Safety

13Nov 2014

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.

11Nov 2014

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.
05Nov 2014

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.

29Oct 2014

You probably suspected that temperature swings are not good for your lithium-ion battery. But what is the extent of the damage, and what are the temperature limits that one should attempt to follow? This is the subject of today’s post.

If you were to review a specification sheet for a lithium-ion battery, it most often has a few (but not too many) things to say about temperature. Incidentally, it will take a great deal of effort and investigation for you to find a specification sheet that corresponds to the battery in your mobile device. These documents are usually not provided by the battery vendors to end users. 

Usually, the specifications will list the test conditions. The vast majority of battery tests are usually conducted in a laboratory with a controlled environment, and a temperature typically between 22 °C and 28 °C (equivalent to 72 °F to 82 °F). The specifications will also provide some additional conditions at temperature extremes such as below 10 °C or above 45 °C. For example, it will state the dependence of the battery capacity on temperature. The following chart shows the dependence of capacity on temperature for a typical polymer lithium-ion battery based on the specification from the battery manufacturer. One can see that the battery is “happiest” near 25 °C to 35 °C (or near 75 °F to 95 °F). Note that this is the internal temperature of the battery itself, not that of the outside case of the mobile device.

The available maximum capacity of the battery has a strong dependence on temperature.

The specifications will seldom provide the effect of temperature on the battery health and its cycle life. Tests have repeatedly shown that the cycle life of the battery tends to degrade with temperature. At temperatures below 15 °C, the cycle life drops very fast. That’s because the lithium ions find it increasingly difficult to make the journey from one electrode to the other at colder temperatures. At higher temperatures, this “ion mobility” is improved and tests show that cycle life is improved up to a point, somewhere near 45 – 50 °C. At such high temperatures, several materials such as the electrolyte begin to decompose causing a rapid degradation of the battery. The following chart exhibits this effect. This particular polymer battery exhibits an excellent capacity retention of its capacity when cycled at 45 °C, but as soon as its temperature is raised to 55 °C, its capacity fades at an alarming rate.

So what practices should you take away? Clearly, avoid operating in temperature extremes. For example, charging your phone on your car dashboard in the middle of the summer heat will undoubtedly cause your battery lots of health problems. 

28Sep 2014

No, it is not “Batteries Made Simple,” nor “Better Make Sense,” though BMS do indeed try to accomplish both in a very indirect and implicit way.

BMS stands for Battery Management Systems. These are electronic systems, both hardware and software, whose primary function is to control the operation of the battery. In order for batteries, and more specifically lithium-ion batteries, to deliver the requisite safe performance, they must operate within some very well defined, and in many cases, strict limits. For example, a lithium-ion battery cannot be charged above a certain voltage specified typically by the manufacturer in the range of 4.2V and 4.35V. Maximum current values and temperature limits are other examples. Failure to observe these limits will result at the very least in performance degradation, and quite likely in a seriously unsafe outcome such as fire or even death. A Chinese flight attendant died in 2013 while using her iPhone 5 during charging; her electrocution was attributed to a counterfeit charger she purchased in China.

BMS cover several functions including charging the battery, measuring the battery’s amount of stored charge, and making many decisions to ensure the battery remains within a safe operating mode. 

The fuel gauge, the device responsible for giving you the percentage of “battery full” in your mobile device, is an integral part of the BMS. Fuel gauges were practically inexistent until a startup company called Benchmarq introduced them in the early 1990s, initially for notebook PCs. Fuel gauge functionality is integrated today in the power management integrated circuits (known as PMIC) manufactured by companies such as Qualcomm and Texas Insruments, yet sadly, there has been very little if any meaningful innovation added since Benchmarq — I will resist the temptation of openly promoting Qnovo here. For example, the accuracy of the fuel gauge in your smartphone is quite poor, and can often be as high as 5 to 10 percentage points. Next time you look at your mobile device and it reads 20% battery remaining, keep in mind that may be as little as 10% or as high as 30%. Worse yet, device manufacturers routinely fail at translating this reading into a meaningful usage number like  hours of remaining use.

Battery charging is another function of the BMS. Yet charging remains extremely primitive. Most mobile devices today charge using a method called constant-current constant-voltage (abbr. CCCV) that was invented in the 19th century to charge lead-acid batteries. Its simplicity certainly made it irresistible; but there is no free lunch. CCCV charging has now been clearly established as a primary cause of battery damage. Next time you look at your mobile device and wonder why it is not lasting you a full day as it did when it was new, you can start by pointing the finger to CCCV charging. Yet, most mobile devices still stick with this archaic charging approach.

If you are a battery user, you also might want to see additional information such as the health of your battery. Nope! You can’t get it from present-day BMS in your mobile device. You may want to charge your mobile device faster. Nope! You can’t do it. You may want to know whether your battery may have been defective from the onset. Nope again! Both you and the device manufacturer are in the dark. Yes, you can walk today into the store of your favorite wireless carrier (or operator) and tell them that your battery was defective, and there is virtually little they can do to prove or disprove your concern. Insist a little and you will walk away with a replacement smartphone or mobile device. And while you are at it, let them know that you want more features such as faster charging!

This is the sad state of battery management today. It’s not because innovation is lacking or the technology is behind. Solutions do exist. Device manufacturers are slow to implement innovation. So let them know what you want!