Guest post written by Noam Kedem
Noam Kedem is VP of marketing for Leyden Energy, a Fremont, California-based company that makes batteries for consumer electronics, electric vehicles and storage applications.
Speculation about the design of Apple’s much-anticipated iPhone 5 got a big boost from the launch of the new iPad. It was apparent that Apple had faced some rather stringent design tradeoffs between feature set, form factor and battery life. The company carried it off with its usual engineering panache, but even so, the new iPad is a bit thicker and heavier than its predecessor, and with less battery life.
What does this portend for the iPhone 5?
Others can speculate more ably on the display and processor choices Apple faces. What I’d like to focus on is the battery. From my perspective, a fundamental problem is that while Apple can count on ever-increasing performance in the silicon involved, the lithium-ion (Li-ion) batteries that power virtually all mobile devices are practically standing still: they use the same chemistry platform as they did 20 years ago. Absent a change in battery chemistry, Li-ion is going to impose some limitations on where Apple can go with the iPhone 5’s design and spec sheet.
The new or upgraded features expected in the iPhone 5 that will require the most from the battery are the display, 4G LTE wireless connectivity and a more powerful processor. Leaked images show the Retina display growing from three and a half to four inches; not surprising, since the average Android phone display is now over four inches. 4G LTE connectivity seems like a no-brainer, too, given the demands of mobile video, iCloud features and the Android competition. We can also expect a faster version of the A5 processor with even more powerful 3D graphics.
Each of these new features can end up drawing more power and generating more heat. Both of those are challenges for Li-ion technology. That’s why the new iPad’s battery ended up being some 70 percent bigger and heavier than its predecessor yet still offers somewhat shorter battery life, and why Apple faces some difficult choices for the iPhone 5. The iPhone 5 battery is going to have to be notably bigger than its predecessor. Even with the increase in battery’s X and Y dimensions made possible by a larger screen, the result could still be shorter battery life—in terms of run-time per charge, cycle life and calendar life.
This is because Li-on battery technology is falling short in two areas: energy density and thermal sensitivity. Energy density determines the amount of run-time you can pack into a give size (volumetric) or weight (gravimetric) of a battery. Unfortunately, Moore’s Law doesn’t apply to batteries. Since the first Li-ion batteries hit the market in 1991, the transistor count in the devices they power has increased a thousandfold in response to consumer demand for more features and higher performance. Li-ion batteries have eked out a mere 3X increase in their volumetric energy density in that same period, and battery manufacturers are having a harder and harder time squeezing more energy into them.
Increased packaging efficiency is one way of getting higher energy density. For instance, the non-removable Li-ion pouch cells now used in the majority of smartphone models eliminate the protective casing needed for user-replaceable batteries. They’re just a sealed bag containing carefully stacked or wound anode and cathode sheets, separators between them, and—permeating all of these layers—a liquid electrolyte. By relying on the smartphone case for protection, there’s more room for the active materials that actually store energy.
Packaging efficiency is where Apple may have a slight advantage when it comes to the iPhone 5. There are two ways to place a battery in a smartphone. One way is two layers of electronics (screen and circuitry) with a space “carved out” for the battery. This is the approach taken in the iPhone 4S. The alternative is to use three layers: screen, circuitry, and battery, as in the Motorola RAZR line (both RAZR and RAZR MAXX).
At first glance, it would seem that the three layer approach, by permitting a larger battery, would deliver longer run-time, but the narrower battery in the carve-out approach actually offers higher energy density. Li-ion pouch batteries have a built-in printed circuit board that is connected to the positive and negative terminals of each cell and provides active protection against short circuits, overcharge and forced discharge. A narrower, rectangular battery can put this PCB on the short edge, leaving more room for the active materials and thus delivering a somewhat higher energy density than a square battery.
My overall estimate is that maintaining the carve-out approach - and with it a narrow X/Y ratio for the battery in the new iPhone similar to that of the iPhone 4S - would offer a limited yet important battery energy density advantage over going to the three layer approach with a more “square-ish” battery.
