Monday, December 22, 2014

45. LITHIUM IS GOLD                                                                        


You have got to be saying, "Wait!", this is non sense. That perennial Periodic Table that we all took in high-school chemistry said that Lithium had an atomic number of 3, yes three, and Gold had an atomic number of 79. Gold is 28 times heavier than Lithium...are we preaching alchemy?

First, let me just say that this not a metaphor for lithium bringing financial rewards. Lithium is fairly abundant in the Earth's crust in the form of lithium salt deposits, and second, I have yet to see a battery manufacturer that is reaping huge financial profits. Financial margins in battery manufacturing are dismally challenged, and the financial markets have not rewarded such performers. Dare I say A123, an old and no-more darling of Wall Street?

This is much simpler. You see, graphite turns into a pretty golden color when lithium diffuses and intercalates inside its carbon matrix. Think of the graphite as swiss cheese. Naturally, this is not evident to most of us since the battery is actually sealed and we don't see what happens inside. But should you develop a transparent battery, then its colors shine. This is precisely what researchers from Michigan did, and while they were at it, they took some nice photographs of the graphite as it turned from black to gold.

Source: S. J. Harris et al., Chemical Physics Letters, 2010 (pages 265 - 274)
The first photograph in the sequence on the far left shows half of a small battery as we look through a transparent window. The battery is discharged, in other words, the graphite is empty of lithium ions. As the battery is slowly charged, lithium ions move from the cathode, the other electrode, with the direction of motion upwards in the screen. The lithium ions are physically intercalating themselves inside the carbon matrix. In the process, the graphite electrode swells to accommodate the physical presence of these lithium ions. This and the chemical bonds that the lithium forms with the carbon change the nature of the material, and consequently its optical properties. The graphite slowly turns from black to reddish and ultimately to gold color. Adding more lithium ions to the carbon ultimately results in the lithium depositing on the surface of the carbon as metallic lithium which has a silver-like appearance.

Now, take a deep look at the third photograph from the left. The bottom of the graphite electrode is red, indicating that it is beginning to fill up with lithium ions, yet the top of the electrode is still black.   This is called diffusion: Put a little red dye in a glass of water and see how it slowly diffuses into the surrounding water. The diffusion of lithium ions creates a steep gradient across the thin electrode (only 0.8 mm thick). Consequently, this puts enormous mechanical stresses across the graphite and is one of several causes of battery failure. This gradient, and consequent battery degradation, is the result of how you charge the battery, and in particular CCCV and its variants such as step charging.  You see, CCCV has no idea what may be the diffusion characteristics of the lithium ions. Charging at faster rates only makes this situation worse unless more clever charging algorithms are incorporated to mitigate this and other degradation effects.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Saturday, December 20, 2014

44. YOUR BATTERY IS FINE, YOUR APPS ARE NOT!                               


It is nearly guaranteed that every consumer has looked at least once at their mobile device in total bewilderment, wondering how their battery came to be depleted. If it happens more than once, they begin to suspect that they battery is defective or has gone bad. In reality, before suspecting that the battery is defective, they should inspect their power usage, examine whether an app has gone rogue and is responsible for the battery drain. Naturally if you battery is old, say one or two years, then you should suspect that the battery may need replacement. So let's do a couple of debugging checks.

1. Has your battery gone bad? 

A few Android phones are capable of reporting the capacity of the battery. For example, dialing the following sequence *#*#7378423#*#* on your Sony Xperia Z device gives you access to a host of tests including reading the actual battery capacity in mAh. But if your mobile device does not have this feature, then one simple trick is to measure the charge time of your mobile device and compare it to the charge time when it was new. Yes, I know, it is not ideal but it gives a good indication of lost capacity. Make sure your screen is off, all of your apps are shut down, and you use the same AC adapter rating. If your measured charge time is 20% shorter than what you measured for a new phone, you can suspect a bad battery.

2. Do you have a rogue app?

Unfortunately, some apps especially in the Android universe are improperly designed and can be suspect of excessive power usage, and consequently faster battery drain. Additionally, the consumer behavior has a big impact on battery drain. If your app is using the radio or GPS (location services) continuously, your battery will drain fast. The good news is you can now detect this in both Android as well as iOS 8.1.

In Android, go to Settings>Battery. In iOS, go to Settings>General>Usage>Battery Usage. You will see screens like the ones shown below.


Both screens will list the approximate usage pattern as well as the apps or processes that are consuming power. I say approximate because each operating system will add its own interpretation on top of inexact measured battery numbers, but the screens do offer a reasonable guidance on what is draining your battery charge. In the case of Android, you will notice that "Screen" seems to top the charts. That's because it does take a lot of power to maintain the lighting of the LCD screen. Scroll through your list and identify what is potentially a rogue app. Then either delete it and reinstall it, or if you can live without it, then do so. In some cases, your rogue app may be operating constantly in the background, so be sure to turn off background services.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Wednesday, December 17, 2014

43. ARE LITHIUM ION BATTERIES BECOMING UNSAFE?                             


I just returned from travel in China. The Chinese airport authorities take very seriously the transport of lithium ion batteries on board of commercial airliners. If a passenger is carrying an unknown or unlabeled or improperly marked lithium ion battery in any form, the authorities will confiscate the battery. I saw a disposal bin past the security check point at Beijing Airport that was full of confiscated battery packs.

Why are the authorities so seriously concerned about the safety of lithium ion batteries? I am not suggesting that all lithium ion batteries are unsafe but under some conditions, from both perspectives of battery design and battery operation, a lithium ion battery can become a fire hazard. That's the topic of today's blog.

1. Can the design of the lithium ion battery make it inherently unsafe?


Absolutely! There are countless stories of battery factories that have caught fire in the past decades. The fundamental reason is that lithium metal (not as ions, but as lithium metal in the form of Li2) is highly flammable in the presence of oxygen or water vapor, both abundantly present in air. Therefore, it comes down to assessing whether the design of the battery can allow the formation of lithium metal inside the battery. Unfortunately, as energy density increases, battery manufacturers are forced to pack more material into the electrodes and compress the battery into smaller volumes. One of  unintended consequences of this trend is increased risk of lithium plating. 

