Electronic Design
Understanding Lithium Battery Tradeoffs In Mobile Devices

Understanding Lithium Battery Tradeoffs In Mobile Devices

Li-ion batteries give mobile-device designers choices that affect product convenience, durability, and style. These choices involve tradeoffs among several parameters, though. Understanding how these factors interact requires some knowledge of Li-ion construction and chemistry. 

Lithium-ion (Li-ion) batteries give mobile-device designers choices that affect product convenience, durability, and style. These choices involve tradeoffs among several parameters, though (see the table). Understanding how these factors interact requires some knowledge of Li-ion construction and chemistry.

Table Of Contents

• Market Demands

• Assessing Batteries Using Radar Charts

• Li-ion Pouch Cells

• Run Time Per Charge

• Application Support

• Charging Rate

• Battery Cycle Life

• Battery Calendar Life

• Thinness

• References

Market Demands

The tablet market is bullish, with smart phones acting as a gateway drug. According to ComScore, at the beginning of 2012, one in four smart-phone users in the U.S. also had used a tablet.1 The ratio is doubtless even higher now. Tablet shipments within the last year reached 73 million units and will reach 275 million units by 2016, as GBI Research reported recently.2 That’s a compound annual growth rate of 19%. All of this is very good news for the battery manufacturers whose Li-ion cells power these tablets.

Yet as with all consumer electronics, such rapid growth depends on Moore’s Law, which is delivering ever faster, more competent processors and more vivid, detailed displays to support demanding applications like streaming video and gaming, for which tablets are preferred. Unfortunately, their batteries can’t always keep up. Li-ion energy density, basically the run time available from a given size or weight of battery, has only tripled since the commercialization of Li-ion batteries in the early 1990s. During the same period, processor power has increased more than a thousand-fold.

This disparity imposes difficult decisions on tablet designers and higher costs on consumers, as perhaps most dramatically illustrated by the latest iPad. Apple, the apostle of “sleek,” had to make its leading-edge tablet heavier and thicker than its predecessor to maintain a similar—but in reality, somewhat inferior—run time per charge.

Things aren’t getting better, either. “Based on analysis of Strategy Analytics’ SpecTRAX database, the average tablet battery provides just eight to 10 hours of run time (Web browsing or video playback) on a full charge, with no significant increase evident in the last 12 months,” says Stuart Robinson, director of the Handset Component Technologies advisory service at Strategy Analytics.

Consumers are not oblivious to this and are demanding a boost in power and performance in tablets. Results from TechBargains.com’s recent iPad/iPad mini Predictions survey show that longer battery life is a feature that 87% of respondents want from the next iPad mini and one that 86% want from the next iPad.

Assessing Batteries Using Radar Charts

Thinness versus run time is just one of the tradeoffs that present-day Li-ion battery technology imposes on both tablet designers and consumers in three key areas: convenience, durability, and style. Radar charts can illustrate how these tradeoffs work and why it’s hard to isolate the ideal solution (Fig. 1).­­

 

1. There are seven tradeoffs to consider in selecting a battery type for a tablet.

Radar charts rank each parameter on a scale of one to 10, with 10 being best. Note that “affordability” and “thinness” are inverse relationships. Smart-phone and tablet designers might use this kind of thinking to spec a battery. You can use it to decide what the right balance of capabilities is for you, depending on how important each parameter is.

The key is to think of the blue area in the center as a kind of “blanket.” If you tug on one or more corners, the blanket pulls in from the other corners. For instance, consider the difference between choosing a smart phone and choosing a tablet on the same radar chart (Fig. 2).

 

2.  Smart-phone and tablet requirements stretch the fabric of the radar chart in different directions.

The relative weighting may be exaggerated, but the chart approximates the decision profile for tablet owners who already have a smart phone, especially if their budget for the tablet is therefore limited. The smart-phone’s emphasis on run time, quick charging, and thinness results in higher cost and a battery that wears out more quickly, which is okay, given an average device replacement cycle of 12 to 24 months.

