, it is very hard to point to one particular technology that is likely to be commercialized in the next 5 years. So the wait and the hope continue.

The fourth observation: Lithium-ion technologies, first commercialized by Sony in 1991, encompass a wide range of energy densities depending on the particular choice of material for the electrodes. Lithium-ion batteries using nickel-cobalt-aluminum oxide (NCA) electrodes — the type used in the Tesla Model S — have over 3X the energy density of lithium-ion batteries with lithium iron-phosphate  (LFP) electrodes. So why is anyone considering LFP lithium-ion batteries: Cycle life! Welcome to the world of compromises.

So by now, you are probably disappointed about the future of batteries! It is true that the progress of batteries over the last 150 years has been slow, and it is true that batteries can’t yet compete with carbon-based fuels…but that does not mean that the incremental progress in batteries is insufficient to meet many needs of our society. Yes, they can be better, but present batteries boasting 700 Wh/l can and are sufficient to provide an electric vehicle with a range of 300 miles. In other words, don’t expect miracles in batteries, but do expect that incremental technologies from materials to algorithms and electronics will be sufficient to address a wide range of energy storage needs, including smartphones that can last an honest day to electric vehicles with a range of 200 – 300 miles.

There is plenty to look forward to here, just be careful about wild claims of amazing discoveries. If there are too good to be true, then there is a probably a good reason to be skeptical.

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.

12Jun 2015

Kevin Gibb of TechInsights published recently an article in EE Times that shows a teardown of the lithium-ion battery used inside the iPhone 6 Plus. While the teardown and the article seemed motivated by determining the cost of this battery — somewhere near $4.00 — it contained some very nice cross sectional photographs taken using optical and electron microscopy of the various layers that make the iPhone 6 Plus.  Anything that carries an Apple logo seems to attract a lot of attention, but the battery inside the iPhone 6 Plus is similar in performance and structure to many other Li+ (lithium-ion) polymer batteries used in mobile devices.  For example, the battery capacity of the iPhone 6 Plus is rated at 2.915 Ah, within a rounding error of the capacity of batteries used in the Sony Xperia Z3 and Z3+, the LG G3 and G4. Let’s use this very nice teardown report of the iPhone 6 Plus battery to shed more light onto the inner structure of a lithium-ion battery and its workings especially in view of fast charging.

I described in an earlier post the various shapes of a lithium-ion battery. A 18650 cell is encased in a metallic cylinder, whereas a polymer one is a thin and flat pancake-like without any external metallic protection. Yet, the insides are nearly identical, all consisting of a set of electrodes called anodes opposing another set of electrodes called the cathodes with both sets of electrodes separated by a porous membrane called a — hold your breath — “separator.” The first picture below shows a cross section of the polymer battery inside the iPhone 6 Plus viewed through an optical microscope. For reference purposes, the iPhone 6 Plus battery is approximately 3 mm thick.

In mobile devices, the vast majority of batteries use a metal oxide called lithium-cobalt-oxide (LCO) deposited on an aluminum backplate to act as the cathode (the positive electrode during charging). You can see the bright white aluminum back layer in the photo above, but it is difficult to see the LCO layer at this magnification. The anode is nearly always made of a thin carbon graphite layer deposited on top of a copper backplate. There is a very thin separator layer that sits between each set of anode/cathode layers. During charging, the ions, yes, the lithium ions, travel from the cathode through the porous separator to the anode, and embed themselves inside the graphite. As every skilled engineer should know, charge balance means that there is an opposing current made of electrons that goes through the external circuit between the anode and the cathode. This means that maintaining a low electrical external path resistance is essential to the operation of the battery — one of the reasons why aluminum and copper conductors are used.

The photo above shows a stack of alternating layers of anodes and cathodes. There are 11 anode/cathode layer pairs, which means the pitch is approximately 275 microns. This particular construction is unique to LG Chem with a stack of parallel layers. Other battery manufacturers use what is known as a jelly-roll, with the layers of anodes and cathodes rolled together like a cigar. This mechanical structure, while seemingly immaterial to the novice, plays a big role in the distribution of electrical current inside the battery, and consequently the governing degradation mechanisms. Let’s zoom in a little more.