For readers who are technically inclined, lithium plating occurs when the voltage of the carbon anode relative to a fictitious lithium reference electrode approaches zero. I explained in an earlier blog the potential contribution of each electrode. Let's re-examine this graph once more. The voltage of the anode is shown in red. Lithium plating happens to the right side of the chart when the graphite is getting filled with lithium ions. Inherently robust designs adjust the geometry of the cathode relative to the anode so that full battery capacity never coincides with an x = 1.0. In other words, the battery is full of charge (i.e., 100% of charge) but the graphite anode is actually at x < 1.0, thereby ensuring that the lithium plating threshold is never reached. The trick, from a battery design standpoint, is to also not sacrifice energy density. This dilemma, avoiding lithium plating vs. increasing energy density, is where battery designs tend to trip and become sensitive to lithium plating.


2. Can one operate the battery unsafely and cause the battery to catch fire?


Absolutely! Even a well-designed battery, in other words, one that is designed to be safe within some given parameters, can be operated in an unsafe manner. Three examples of bad operation come to mind:

i) Charging the battery to voltages above its rated maximum, often 4.35 V: When this happens, the cathode voltage increases above 4.35V and the anode voltage drops below zero, thereby causing lithium plating.
ii) Charging at high charge rates using CCCV or some derivative of CCCV: The high charging current, if not applied properly with the right control algorithms, can also cause the anode voltage to dip below zero and result in lithium plating.
iii) Charging at low temperatures: As the battery temperature drops below 10 °C, the electrolyte becomes viscous, think of gummed up, and consequently, the ions have difficulty in making their journey from the cathode to the anode. This also creates the conditions necessary for lithium plating. 

Fortunately, modern battery protection systems are there to ensure that these unsafe operations are not allowed -- that is if they are well-designed; hence I suspect the origin of the caution by the Chinese airport authorities.

Lastly, one might ask: If the lithium plating happens inside the sealed battery and is never exposed to air, why is it a hazard? The answer is quite simple. Lithium metal plating will grow in time as the battery is used. Once this metal deposit or dendrite grows sufficiently long, it will form an electrical short between the anode and the cathode....and boom, catastrophic failure ensues.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Monday, December 15, 2014

42.  WHY STEP CHARGING IS MISLEADING                                              


You are now asking, what is "step charging?" The standard charging methodology for lithium ion batteries has been something called CCCV, which stands for constant-current-constant-voltage. As the name implies, the charging starts with a phase of constant charging current into the battery, until the battery terminal voltage reaches 4.35 V, at which point the charging circuitry switch to maintaining a constant voltage at the battery terminals. This ensures that no excessive and unsafe voltages are applied. As I indicated in an earlier blog, CCCV has proven to be a major source of headaches for the battery. Over the past 20 years, a variation on CCCV called step charging, continues to come in and out of consideration.  The figure below illustrates how step charging introduces an additional charging step where the constant current value is reduced so that the charging is gentle on the battery.



But step charging is very misleading and does not solve the problem in fast charging. Instead, it forces the manufacturers to create more compromises that are covered up with marketing gimmicks. Here are a couple of reasons.

1. IT REALLY IS NOT FAST CHARGING:
 Step charging includes an initial phase of increased charging current, then drops into a second phase of much reduced charging current to be gentle on the battery. Therefore the fast charging is only limited to the first phase of charging. But this phase tends to be relatively short, typically taking the battery to only about 30 or 40% of charge. The overall charge time for the battery remains very long.  In other words, once the battery reaches 30 or 40%, the charge time slows down and your anxiety takes off. In fact, the overall charge time for step charging ends up being worse than that of CCCV. And users notice this very quickly. Browsing the comment sections on the Moto X (one of very few phones that use step charging) illustrate how users notice that the fast charging is really limited. Considering that very few consumers will wait to charge when the battery is below 20%, the benefit of step charging becomes very small.

2. CHARGE TIME GETS WORSE WITH USE:
One of the hidden secrets of step charging is that its charge time grows with battery age. If we look in greater detail at the current profile of step charging in the figure above, the duration of "fast charging" is directly related to the duration of the first constant current phase, i.e., when the charging current is high. The onset of the "step" is when the current drops in value, at which point the charging really slows down. Well, the hidden secret is that as the battery ages, the onset of this step moves to the left. In other words, the duration of the fast charging phase shrinks as the battery ages. So instead of reaching 40%, now this step happens near 20%. So it is a double whammy: as the battery ages and its amount of charge dwindles, and the user loses battery life, it now takes far longer to charge the battery. How much longer? The figure below shows after 500 cycles, the charge time from 0 to 50% of the battery, CCCV and Qnovo charging is about 32 minutes, whereas step charging has grown to 50 minutes! This is not acceptable!


So how can you tell whether your mobile device is using step charging. For now, the only devices on the North American market that are using step charging are made by Motorola. In particular, the Moto X makes an additional and very serious compromise: it reduces the capacity of the battery in order to use step charging. The Moto X has a battery capacity of 2,300 mAh, greatly insufficient for a full day of use, especially for this 5.2-in giant.  In other words, step charging creates more reasons for more compromising. This is not innovation!

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Saturday, December 6, 2014

41. LCO, LFP, NMC...CRYPTIC LIVES OF THE CATHODE                              


What flavors does the lithium ion battery come with? The answer is: quite a few. They represent the various combinations of materials that are used for the two electrodes, the anode and the cathode.

It turns out the available choices for the anode are quite limited. Different forms of carbon, in particular graphite, are the common choice in all commercial applications. Silicon and silicon-carbon composites as well as tin are candidates for future anode materials but they are not presently in wide commercial use.

But the cathode has enjoyed a longer list of candidate materials, typically known by their acronyms.  All of these materials are lithium metal composite alloys, with many them using heavy metals such as cobalt and nickel.Each material seems to fit a particular application. Lithium-cobalt-oxide (LCO) is most commonly used in applications where high energy density and high capacity are needed, in particular mobile devices. Lithium-iron-phosphate (LFP) is of particular interest to the automotive industry especially in China. Its safety, long cycle life and low cost make it attractive to electric vehicles. Nickel and manganese composites have been rather limited in their utilization primarily due to cost considerations. For example, the Tesla Model S uses 18650 cells made by Panasonic with nickel-cobalt-aluminum (NCA) alloy for the cathode material.



Another salient difference between these materials is their open circuit voltage; in other words, how their terminal voltage varies with the amount of charge stored in the cell (the state of charge or SOC). The graph below shows that dependence during charging. As more charge is added to the battery, its terminal voltage rises.


One will quickly notice that LFP has the lowest average voltage, near 3.2V, considerably lower than the voltage for NMC or LCO, both hovering near 3.7V. In fact, modern cells made with LCO have a maximum voltage of 4.35V, up from 4.2V, thus raising the average voltage to 3.8V. This higher voltage, and consequently higher energy, makes LCO an attractive material for use in mobile devices.

