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.

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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. 

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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!

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