The second photograph shows a scanning electron micrograph (SEM) of two sets of anode/cathode layers. Now we can see the individual structural materials. The separator is typically near 10 to 20 microns in thickness. The graphite and LCO layers are often around 50 microns but can vary depending on battery capacity and current rating. This SEM now shows that the LCO layer is granular in nature. The graphite layer is granular too.  The grains, varying in size from a few to several microns in diameter, consist of crystalline layers — a lattice-like — where the lithium ions can embed themselves. In charging, they embed themselves in the graphite lattice, and in discharge, in the LCO lattice. The graphite lattice is pictured next using a transmission electron micrograph (TEM). The lattice is made of atomic layers that are a mere 0.34 nm apart — think of it as atomic Swiss cheese.

The LCO and graphite have a limited capacity of how many lithium ions they can “hold” inside their lattice. This determines the amount of LCO and graphite material that is needed for a battery of a given capacity, i.e., of a given mAh. This in turn determines the energy density. Well, sort of, because there is another kink in the design of the battery, and that is the size of the grains (both LCO and graphite) and how tightly packed they are in the electrode layers. If the grains are too tightly packed, then the lithium ions will find it difficult to travel through all the grains; in other words, the maximum current capability of the battery is impaired. So you are hopefully getting a little taste of the various compromises a battery designer needs to go through….and we haven’t even yet gotten to charging.

Now let’s talk about the headaches that come with degradation of this structure especially with fast charging. High capacity and/or faster charging means a lot of ions need to zip in and out of the anode layers — since the anode is primarily responsible for storing the ions during charging. Think of cars on a highway at peak rush hours….it’s not easy; every pothole in the road now contributes to traffic flow. For example, small perturbances in the uniformity of grains means more ions will flow into one grain vs. another, thus creating differences in current density, and excessive stress on some grains (ultimately causing mechanical fracturing of the graphite lattice and loss of capacity). Small disturbances in the voltage distribution across the layers means some portions of the stack may see a potential difference between the anode and cathode that will promote the metallic plating of lithium — a very detrimental failure mode especially present with faster charging. These are only but two examples of the degradation mechanisms. There are several more that are becoming prevalent in modern batteries with high energy density and faster charging. The task is to tame these degradation mechanisms to extract maximum performance, and that is now falling onto the next frontier of clever charging algorithms — and that is what we do at Qnovo.

Fast charging a battery clearly involves a high degree of optimization in order to manage the large flow of ions. Historically, battery vendors did it while sacrificing grain size, or packing density of grain; in other words sacrificing energy density and overall battery capacity. This compromise is no longer acceptable.

14Apr 2015

We have covered in prior blogs the operation of batteries in smartphones. The vast majority of such devices use single-cell batteries. In other words, there is one physical cell that is the battery. As such, it has a given charge capacity measured in mAh or Coulombs, and it has a voltage range that is between 3.0 and 4.35V. If we stack multiple cells in an electrical configuration, then in principle, one can obtain a multi-cell battery configuration, called a pack, that can deliver more charge capacity.

The electrical configuration of such cells defines the nomenclature – see figure below. If the cells are electrical tied in series, then the pack is called s-configuration. If they are tied in parallel, then they are in a p-configuration. The former serves to raise the maximum voltage of the pack in multiples of 4.35V, whereas the latter serves to increase the maximum current through the pack without increasing the voltage.

Now let’s examine what happens if the cells in a multi-cell pack are not identical. For example, they could be slightly different from the onset, or perhaps aged at different rates. In a parallel configuration, the voltage is always equal for both cells. Any difference in charge capacity between the cells will manifest itself as a difference in current in the two branches. In particular, this parallel configuration always guarantees that the cells do not exceed their maximum safe voltage, often 4.35V.

But a series configuration creates a different and more challenging situation. The current is shared and equal to both cells, and hence, each cell will manifest a different voltage. Let’s first examine the charging of two cells in series. If the two cells are truly identical, then they will reach their maximum capacity and their maximum voltage at the same moment. But if there is a difference in capacity between them, then the cell with a smaller capacity will reach 4.35V before the other cell does. At this point time, one cell is 100% full while the other one is not. If the charging is not disconnected immediately, one cell will certainly get overcharged and cause a hazard.

