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Editor's note: Apple's recent acknowledgment that it slows some older iPhones because of battery issues has spurred reader interest in battery technology. This story from our archives explains the science of batteries.

When Apple redesigned the MacBook Pro in 2009, it unveiled a new type of battery that ran a whopping 40 percent longer than the previous model.

The laptop lasted as long as seven hours, almost enough time to watch the epic movie "Lawrence of Arabia" -- twice. Phil Schiller, Apple's marketing chief, called the battery "revolutionary." But was it really?

Technological leaps over the past two decades have been astounding. Computers have transformed from utilitarian boxes into svelte rectangles of shiny metal and glass that fit in our pockets. Today's devices are also far more powerful. A new smartwatch has more computing power than the Apollo moon landing spacecraft. Batteries are a different story.

Even though consumer electronics makers, from Apple to Samsung, pour millions of research dollars into eking out more battery life for devices, the technology isn't expected to advance much in the next few years. But that won't slow the rising tide of gadgets that rely on batteries.

Why battery tech has stagnated is a topic of debate among researchers, many of whom claim we're reaching the limits of what science can muster. No matter the reason, consumers will need to find ways to squeeze more juice out of their battery-powered devices.

Two evolutionary trails

To understand what's going on, consider where battery makers have been, where they are now, and the challenges they face.

Michael Sinkula of Envia Systems, an advanced battery startup in California, crunched the numbers and found the energy stored in a battery in 1995 didn't double until more than a decade later, in 2007. Since then, a battery's energy hasn't even risen by 30 percent. And Envia believes most batteries likely won't have doubled again even by 2021.

But a typical laptop now runs about 10 hours, up from just four hours when President Barack Obama was sworn in for his first term. How's that possible?

Tech advancements generally come from two separate forces: a relentless drive to shrink every part's size and ever improving software to manage it all.

The brains of a computer are its microprocessors, the chips that do the complex math needed for drawing images and for helping Facebook update you about a friend's birthday. For decades, the industry has been shrinking processor size. As they get smaller, they consume less energy, and battery life gets longer.

Batteries are different. Basically, they're collections of metals and chemicals. When they're connected, electricity flows. The problem with chemistry is that making it smaller doesn't always make it better. Think of it like a drink: if you put less beer in your mug, you just have less beer.

Until now, major battery advances came from using new materials. Consumer electronics batteries began lasting longer when they switched from relying on nickel, a type of metal, to lithium.

John Goodenough, a key scientist in the development of modern batteries, says research now is focused mainly on improving lithium batteries. "The periodic table is limited," he says, and advancements are becoming increasingly tough.

Even though more people are working on these problems than when Goodenough announced the breakthrough that made modern batteries possible in 1979, scientists are simply running out of new stuff to work with.

A smartphone that lasts a week -- instead of a day -- requires a radical new technology that hasn't even made it to the drawing board. "The strategy for the next step isn't here," Goodenough believes.

It's possible that in 250 years, when Capt. James T. Kirk hails the starship Enterprise, his communicator may need a recharge first.

The path to lithium

Modern batteries date back to the 18th century, when scientists stumbled upon a way to harness static electricity by inserting a metal rod into a jar coated with foil on both sides and filled with saltwater. Touch the outside of the jar with one finger and the rod with another, and -- zap!

In his book "The Battery: How Portable Power Sparked a Technological Revolution," Henry Schlesinger describes scientists who played with these devices, known as Leyden Jars. One prominent tinkerer was poet Percy Bysshe Shelley. When he was young, he experimented with help from his sister. He also inspired his wife, Mary, who used electricity as a primary plot device in her novel "Frankenstein."

Shortly before people were reading about Mary Shelley's monster, Alessandro Volta invented the first widely used battery, the Voltaic Pile, by stacking plates of zinc and copper separated by cloth or cardboard soaked in saltwater.

Today's batteries haven't changed much. Cut one in half, and you'll see a material made of metal, such as lithium, on one side, and another material, typically carbon, on the other. In between is the equivalent of the cloth Volta used 200 years ago: a plastic surrounded by a gel or a liquid designed to keep the metals from interacting with one another, but that still lets atomic particles move around.

Mark Hobbs / CNET

When a connection, or circuit, is created by touching a wire from one side of a battery to another, electrons flow out, and the light bulb turns on, the stereo blasts sound or the car's lock beeps.

For today's devices, the most popular rechargeable battery, lithium-ion, has been widely used for about two decades.