Tablets are expected to last longer and support more high-powered applications like video and gaming. Given a previous expenditure on a smart phone, affordability may also be a larger factor. Of course, only you can decide the shape of the “blanket” required for your dream tablet.

Li-ion Pouch Cells

The Li-ion pouch cells used in most smart phones and practically every tablet to date are sealed bags enclosing multi-layered anode and cathode sheets (the active materials) with separators between them, all permeated by a liquid electrolyte that supplies the lithium ions, whose exchange between anode and cathode is the basis of the charge/discharge cycle. Smart-phone batteries are generally single cells. Tablet batteries use several (e.g., three in the Apple iPad).

The tradeoffs in Li-ion battery design are due in large part to the chemical instability of the electrolyte used by virtually all manufacturers. It is based on a lithium compound, lithium hexafluorophosphate (LiPF6), which tends to react with residual moisture left over from the cell manufacturing process to create hydrofluoric acid, perhaps the most corrosive of all chemical compounds. The resulting damage to the active materials causes a host of problems.

Worse, like all chemical reactions, this happens faster at higher temperatures, which are a fact of life for mobile devices with high-powered microprocessors and consumer abuse being but two of the heat sources. Just a short exposure to high temperatures, like leaving a tablet in a car parked in the sun, can dramatically reduce battery performance and lifespan.

Run Time Per Charge

How long before I have to find a power outlet?

Run time per charge depends on the energy density of the battery and its size. Tablets have a natural advantage since they have more space for a big battery. But energy density is a “day-one” specification that starts declining the moment the battery is manufactured, a process that’s accelerated by heat. Generally, the more powerful the electronics, the hotter they run (Fig. 3).

 

3. A heat map shows the hotspots in a typical tablet. Protecting the battery against hotspots requires careful layout and shielding, which increases cost and makes the tablet thicker.

Application Support

Can this tablet support the features and applications I want when I want them?

Applications such as 3D gaming or 4G LTE video streaming, for which tablets are the preferred mobile platform, can affect both run time and battery lifespan because they require high-powered processors that suck more energy and generate more heat. Running multiple applications makes this worse and may produce an irritating cool-down message or an exhausted battery.

Again, tablets have a bit of an advantage, since they don’t generally have to support voice, which, by its very nature, is a constant power drain—even more so in weak signal areas. However, there’s a bigger problem: the sudden peaks in current draw that such applications tend to produce are hard on current Li-ion technology and can impact battery life.

This is a fundamental limitation of the legacy chemistry used in virtually all Li-ion batteries for mobile devices. It is especially a problem for tablets, since these applications make this form factor so attractive. For instance, according to Flurry Analytics, consumers spend 71% more of their time using games on tablets than they do so on smart phones.

Charging Rate

How fast can I get this tablet back to a useful charge level?

The faster you charge a battery, the more you stress it chemically. Fast charging also generates more heat. Both will shorten battery lifespan. If you’re typically using your tablet where power is available, accepting a slower charging rate is a sensible option. Of course, since tablet batteries are larger, with a given charge current and voltage (e.g., 1.5 A at 5 V, which is the typical through a USB 2.0 port), they’re going to take longer to fully charge in any case, making this a hot topic (sorry!) for tablet owners. For instance, the latest iPad has a 42.5-Wh battery and a charger that delivers 5 V at a maximum 2.4 A (i.e., 12 W). Charging a nearly exhausted iPad, then, still may take about eight hours, even with the larger charger.

Battery Cycle Life

How many times can I recharge the battery before I have to replace it or the tablet itself?

Every time you charge and discharge the battery, it loses a tiny bit of capacity, so you don’t get as much run time. Eventually, it can’t keep your smart phone or tablet charged long enough to make it between power outlets. Again, this happens faster when you don’t keep your tablet cool. Since practically all tablets use non-removable batteries to help keep them thin, shortened battery life becomes much more of an inconvenience.