To remedy this situation, electrical circuits called cell balancing are used. In principle they are simple They add a little switch and a small resistor across each cell in series. This added circuitry provides the ability to “bleed off” additional charge from the “strong” cell, so that its voltage stays about equal to that of the weak sister. This type is called “passive balancing.” Naturally, this is not a very energy-efficient nor cost-efficient method, but at least it guarantees that the weak cell will not be overcharged. As we covered in prior blogs, lithium-ion cells, unlike lead-acid batteries, risk catching fire or exploding when they are overcharged above their maximum voltage, typically 4.35V for one individual cell.

Let’s now examine discharging two cells in series. The figure below shows the voltage vs. charge curve for two similar but slightly different cells. They are both nominally 7,000 Coulombs (or about 1,900 mAh) but in reality, one cell is 7,200 Coulombs and the other one is 6,800 Coulombs. This is about 5% difference in capacity, and can readily happen in a pack without the proper precaution.

Let’s now assume that both cells are charged to an identical voltage. For the blue cell, this will correspond to a stored electrical charge of 3,600 Coulombs, or about 100 Coulombs more than its sister cell. Let’s now start discharging the cells in series; in other words, the exact same discharge current flows through both of them for exactly the same duration of time. This means that both cells will lose the same amount of charge; for the purpose of this discussion, we assume it is 3,000 Coulombs. We notice from the figure above that the blue cell will have a terminal voltage across its cells that is higher than the red cell (the more aged cell). Any further discharge will cause the red cell to drop precipitously and cause it further degradation, effectively over discharging the cell. This is not an unsafe event but it is a phenomenon where the weak cell (the red cell) will actually degrade at a faster rate in a series configuration. This is why it is always said that a “pack is only as good as its weakest cell.” In other words, without the use of clever algorithms and balancing, the cycle life of the entire pack will be equal to the cycle life of its weakest cell.

Battery-pack manufacturers try to minimize this problem by matching the cells in a pack as much as possible. It is very common for pack manufacturers (including makers of electric vehicles) to measure the capacity of each and every cell in a pack, and matching the cells to within less than 1% in charge capacity. But as one will immediately observe, this gets very expensive especially for large packs as the yield of useable cells can be quite low.

In some extreme cases, some packs can utilize “active balancing.” This includes more sophisticated electronic circuits that will actually shuffle charge from the strong cell to the weak cell. The effect is to increase the cycle life of the pack by shoring up this weak cell and ensuring that it does not get overcharged nor over-discharged.

It is important to close here by saying that the vast majority of mobile devices use single cell configuration, and hence do not implement cell balancing. Most laptop computers and some tablets use 2S configurations (two cells in series). They often implement rudimentary passive balancing. For example, the Apple MacBook series of products often use the bq20zxx family of fuel gauges with integrated cell balancing from Texas Instruments — such consumer-grade fuel gauges can handle cell balancing for small packs up to 4 cells in series.

15Mar 2015

You are shopping for a new smartphone and you are trying to understand how long the battery will last. But you can’t seem to get a straight answer. Apple says the iPhone 6 will last up to 14 hours of talk time on 3G but you are really not going to have your iPhone glued to your ear for 14 hours. Motorola is more subtle about its claims: up to 24 hours of mixed use. Other manufacturers follow the same strategy of being vague about their claims with the operative word being “up to.”  

The reason nobody wants to commit to battery life is you, the user and consumer. We each use our mobile device differently. Some of us use the device as a phone more frequently, others use apps more intensively. Some of us turn off plenty of background services such as data refresh, whereas others want their GPS operating with as many apps that request it. This creates an infinite number of combinations of use, and hence makes the “average user” profile somewhat of an oxymoron. This blog will shed some insight onto what components and features in your mobile device are power hungry and what you can do to limit the times that these power-hungry features are allowed to access your limited battery capacity.