Additionally, the voltage behavior near empty (below 15% SOC) plays a big role in the utility of the material. LCO can sustain a useful voltage above 3.6V down to about 5% remaining charge, whereas NMC drops below 3.5V when the SOC is at 10%. This is yet another reason why the mobile industry continues to choose LCO.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Thursday, December 4, 2014

40. WHAT ARE THE REAL  0% AND 100% POINTS?                                                      


Your fuel gauge indicator, whether it is in your smartphone, tablet or even electric vehicle, gives an indication of remaining charge in your battery. That indicator is universally given in percentage. It is assumed that at 100%, the battery is full, and at 0%, the battery is empty. But what is the definition of full and what is the definition of empty. That's the topic of today's post.

In an earlier post, I showed how the voltage across the terminals of an individual battery cell is really the composite voltage contributions of both electrodes, the anode as well as the cathode. The voltage contribution for each electrode varies with the fraction of lithium ions that are embedded inside the electrode. A user can only measure the composite voltage sum of the two electrodes when he or she measures the terminal voltage of the cell.

The first point to understand is the relationship between the fraction of lithium ions inside the electrode material vs. the notion of empty or full. When the graphite or carbon anode is completely devoid of lithium ions, the cell is truly empty. In other words, there are no available lithium ions and consequently, no "stored" charge. From that earlier post, one can see that the composite cell voltage can be very low, somewhere near 1V or even less. Cells never operate near that low voltage point. A truly empty battery cell has most likely incurred serious damage to its internal structure. If any of your lithium ion cells measure less than 2V, it is time to discard them. As a result, most battery cells consider a safe lowest operating voltage to be between 2.5V and 3.0V. This is the definition that one may get from the battery manufacturer. In practice, however, a smartphone will display 0% when the cell voltage is near 3.3V. This is because several electronics components, most notably the power amplifier for the radio, will not operate efficiently below 3.3V. Consequently, your mobile device shuts off at that low voltage threshold. This is the definition of zero as made by the mobile device maker. In either case, you will observe that the definition of empty is usually related to a low operating voltage threshold, and less so about being "empty."

The chart below illustrates the dependence of voltage on the amount of charge taken out of the cell during discharge. When the battery is fully charged -- to the far left of the chart -- the voltage is at its maximum. As charge is slowly removed from the cell, the voltage declines. At some point near 3.6V, it begins to drop precipitously; in other words, one needs to take out only a small amount of charge before the voltage drops rapidly. The rate at which the voltage declines depends on the choice of material. The chart below shows the voltage dependence for a battery that is made with a carbon anode and a lithium-cobalt-oxide (LCO) alloy cathode. The battery nominally stores 3,000 mAh.



If you think about this chart for a brief moment, you will quickly realize that the area under the curve is the amount of energy stored in the battery -- after all, energy is the product of charge and voltage.  So let's compare this to NiMH batteries that have a nominal cell voltage of 1.2V. Which one has a higher energy density? Naturally, lithium-ion: by at least a factor of 3X.

What about the definition of the 100% point? Is the battery full and hence cannot accept more charge? Not really. The definition of 100% is simple: the terminal voltage of the battery has reached 4.35V (sometimes, it is 4.2V but more often in consumer devices, it is 4.35V). This voltage threshold is strictly due to safety. Above 4.35V, three unsafe mechanisms begin to take place. First, lithium plating occurs in lithium ion batteries that use a carbon-based anode. These lithium metal deposits can short the cell and cause a fire. Second, the electrolyte, being liquid or gel-based, deteriorates rapidly and decomposes. The electrolyte is the medium through which the lithium ions can travel from one electrode to the other. And finally, the structure of the cathode itself begins to change its material phase and it becomes unstable. 

So there you have it, empty is really not empty, and full is really not full. Both limits are defined primarily on the basis of safety and practical utilization of the battery.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Tuesday, December 2, 2014

39. THE BATTERY SCANDAL OF THE BOEING 787                            


The National Transportation Safety Board (NTSB) released today its incident report on the battery fire that led to the grounding of the Boeing 787 fleet for many months. It is a thorough report, nearly 100 pages long, that examines in great detail all aspects of the lithium ion battery and its systems. The result is shared responsibilities for the battery fire, with blame targeting GS Yuasa, the Japanese manufacturer of the lithium ion individual battery cells, and Thales, the French system integrator responsible for the power conversion subsystem, as well as Boeing. Even the FAA shared some of the blame in how it managed the certification process.

The report reserved special blame on the poor design, test and manufacturing practices at GS Yuasa. In essence, the NTSB reports that defects in the assembly and manufacturing of the cells led to internal shorts in the battery. Inadequate testing during certification throughout the chain did not catch these problems. Specifically, these defects led to shorts and consequently thermal runaway in cells 5 and 6. The battery contained a stack of 8 cells connected in series for a nominal voltage of 29.6 Volts.  CT scans and disassembly of defective batteries showed the failure points -- defects in the form of protrusions or wrinkling in the cathode-separator-anode winding that effectively created minute electric shorts between the cell's terminals.

The first figure below shows a schematic from the NTSB report of the design of an individual cell. It consists of a sandwich of the lithium cobalt oxide (LCO) cathode layer, the separator layer, and the carbon anode layer. Aluminum and copper foils act as the electrical conducting planes for each of the cathode and anode, respectively. This material construction is nearly identical to the ones used in the consumer industry. These layers are then folded together to form the cell windings.



The next photograph shows how the cell windings are flattened before insertion into the metal packaging can. The NTSB pointed to this winding formation and flattening process as one of the culprits in the origination of the defects and shorts. This manufacturing process created wrinklings; this is a highly technical term referring to the buckling of the electrode foils, thus compromising their integrity right during the cell manufacturing steps. Additionally, the NTSB found that the electrolyte filling process was inconsistent with the practices used in the industry and possibly led to the incomplete formation of the SEI layer (it is a thin layer that forms between the anode and the electrolyte). In other words, the NTSB pointed a clear finger at the poor cell manufacturing at GS Yuasa.


So why weren't these defects caught during the test and certification phases? Well, apparently the battery that made it on the actual B787 airplanes was different in design and properties from the batteries that underwent the tests. It seems that lots of heads at GS Yuasa and up the food chain to Boeing will be rolling -- if they have not already. Shame on the engineers and the managers who supervised and led this process.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Wednesday, November 26, 2014

38. WHAT IS QUICK CHARGE 2.0 FROM QUALCOMM?                                


We see it in Qualcomm's tweets. We see it in some published articles. We see it in fine prints from a few smartphone makers. It says "fast charging using Quick Charge 2.0." What is it?