The other factor here is bulk. Going to a larger screen means that Apple will probably try to keep the iPhone 5 as thin, if not thinner, than the iPhone 4S, as indicated by recent leaks about a thinner screen with integrated touch sensors. This will be easier to do with the carve out approach Apple presently uses, especially given the company’s skill in reducing the size of the PCB in each succeeding generation of iPhone and iPad (generally by folding more functions into each chip and using newer manufacturing processes). This approach makes more room for a battery that nonetheless maintains the optimal X/Y ratio and thickness to maintain maximal energy density and thus run-time per charge.
The other major battery design problem, thermal sensitivity, also militates against a three-layer design. The standard Li-ion battery chemistry depends on a chemical compound that unfortunately reacts with residual moisture to create hydrofluoric acid, the most corrosive of all chemical compounds. Like all chemical reactions, this process doubles in speed with every 10° C increase in temperature, and smartphones run hot.
The hydrofluoric acid gradually degrades the battery, resulting in reduced cycle and calendar life. Each charge and discharge cycle reduces run-time until the battery just doesn’t last long enough between charges. Even without cycling, battery age reduces run-time. Heat makes all this happen faster, which is one reason why consumers too often end up “locked out” of their phone until it cools down.
In addition, charging or discharging a battery also generates heat. The higher power requirements of the new features in the iPhone 5 will mean faster discharge. Consumer impatience requires making charging happen as fast as possible, but the more powerful battery needed in the iPhone 5 will naturally take more time to charge when using the same charger as the earlier iPhones. Trying to reduce charge times with a more powerful charger would likely generate even more heat and further shorten battery life.
With a three layer design, the battery lies over every heat generating component. Metal heat shields or thermal heat spreaders can be used, but this adds weight and thickness. A two-layer design makes it just a bit easier to keep the hottest parts of the device away from the battery. A miniscule advantage, perhaps, but Apple is known for squeezing every last bit of engineering advantage out of its designs to maintain the Apple experience. (Even so, the new iPad’s battery has a built-in thermal spreader, underscoring the sensitivity of current Li-ion chemistry to heat.)
Looking farther down the road (beyond the iPhone 5), there are certainly other ways Apple could better tackle the double-whammy of energy density and thermal sensitivity. An interesting Apple patent reveals ways to pack battery material into places, like the iPhone bezel, that are presently impossible to use and farther from heat-producing components.
More promising are possible changes in the battery chemistry, where there’s a lot of ongoing research and some new solutions already on the market. One of these, Li-imide, doesn’t generate hydrofluoric acid and thus delivers a dramatic improvement in thermal stability and battery life. It also permits effectively thinner batteries by eliminating most of the swelling in thickness characteristic of current Li-ion pouch cells over their useful life, which forces designers to sacrifice cavity space to accommodate the swelling.
Given how fast the consumer electronics market moves, and how hard it’s getting to stay ahead of the competition, one would hope that the iPhone 5 will represent the last time Apple relies mainly on improvements in electronics miniaturization and packaging to maintain its dominance. The iPhone 6 should have at least one breakthrough technology in it - and it might just be the battery.
In the meantime, the iPhone 5 will squeeze the ultimate out of the two-layer carve-out approach, the greater real estate offered by a larger screen, and additional PCB density. There will be more area (X*Y) for the battery in the optimal ratio for the sake of energy efficiency. This is a necessity if Apple is to at least maintain the same run-time per charge as the iPhone 4S, as I think they must.
All of this suggests that Apple may succeed in further slimming down the new iPhone even with a higher-capacity battery, thanks, in part, to the optimal X-Y ratio of the carve-out design. It also implies that the phone will be heavier due to both greater screen size and the bigger battery needed to satisfy the dazzling graphics performance and improved application response that Apple users will be expecting in order to pull the upgrade trigger.