The most power intensive components in a smartphone are the display, the processor (or CPU), the various radio functions (and there are several radios in your smartphones), the location services, in particular the GPS system, and the memory, in particular, writing into memory. Naturally, they are not equal in their power consumption, so we will attempt to put some power figures for each of them as well as rank them in terms of their power needs.

1. Displays: The display and its associated electronics (backlight, touch screen controller, graphics processor) are by far the most power hungry component in your mobile device. Modern smartphones have some pretty impressive displays but the more pixels they pack, the more power they consume. The Galaxy S6 has a spectacular 2560 x 1440-pixel Quad HD display but I can imagine it will be a serious power hog. Naturally, Samsung will not share these power figures with the public but one can estimate from various publications and lab tests these power levels to be about 1,000 mW for a standard 5-in HD display, and rising to 1,500 mW for the quad HD screens. A battery with a capacity of 2,600 mAh or equivalently 10,000 mWh, this translates to about 6 – 8 hours of active screen time. A couple of years back, these power levels were nearly half what they are now because the screens were smaller and were at most 720p. Tidbit #1: If you must have a large screen, reduce the backlight screen intensity. Backlights can consume several hundred milliwatts.

2. Processor: A an octa-core running at 2.4 GHz all the time will most likely cause a thermal shutdown of the smartphone quite rapidly. A processor running at full steam will consume 3,000 mW at its peak — and generate a lot of heat. Fortunately, these peak events are short-lived and may be infrequent depending on your usage. But still, applications that are processor intensive will invoke that processor horsepower more often than you desire and deplete your battery. Rogue applications are clearly detrimental to battery life. On average,  iOS is more power-conscious and tries to reduce the demand on the processor. The new Android 5.1 Lollipop has gotten much smarter in this segment than its predecessors, but can still benefit from more improvement.  Tidbit #2: While the OS should in principle terminate applications not in use, keep an eye on rogue apps that continue to run in the background or when you don’t need them. Shut them down or better yet, remove them from your device.

3. Networking and radios: Your smartphone contains several radio systems. A modern device will have a LTE radio and a separate 3G radio, and possibly an older 2G radio. It will have a separate WiFi radio and a bluetooth radio, albeit these two are usually low-power, relatively speaking. These radios have power amplifiers for their transmit-receive functions. These power amplifier consume a lot of power — to amplify the signal — when the network signal (the number of bars on the top of your screen) is really low. In other words, if your signal level is low, the smartphone will compensate for that by boosting its own transmission power, hence more power consumption. How much power: what is an average power of 1,000 – 1,500 mW could double or more. Tidbit #3: Turn off unnecessary radios (WiFi, bluetooth or LTE radio if there is no LTE signal). Turn off background data refresh and do not let apps have unfettered access to the network radio (especially 3G and LTE) in the background. 

4. Location services: The location services utilize an integrated chip that includes a GPS transceiver complemented by another integrated chip with accelerometers and gyroscopes. A GPS chip will consume approximately 25 mW and the accelerometer/gyroscope will consume another 25 mW — that’s 50 mW in total. It surely is far less than the radio and screen, but in a world of limited power budgets, every mW counts. Tidbit #4: More and more apps are requesting to access locations which turns on these services and consumes power. If you don’t need them, turn these background location services off, and limit them to only the apps that are essential, such as navigation. Also, terminate the apps that use location services if you are not using them. Google Maps and other map apps are apps that like to check your location frequently. Terminate it if you are not using it.

5. Data storage: For most users, we don’t write into memory very frequently. Memory includes the flash memory in your device (those 32 or 64 GB that hold your files and music and photos), as well as the SD Card that boots your storage by a large amount. But if you are a user who loves to use the camera feature continuously, more importantly the video, then you may be in for a surprise. Each MB file consumes a peak of 400 mW of power to be written into memory. Uncompressed standard HD (1080p) video file is 3 MB per second. Assuming an optimistic 10:1 file reduction after compression, that translates to 120 mW for each second of recording. The newer 4K video format has a whopping uncompressed bitrate of 40 MB/sec. That will be a serious power hog! Tidbit #5: If you want to record video on your smartphone, reduce the resolution to the minimum you are willing to live with. You will be surprised to see how great the 720p quality looks on the screen.