A market surge

Batteries are the lifeblood of tech. In 1990, just as lithium-ion was poised to flood the market, worldwide demand for batteries reached nearly 200,000 megawatt-hours, according to estimates from consulting firm Avicenne Energy. That's the equivalent of 44.4 billion Energizer Ultimate Lithium AA batteries, enough to circle Earth nearly 57 times.

By 2013, just two decades later, demand had nearly doubled.

Lux Research predicts that spending on batteries to power electronic devices alone could reach $26.6 billion by 2020, up nearly 30 percent from this year. Most of that demand will come from phones and tablets, with both expected to jump about 45 percent over the next six years. Battery spending for transportation, such as cars, will double to $20.9 billion.

Given all the money at stake, many researchers are working to improve batteries. Even so, few breakthroughs have materialized. Plus, almost all major research has shifted to cars and power grids.

Enterprise tech giant IBM, for instance, has a team of scientists at its Almaden facility in San Jose, Calif., working on battery tech. In 2009, IBM pledged $500,000 and a few researchers to work on what it calls Battery 500: an attempt to invent batteries to propel a car 500 miles. That's enough to go from San Francisco to Los Angeles on a single charge and have some juice left for a trip to the beach.

A key focus is a so-called lithium-air battery. Instead of relying on carbon and other metals, as in a lithium-ion battery, IBM and its partners believe they can create a container filled with air that interacts with a piece of lithium to produce electricity. If they're right, it could potentially halve the weight of a battery.

But there's a hitch: to keep the energy consistent and enable recharging, you need pure air. The air we breathe is filled with pollutants and water.

"You would need machinery to clean the air," says Winfried Wilcke, a researcher leading IBM's battery efforts. That adds size, weight and complexity.

Others, including researchers at the Massachusetts Institute of Technology and the University of Texas, are considering materials such as silicon, sulfur and sodium. But many R&D efforts are targeting these designs for cars first. It will likely be years before such tech powers consumer electronics.

As for efforts to improve lithium-ion batteries, Stanford University in July said it created a battery with pure lithium that can hold more energy. But this battery still has a long way to go as well.

Some scientists paint a dire picture, saying we're hitting the limit of what a battery can do and how much it can improve. "There is no order of magnitude to be had," Wilcke says. Others, like Bill Watkins, head of battery startup Imergy Power Systems in California, are more hopeful. "Never underestimate a bunch of Ph.D.'s with a lot of money," he says.

Dealing with today's realities

The good news is that companies are finding ways to extend battery life while they wait for new battery technologies.

At Apple, many improvements are coming through software. Its OS X Mavericks operating system, released in late 2013, looks for moments when computer users have several programs open that they're not accessing. The Mac then strategically reduces the processing put toward running programs in the background. Overlay a window on top of a movie playing on YouTube, for instance, and the sound continues, but the video stops updating.

Inside Apple's software is a technology it calls "timers," which wake up the computer's chips from low-power mode so they can complete certain tasks. As Apple's software team built Mavericks, they realized they had too many timers waking the machine too often. Combining them reduced the processor's activity by 72 percent, making the computer more efficient.

OS X Yosemite, released this year, adds even more power-saving features. One tweak lets users get up to two more hours of battery life on a MacBook Air when streaming movies in full 1080p HD.

Mark Hobbs/CNET

Companies are making similar strides with mobile devices. Samsung created an "ultra power-saving mode" that can allow up to 12.5 days of battery life for its Galaxy S5 smartphone. The mode switches the screen from color to black-and-gray images and also limits apps, phone calls, messaging and basic Web surfing.

Some companies, including Samsung SDI, are trying to make batteries safer and more robust. Samsung is working on a type of battery that replaces the gels and liquids of today with solid material. The company hopes to make batteries safer, flexible and less likely to explode. It aims to deliver these batteries by 2015.

Apple, meanwhile, also has focused on squeezing as much battery into its devices as possible. In 2009, when Schiller announced battery life breakthroughs for MacBooks, he showed how underneath the hood the typical brick-shaped battery had been replaced by ones designed like puzzle pieces to fill every available space.

As part of that design, Apple eschewed its decades-old practice of letting consumers swap out the battery. This created even more space that would otherwise have been taken up by battery housing and protective shells built to keep the battery safe while outside the computer.

The result: a battery Schiller could call a revolution. Maybe that's as revolutionary as it gets.

This story first appeared in the winter 2014 edition of CNET Magazine and was first published online in December 2014.