In my earlier posts, I spoke about two pieces that are essential for fast charging. The first piece is power delivery from the wall socket to the mobile device, and the second piece is battery management that ensures optimal operation of the battery itself under the stressful conditions of high charging power.

Well, Qualcomm's Quick Charge 2.0 is about the first piece, and only the first piece, i.e., ensuring power delivery to the mobile device and the battery. It does not perform any meaningful battery management functions and certainly does not ensure the battery's health.

What is its purpose and how does it work? Standard AC adapters and power delivery mechanisms into mobile devices have been limited to 5V and 1.6A. In other words, the AC adapter takes 120V/240V at the input and outputs a maximum of 8W of power at 5V. This has been the de facto standard for several years. This power output was plenty sufficient when mobile devices had batteries that were small, about 1,500 mAh or less. But as tablets first came on the market with batteries in excess of 4,000 mAh, and smartphones followed with batteries in excess of 2,000  mAh, it became clear that higher power output was needed from these AC adapters.

Naturally, the easy answer would be to just increase the current above 1.6A but still maintaining an output voltage of 5V. But this is highly impractical. First, the USB cable cannot be realistically changed -- even though the new USB 3.0 standard aims at changing the cable dimensions, it will be years before such new standards make it through the industry. In other words, power delivery had to co-exist with present-day USB cables. Additionally, if you recall from your high school physics class, electric losses go as the square of the current. So increasing the current from 1A to 2A, for example, quadruples the losses, all going into heat. That's not good! So if one cannot raise the current, then the next logical answer is to raise the voltage.

I will digress briefly here. Take a look at how power is transmitted from the power stations to your house. You will find, somewhere near your house, a large power switching station whose primary responsibility is to step down the voltage from the transmission lines, typically at 66 thousand volts, down to what ultimately becomes 240V at your house. Transmission lines use 66 kV precisely to limit the losses and use cables of reasonable dimensions and cost.

So QC 2.0 establishes a new methodology to deliver current at or near 1.6A, but raise the voltage to 9V or 12V, up from 5V. That effectively raises the maximum to nearly 20 Watts. Plenty for now. So you can see, there is nothing earth shattering about the concept. In fact, Apple had been doing precisely this on their Macs for many long years.

How does it work? The question specifically is: If the AC adapter can deliver 5, 9 or 12V at its output, how will the mobile device know which voltage to select? If you take a look at the terminus of a USB cable, you will see four electrical contacts. Two pins are for power and ground, and two pins are for data (called D+ and D-). The communication between the AC adapter and the mobile device happens over this pair of data wires. The combination of the D-/D+ voltages is the signal between the two devices on the choice of power voltage requested from the AC adapter (see picture below). If this voltage "wiggling" over the D+/D- is not present, then the mobile device defaults to the standard 5V. That's really it!



So why is Qualcomm making such a big fuss of it? Of course, there is a marketing element, which is welcome. It's great to see Qualcomm throw its weight behind the ecosystem of fast charging, even when QC 2.0 solves only one half of the fast charging problem. By now, you know that Qnovo solves the other half of this problem, the battery.

So to sum it up, if your phone is capable of QC 2.0, then it is great. You are capable of fast charging. But before you start fast charging, make sure that your battery will not get damaged in the process.

Happy Thanksgiving!

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Saturday, November 22, 2014

37. CHEATING THE REPORTED CHARGE TIME                                    


I doubt that anyone will resist fast charging from becoming standard. Once a norm, I also doubt anyone will even refer to it as fast charging. The comparison then becomes one between "normal charging" vs. "slow charging." This is a day we should all look forward to.

It merits a post on measuring and reporting charge times. Given the lack of standards, the temptation is substantial to "round down" charge times and project numbers that are better than actual. At the end, of course, consumers are savvy and will not be fooled by such gimmicks.

First, how can you measure your own charge time? It's quite simple. Discharge your smartphone or mobile device battery to near zero (say 1% of remaining charge). Dim your screen if you must keep it on, set your device on "airplane mode", and shut down the GPS (location finder) and all operating apps; you want all the power to the phone to go into charging the battery. Mark the start time, connect your device to the charging AC adapter, then record every 5 or 10 minutes the percentage read by the fuel gauge. After your mobile device reports that the battery is fully charged, you will make a chart: on the vertical axis, show the measured percentage of charge in the battery, and on the horizontal axis, show the recorded time. You will get a graph that looks like the one below.

This particular chart includes data specific to a battery with a capacity of 2,500 mAh charging at a rate of 0.7C using a 9-Watt AC adapter. You will notice that the charge increases monotonously for the first hour or so, then near a charge level of 90%, it slows down quite a bit. For the readers who are technically inclined, this transition happens when the battery voltage reaches its maximum value of 4.35V, and the charging switches to a slow constant-voltage phase.

Let's plot this chart in a little different way. The following chart shows, for the same battery above, the amount of time required to charge the battery in small increments. It takes about 8 minutes to charge the first 10%, and all subsequent increments of 10% until the battery reaches the transition point. And then, the charge rate really slows down to a dismal 35 minutes for the last 10%. Worse yet, it takes nearly 15 minutes to charge from 98% to 100%. This is quite a bit given that the entire charge time is about 2 hours.

Charge time in minutes for increments of battery charge levels

This is precisely when the opportunity and temptation to cheat are greatest. Many smartphones actually declare they are full when they are near 98% of charge thus saving 15 minutes from the actual charge time. Now, one may reasonably argue that the difference between 98% and 100% is miniscule, and that is indeed true. But beware of making comparisons among different mobile devices by looking at their 100% charge times. They are likely to be inconsistent. Instead, you are better trusting the charge times at lower charge levels, say to 50% and to 80%. There is far less room to manipulate these figures.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Friday, November 21, 2014

36. DON'T ALWAYS TRUST YOUR ENGINEERS                                                  

I described earlier the two pieces of the fast charging puzzle. There is first the power delivery from the wall socket to your mobile device, and then there is managing the charging power into the battery so it does not get damaged.