Battery Calendar Life

How long before I have to replace this tablet or its battery, regardless of how much I use it?

Even if you don’t use your tablet, the clock is ticking on the battery, faster when it’s hot and even faster when it’s fully charged. Battery manufacturers ship batteries at about 50% charge because they last longer in unpredictable shipping and storage conditions, among other reasons. It’s also why you shouldn’t use your tablet for a long period of time while it’s plugged in, which exposes a battery at 100% charge to elevated operating temperatures.

Thinness

How sleek is this tablet?

This is where the blanket gets really tight. First, the inevitable decomposition of the LIPF6 electrolyte generates gas that causes the battery to swell. This can amount to 10% to 12% growth over the life of the battery. Tablet designers have to leave space for this swelling, which reduces the battery’s effective energy density and tends to make the tablet thicker. In some instances, the LiPF6 breakdown can cause uncontrolled swelling, or ballooning, that can destroy the tablet or even rupture the battery.

Second, because thinness is so highly valued, virtually every tablet uses a “built-in” battery that relies on the tablet case to protect the soft pouch against punctures, which could result in a fire. This eliminates the weight and thickness of a protective casing for the battery that would otherwise be required for safety and allow consumers to replace the battery. While this strategy may reduce manufacturing expense a bit, the eventual buyer ends up paying for it at the other end when the battery wears out.

Finally, the thinner a battery is, the lower its energy density (Fig. 4). As noted above, shielding the battery against heat-generating components adds thickness as well. It takes really good design to overcome these factors, and that costs money.

 

4. Some components within a battery do not provide energy, such as the separator, protective circuitry, and connectors. As the battery gets thinner, these components take up a greater percentage of the total battery volume, which reduces the energy available for a given space.

Affordability

How well does this tablet fit my budget?

Given current Li-ion battery chemistry, how much you are willing to pay pretty much determines the size of the “blanket” in the charts above. But even then, as the latest iPad demonstrates, there will be tradeoffs. However, improvements in Li-ion technology coming in the next year or so may require you to make fewer tradeoffs to get your dream tablet.

One of these innovations uses a new electrolyte that doesn’t generate hydrofluoric acid. Li-imide technology is heat resistant, delivers longer-lived batteries, withstands high current draw, and eliminates most of the lifetime swelling that forces designers to sacrifice cavity space, making thinner tablets available at a lower price point.

In addition, being more chemically and thermally stable, Li-imide can be used with a wider range of active materials, such as silicon anode (Si-anode), with the appropriate high-density cathode material. This particular chemistry is expected to deliver a 20% or higher boost in energy density, stretching out the blanket so style, convenience, and durability aren’t mutually exclusive in your next tablet.

References

1. ComScore, “Majority of Tablet Users Watch Video on their Device, 1 in Every 4 Viewers Pay to Watch,” www.comscore.com/Insights/Press_Releases/2012/6/Majority_of_Tablet_Users_Watch_Video_on_their_Device

2. GBI Research, “Tablets Closing the Gap on Laptop Market,” www.gbiresearch.com/pressreleasedetails.aspx?title=Technology&prid=144

3. Flurry Analytics, “The Truth About Cats and Dogs: Smartphone vs Tablet Usage Differences,” http://blog.flurry.com/bid/90987/The-Truth-About-Cats-and-Dogs-Smartphone-vs-Tablet-Usage-Differences

Dan Friel is the director of systems engineering at Leyden Energy. He has been involved with consumer rechargeable batteries since the early 1990s while at Duracell when he co-developed the smart battery standards with Intel for laptop computers. Since then, he has worked for multiple firms in the battery and power management industry, including PowerSmart, Microchip Technologies, Honeywell Batteries, PowerPrecise Solutions, Texas Instruments, and now, Leyden Energy. He has authored four book chapters on rechargeable battery technology and has more than 10 U.S. patents. He has a BSEE from Purdue University and a MBA from Boston College. He can be reached at [email protected]

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