The first piece of the puzzle is all about power circuitry, something electrical engineers enjoy and revel in. But the second piece of this puzzle is about managing the chemistry in the battery. Here, I have some bad news: electrical engineers, even the most successful ones, are likely to have scored a C- or a D+ on their chemistry college classes; in other words, they are not comfortable with chemistry. Like all of us humans, we drift towards our comfort zones, so chemists drift to chemistry, and electrical engineers drift to building electronics. Getting the two disciplines to work together takes hard work and lots, really lots, of discipline. As a result, it is rare to find interdisciplinary development in the mobile world, especially between battery chemistry and electronics. We, at Qnovo, happen to be one of these rare exceptions.

Well, enough self-promotion; let's get back to real life. What happens if overzealous electrical engineers decide to charge a battery as fast as they can. After all, they are electrical engineers and know how to design circuits that can pump a lot of energy in the battery -- some even give it great names like Quick Charge. The result is simple: They will destroy the battery (if you use CCCV charging). The data in the next graph illustrates the extent of the damage.


The graph is a standard capacity fade curve for a battery. In this case, it is a 2,400 mAh lithium-ion polymer battery from one of the leading manufacturers. The vertical axis shows the remaining capacity of the battery (in Ah) as the battery undergoes cycling (shown on the horizontal axis).  As the battery is repeatedly charged and discharged, it loses its ability to hold charge, hence the degrading curve. This graph shows the capacity degradation for four distinct charge rates, varying from a slow 0.5C up to a superfast 1.5C, all using CCCV. At 0.5C, the battery charges in about 3 hours, and at 1.5C, it charges in a little over an hour.

So what is the graph telling these overzealous engineers? It says that at the slow charge rate, the user will comfortably obtain over 600 cycles of operation. That's plenty to cover a year and more. But the superfast charge rate of 1.5C does so much damage to the battery that the battery will barely last a couple of months. In other words, fast charging with CCCV is bad! Isn't time to use new methodologies that give us fast charging without damaging the battery.

The moral of this post: Don't trust everything your electrical engineers tell you...well, that is unless they really scored A's on their chemistry college courses!

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Thursday, November 20, 2014

35. I WANT TO CHARGE WITH A SOLAR CELL - OR NOT!                        


I am sure many have wondered about the possibility of pasting a solar cell on the back of their mobile device to charge their battery. The idea is elegant but, it turns out, is not terribly practical. We will go through the analysis together.

First, let's establish the practical size of a solar cell that could reasonably fit on our mobile device. For the sake of simplicity, I will assume that we can somehow glue and connect one such cell on the backside of our mobile device. The table below provides the approximate dimensions of common mobile smartphones.

Next, let's examine the power flux from the sun; in other words, how much solar energy one would reasonably expect to receive. Again, I will simplify. I will assume sunny days and the surface is perpendicular to the sun rays.



The Earth receives from the Sun radiation at approximately 1,340 Watts for each square meter of surface (W/m2). This is called the solar constant. For comparison, it is about as much radiating power as you will get in your microwave oven. But that's in space, right at the edge of our atmosphere. By the time the radiation reaches the surface of our planet, it is now down to an average of approximately 340 W/m2, or less than ¼ of its value in space. The table above calculates the average amount of power that this hypothetical solar cell will deliver to the mobile device, again, assuming a sunny day -- once again, this means outside, in direct sunlight with your mobile device facing perpendicular to the sun rays.

The math tells that we should not expect more than 2 to 5 Watts of peak available charging power. In reality, once your mobile device is indoors or in the shade, or worse yet, in your pocket or purse, that amount of energy will drop by a factor of 100 or even more.

So, remaining optimistic, and willing to sit in direct sunlight while my mobile device charges, how long would it take to charge each of these mobile devices given this amount of solar charging power? The following table calculates these approximate times to full charge (100%). On average, it takes about 4 to 5 hours of charging. On the positive side, you will walk away with a nice tan!


Of course, the question can be posed a little differently: How much surface area would one need in order to charge their device using a solar cell in a reasonable amount of time? Assuming that somehow this solar cell can be mounted outside in direct sunlight and a charging wire can be brought inside, and assuming again that you are willing to charge your device exclusively during the peak day hours near noon, then one requires a surface area about 200 to 300 cm2, or a square roughly about ½ to ¾ foot on each side.

I will leave you with some food for thought. The power levels available from a solar cell are about the same as the power levels that one gets from wireless charging pads. In other words, if you put your mobile device on a wireless charging pad, it will take 4 to 5 hours to charge your device. Of course, you are not tanning at the same time, but it is awfully slow! Now why is it that fast charging is not yet a standard?

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Tuesday, November 18, 2014

34. WHY IS FAST CHARGING SEEMINGLY DIFFICULT? OR IS IT?             


We know and complain about the dismal charging times of modern smartphones! But why is it that these marvels take several hours to charge? That's what we will explore today.

There are two pieces to this puzzle. The first piece relates to power delivery, and the second piece relates to maintaining the health of the battery.

Let's start with power delivery. That's the circuitry, cables and necessary components to deliver sufficient electric power from the wall socket to the mobile device. Specifically, this includes three key elements: i) the AC adapter, ii) the USB cable, and iii) the circuitry, integrated circuits and other passive components such as inductors, all residing inside the mobile device itself.

Presently, the AC adapters are rated 5 Watts, taking 120V from the wall and delivering an output of 1A at 5V into the smartphone. Charging faster means more power. How much more? With 3,000 mAh becoming common, we are looking at a minimum of 15 Watts.  Therefore a 15-Watt AC adapter is required, but most importantly, it has to be at a cost that is nearly equal to the earlier and smaller AC adapter. 

We also need a USB cable that can take the power from the AC adapter to the device. Standard USB cables are typically rated at or near 2A. This automatically says that the higher power should be delivered at a higher voltage. This is very similar to how power is transmitted from large power stations to your house. Power leaves on high-voltage transmission lines (these are the large towers that you see in open fields or on top of hills). When the power gets close to your house, the voltage is then stepped down to the standard 120V that you are familiar with -- you can see these little transformers perched on top of utility poles up or down your street. 

It is a similar concept in the mobile device. The new 15-Watt adapter operates at higher voltages, typically 9V or even 12V, up from the standard 5V. Take 120V from the wall, then step it down to either 5V (the old way) or 9V or 12V. Of course, the circuitry within the smartphone itself now has to be able to take the 9V or 12V. Voila! These AC adapters, USB cables and proper internal circuitry capable of operating at the higher voltages are just getting rolled out in 2015....so the first hurdle in fast charging is solved.

The second piece of the puzzle is the health of the battery. How do we make sure that the battery will operate properly for the life of the mobile device when we dump 15 Watts of power into it? This has been the challenge of the industry -- and fortunately, one of several problems that we have solved here at Qnovo.

The problem with dumping 15 Watts of charging power into the battery is that it will destroy the battery's cycle life. This older post will explain what cycle life is. Poor cycle life manifests itself with rapid loss of battery capacity and increasing warranty returns. That's when the consumer (you!) notices a rapid deterioration in the battery life from one day to the next, and you start b*$%!ing.

Right now, the battery manufacturers are struggling with making the cycle life specifications, albeit trying to deny it. They are promising new generations of batteries that can take the higher charging power, but in reality, they are failing to deliver the requisite performance. This highlights the industry's idiom about "liars, liars and battery suppliers." The failure to deliver is due to fundamental limitations about the underlying physics of charging the battery. Instead, the battery manufacturers are scaling back the battery capacity (i.e., reduce the number of mAh) in order to charge faster, but that's not what consumers want. We want high battery capacities AND faster charging, not a choice of one or the other. 

The good news is that this problem is also being solved and we expect these solutions will also be rolling out in 2015. In other words, expect to start seeing fast charging becoming increasingly common some time next year. Yet I constantly wonder, why in the world did it take this industry so long!

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Friday, November 14, 2014

33. THE SCIENCE BEHIND WHY THE BATTERY VENDORS ARE HITTING THE WALL                           


You have heard it before from me: battery performance is getting seriously challenged. The battery vendors are having a difficult time, and I am being kind, in meeting the expectations of the mobile device manufacturers. Specifically, it is getting extremely difficult for the battery vendors to simultaneously deliver high capacity, fast charging, and long cycle life. But why is that? 

Let me digress briefly here and examine the average cruising speed of commercial airplanes over the past century. One will rapidly notice that airplanes have not gotten much faster in the last 50 years. They are more fuel efficient; they travel longer distances in greater comfort (well, if you pay for it); and they are far safer than they have ever been...but they are surely not faster. That's because it gets very expensive to travel faster than the speed of sound (667 knots). The Concorde is a stark reminder of this economical limit. Chuck Yeager broke the physical limit of the sound barrier decades ago, but we can't really change the economics around this limit.


Lithium-ion batteries are rapidly approaching a similar limit for energy density. Short of a major breakthrough in a new material system, we are staring at a difficult barrier somewhere between 600 and 700 Wh/l. With that, I mean achieving large-scale manufacturing with affordable economics that match the requirements of the mobile device industry. A key limiting factor is now the carbon anode material. It is possible that new carbon-silicon composite anodes can change this equation, but for the foreseeable future, these new composites will continue to suffer from poor cycle life and high manufacturing costs. Until then, the economics of rising energy densities will be severely disadvantaged.

Is there a scientific origin for this limit? There are plenty of reasons, but it is best to illustrate one: the impact of battery voltage on energy density. In a very simplistic description, higher energy density comes from packing more lithium ions inside the battery, as well as raising the voltage at the terminals (if you recall, energy is the product of charge, or ions, and voltage).

The voltage at the terminals is the difference between two voltages; that of the cathode voltage in reference to a fictitious lithium contact, minus the voltage of the anode, also in reference to that same fictitious lithium contact. This is illustrated in the graph below.


On the vertical axis, I show the voltages of both the cathode vs. lithium (top) and the anode vs. lithium (bottom). On the far left of the chart, i.e., when x is approaching zero, the graphite is void of lithium ions and cobalt-oxide is totally full of lithium ions. This is when the battery is "empty." On the far right, when x is approaching one, the opposite is true; the battery is "full." I specifically refrain from saying x = 0 or x = 1. At these extremes, bad things happen. When x = 0, the physical structure of the cobalt-oxide alloy is greatly damaged. This limits the low end of the battery voltage to about 3.0 V. In other words, never discharge your lithium-ion battery below 3.0 V; the risk of irreversible damage is great. Most smartphones actually cut off near 3.3 V (this is really when your phone reads zero percent).

At the opposite end when x = 1, lithium ions combine to deposit (or plate) as a metal.  The anode structure is also under immense mechanical stress. Additionally, when the cathode voltage rises past 4.2 V, the electrolyte begins to oxidize (and ultimately decompose). This effectively limits present-day lithium-ion batteries to a maximum voltage of 4.35 V with the understanding that the "bad stuff" begins to occur past 4.0 V, and becomes unsafe past 4.35 V.

To raise the energy density in the carbon/cobalt-oxide material system, one needs to raise the voltage and/or pack the electrodes as close as possible to each other. Well, raising the voltage past 4.35 V is getting very difficult. Finding electrolytes that can handle such voltages is no small feat. Additionally, the battery is now awfully close to the x = 1 point; in other words, the risk of lithium metal deposition  at the anode is dangerously large at high energy densities. Life is getting tough, and there is very little room left for the battery vendors to maneuver.

These are just a few physical insights behind the challenges that the battery designers and manufacturers are facing. Finding solutions to these challenges via brute-force material development is not the answer. If you find yourself stuck with these limits, talk to us at Qnovo!

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Thursday, November 13, 2014

32. ELECTRICAL CHARACTERISTICS DURING CHARGING                               


If you own a mobile device and in need to charge it, the first thing you do is to find a (or your) AC adapter with a USB cable, then plug it into the appropriate USB charging port on your device...et voila! Come back some hours later and your battery should be full.

It is simple, as it should be. But have you ever wondered what happens behind the curtains? I will cover some of these details in this and additional future posts.

So first, before we delve into the electronic circuitry responsible for charging the battery, let us examine the electrical characteristics of the lithium-ion battery during charging. The battery is a complex chemical device, but electrically, it can be simplified into a two-terminal component; in other words, there are two electrical values of importance to us: i) the voltage across the battery terminals, and ii) the current flow, either into (i.e. during charge) or out (i.e. during discharge) of the battery.

The voltage across the terminals of the battery are directly correlated to the state-of-charge (SoC) of the battery -- if you recall from this earlier post, it is the fraction of the battery charge relative to full. 

During the charging phase, one would expect the voltage to rise across the terminals of the battery from the "empty" level (typically around 3.3 V) up to the "full" level (typically around 4.2 V or 4.35 V depending on the type of battery). This is precisely what the next chart illustrates for a lithium-ion battery with a nominal charge capacity of 720 mAh.



This chart of the battery's charging characteristics looks rather busy but we can dissect it quite readily to glean some valuable information. Every lithium-ion battery, without exception, will have a similar chart, often included in its data sheet. 

The right vertical axis shows the charge capacity (or SoC) as a function of charge time. It is shown in the long-dash curve. Zero is at zero, and 100% is reached after about 2.5 hours of charging.  The charging current itself is represented by the dotted line, and its values are on the far left vertical axis. 

One can make a few key observations. First, the charging current has a steady value of approximately 720 mA, then begins to decay after less than one hour of charging. This first phase is called the constant-current (CC) phase; the second phase where the current is steadily decaying is called the constant-voltage (CV) phase. Some publications and blogs incorrectly label them as the "fast charging" phase and the "trickle charging" phase...this is absolute non-sense and illustrates total ignorance on the part of the writer. I will revisit this type of CCCV charging later -- it is at the core of many ills that plague modern lithium-ion batteries.

The second observation we make is that the voltage of the battery indeed stays between 3.3 V and 4.2 V, but that somewhere around 50 minutes, the voltage is held steady at 4.2 V and remains there. This is precisely what the constant-voltage phase does; the internal charging circuitry will actually pin the charging voltage to a value of 4.2 V and keep it there until the charging is complete. 

This maximum voltage value comes straight from the chemistry. At higher values, the electrolyte inside the battery begins to oxidize and decompose, thus posing a serious safety hazard. This is one of several reasons why an end user cannot, and should not, mix chargers (AC adapters) used for NiMH batteries and lithium-ion batteries. The voltages for each battery type are vastly different.

Finally, one wonders at what point is the charging process deemed complete? Naturally, you will say "100%", but how is 100% defined? During the charging process, the convention is to halt charging when the decaying current reaches 1/20th of the capacity of the cell. In this particular battery, it corresponds to the current decaying to 720/20 = 36 mA. From the chart above, this is reached after 2.5 hours. But mobile device manufacturers are in a hurry and often fudge their numbers, so that's why you will see the green light turn on much, much, earlier, shaving 30 or 45 minutes from the actual charging time.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Tuesday, November 11, 2014

31. A PEEK INSIDE THE BATTERY OF A TESLA MODEL S                           


A single lithium-ion 18650 cell is relatively small in size and in capacity. So how does Tesla pack 85,000 W.h in the battery pack of the Tesla Model S? The answer is very carefully.

The battery pack in a Tesla S is a very sophisticated assembly of several thousands of small individual 18650 cells connected electrically in a series* and parallel combination. A colleague alerted me recently to an outstanding teardown activity that an owner of a Model S is performing on his battery pack. This offers a great peek into how Tesla designed this battery pack.

First, it is very important to note that 85 kWh is a huge amount of energy. The voltages at the terminals of the pack are high, and the currents can also be dangerously high. In other words, safety is of paramount importance in the design, assembly, and equally the teardown of a large battery pack of this size.

The photographs that this owner has published are very telling and provide a great insight into the design of this pack. The first photograph shows the entire pack with its top cover removed.

Photograph of the battery pack for a Tesla Model S electric vehicle. Courtesy: Tesla Motors club user [wk057]. 

This photograph shows a total of 16 sections, or modules, electrically connected together. A closer inspection of one individual module shows that it contains a number of 18650 cells, all sitting next to each other in a vertical position. One can diligently count a total of 432 individual cells in one single module. 

Therefore, the first observation we can make is that there are a total of 16 x 432 cells = 6,912 cells. The capacity of each cell is 85,000 / 6,912 = 12.29 W.h., or equivalently, 3.4 A.h. The individual cells are of the 18650 type, manufactured by Panasonic. They use an anode made of graphite, and the cathode is made of NCA (nickel-cobalt-aluminum alloy). The NCA-graphite architecture has a lower nominal voltage than the cobalt-oxide alloy commonly used in mobile devices. The nominal voltage of a NCA-based cell is 3.6V.  The owner measured the module voltage to be 19.63V when the battery was virtually dead. A dead (i.e., empty) cell has a voltage near 3.2V. 

Photograph of one individual module from the pack.
Therefore, our second observation is that each module contains 19.63V/3.2V = 6 cells in series. Consequently, the module is configured as 72 parallel legs, each containing 6 cells in series (abbreviated as 6s x 72p).

The energy of one single module is 85,000 / 16 = 5,312 W.h. This is equivalent to the energy contained in about 100 (yes, one hundred) laptop PCs. A closer examination of the module assembly shows that each cell is wired to the main bus (the primary electrical path) through little fuses...this is an outstanding safety feature that will disconnect an individual cell that may have shorted with time.

Photograph showing the fuse wires connecting individual 18650 cells to the main bus.
Our third observation is that the entire pack consists of 16 modules connected in series, therefore the overall architecture is 96s x 72p. The stack voltage is nominally 96 x 3.6 = 345 V, but would be as low as 310V when the pack is nearly empty, and 403 V if the battery is at 100% full (but Tesla does not recommend that you charge the battery to 100%).

Our fourth observation is about weight. Panasonic specifies a weight of 46 g for each 18650. The weight of all 6,912 cells comes out to be 318 kg or about 700 lbs. The weight of the entire battery pack is estimated by various sources to be 1,323 lb. So the 18650 account for approximately 53% of the weight of the pack -- the rest is due to electronics, cooling systems, wiring and safety.

Judging from my earlier post on cost trends, the estimated cost is about $1.50 for each 18650 cell. I am assuming that the Tesla sourcing team is very influential in demanding attractive pricing from the cell manufacturer, Panasonic. This equates to a cost of approximately $10,000 for the cells used in the pack. Given a delivery rate of about 35,000 cars for 2014, that equates to nearly $350 million that Panasonic will collect this year from selling cells to Tesla...and the number may grow to $1B in 2015.

Adding another estimated $5,000 for the cost of the electronic battery management systems, and one has a preliminary material (BOM) cost of $15,000 for a pack used in the Tesla Model S. That equates to less than $200 for each kWh of stored energy. It also works out to about $50 of battery cost for each mile of driving range. It's amazing what one can derive from a handful of photographs!

* A series configuration means the positive terminal of the first cell is connected to the negative terminal of the second cell. The voltage at the free terminals is now the sum of the voltages at each cell. The series combination allows raising the voltage of the battery pack to much higher voltages.

† A parallel configuration means electrically connecting the positive terminal of the first cell to the positive terminal of the other, and the negative terminal of the first gets wired to the negative terminal of the other cell. The voltage at the terminal of one cell is identical to the voltage at the terminal of its sister cell. A parallel configuration allows the addition of capacity without raising the voltage.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Saturday, November 8, 2014

30. COST, COST AND COST.                                                            


The cost of today's lithium ion battery constitutes a very large fraction of the total cost of electric vehicles, and a very visible portion of the total BOM (bill of materials) cost for a mobile device. It is estimated that the battery pack for a Chevy Volt costs nearly $10,000. That's nearly 25% of the total price of the vehicle. It's no wonder that the CEO of Fiat, Sergio Marchionne, requested earlier this year that customers stop buying his electric version of the Fiat 500 because he is tired of losing $14,000 on each vehicle. 

The small battery of a mobile high-end smartphone from the top mobile device manufacturers can cost upwards of $3. Batteries made in China will cost less but often come with some serious performance drawbacks. Given that a high-end smartphone has a BOM near $200, that makes the battery cost a visible line item on the list of components. 

Naturally, this attracts attention, scrutiny and further examination of where the cost curve has been, and where it is headed. The next chart shows the pricing trends that the industry has been subjected to. As deployment of batteries accelerated driven by demand in both the mobile and EV sectors, the cost of the batteries, at least for consumer devices, measured in dollars per unit of energy (Watt-hours), plummeted nearly 10X in the past 20 years. The forecast is for further decline especially as more production capacity comes on line in China. The cost curve for electric vehicles remains more recent with a modest deployment of capacity in new electric vehicles such as the Tesla Model S, Chevy Volt, Ford Focus Electric and the Nissan Leaf.

Cost curve for lithium-ion batteries. Source: Bloomberg New Energy Finance, Qnovo

Let's further examine the curve corresponding to consumer batteries, driven primarily by mobile devices and laptop PCs.  Present cost for consumer batteries hover near $0.20-$0.30 per Wh. Higher performance batteries can command substantially more. The annual production and consumption capacity for this market segment is approximately 20,000 MWh. Judging from the historic trend, it may be another 3 or 4 years before the 100,000 MWh threshold is reached. In other words, it may be another 3 or 4 years before we see these cost figures break the $0.10 per Wh mark. That is of course assuming the trend continues at its present pace. Any excess production capacity in Asia, and particularly in China, could easily accelerate the price decline.

Paralleling this pricing pressure is the continued trend to improve the energy density of batteries. Over the past 20 years, the energy density has increased by approximately 2X. For the most part, the progress was incremental translating to an annual gain of about 5%. A state-of-the-art battery in a high-end mobile device may have a density above 600 Wh/l. But as energy densities continue to climb, both development and manufacturing costs are accelerating. The present material system using graphite based anodes and cobalt-oxide cathodes is reaching its limit. Research in new materials is needed. These enormous research and development costs accompanied with the large capital costs of manufacturing are reducing the field of battery vendors to a select few, notably the large chemical conglomerates based in Asia.

So will the past trend of increasing energy density and decreasing costs continue indefinitely? The challenges of the past couple of years indicate that a change in this trend may be quite likely. If the rising costs of developing and manufacturing new high-energy density batteries are not tamed, then expect a slowdown in the rise of energy density, and commoditization and rapid decline in pricing for lithium-ion batteries. That, folks, is the definition of an inflection point!

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com

Wednesday, November 5, 2014

29. THE DIFFERENT SHAPES OF A BATTERY...                                      


That is of a rechargeable lithium-ion battery, of course....We all know that lead-acid batteries, the type you have under your hood, tend to be of a standard size, but lithium-ion batteries can come in a multitude of packaging and shapes.

One of the most common misconceptions is that polymer batteries are different. In fact, they are one of the common types of lithium-ion batteries, assembled and packaged in a flat, pouch-like shape.  Their core design is based on the standard lithium-ion chemistry. They are called "polymer" batteries because they tend to use an electrolyte that is gel-like than liquid-like. The outer package is a thin foil that holds the internal structure together. Consequently, they can be prone to damage or puncture, and are often if not always embedded within the mobile device for mechanical protection.

One of the advantages of polymer batteries is that they can be manufactured in nearly arbitrary custom dimensions or shapes. This ability to make the battery fit the mobile device (instead of the other way around) gave polymer batteries their great appeal. Polymer batteries can also be made very thin. The photograph shows a polymer cell made by Sony for use in their Xperia Z2 smartphone. It is only about 4 mm thick. The downside of polymer batteries is the lack of standardization, and consequently, higher cost of polymer batteries; each battery model has to be designed and shaped to the particular dimensions required by the manufacturer of the mobile device. A polymer battery can be nearly twice more expensive (for the same amount of stored energy) relative to their older sibling, the standard 18650 battery cell.

Three different types of rechargeable lithium-ion batteries. From left to right: Prismatic (used in a Samsung Galaxy S5), Polymer (used in a Sony Xperia Z2), and an 18650.

The 18650 cell was named with very little creativity. It comes as a standard cylinder with 18mm in diameter, and 65mm in height, hence the naming. The standard size of these cells made them immensely ubiquitous and inexpensive in the past decade. They were widely used in laptop computers but proved less practical for smartphones with thin profiles. Tesla Motors took advantage of the large-scale manufacturing and low cost of 18650s, and adopted them for use in their electric vehicles. The battery pack in a Tesla Model S contains nearly 7,000 such cells. The photograph above shows an 18650 cell with a capacity of 3,400 mAh made by Panasonic; it is similar to the one used in a Tesla vehicle. The other major manufacturers of electric vehicles have elected to use large size polymer-type batteries. Nonetheless, 18650s are here to stay. There is so much manufacturing oversupply of 18650s that their price continues to plummet, making them an attractive commodity.

The third type of cells are called prismatic. They are, at their core, very similar to the polymer cell but are packaged inside a solid case or can, typically made of an aluminum alloy. This offers added mechanical protection and the requisite safety. Mobile devices that offer replaceable batteries use prismatic cells. The photograph above shows a prismatic cell used in the Samsung Galaxy S5. Owing to the walls of the external can, they tend to be thicker than polymer batteries. 

Back to the photograph above, the keen reader might ask about the connector attached to the Sony polymer battery. It is indeed an electrical connector made using a thin flexible cable. At the tip of this cable, one can observe some circuitry that provides the necessary electronic protection for the battery. In particular, this circuitry ensures that the battery does not experience excessive voltages or excessive currents. A built-in fuse disconnects the battery should it get exposed to adverse conditions. Similar circuitry is also embedded inside the case of a prismatic cell. However, the 18650 cell is bare, i.e., does not include any such protection circuitry which must be included in an external battery management system before the battery is put to use.

© Qnovo, Inc. 2014 / @QNOVOcorp @nadimmaluf #QNOVOCorp    http://www.qnovo.com