NJGPA Practice Test 11

Why Your Battery Gets Tired — by Gabriel Ashdown

¹Every phone owner knows the moment. The device that once lasted from breakfast to bedtime starts begging for a charger by mid-afternoon. Then by lunch. The screen still glows and the apps still open, but the battery seems to have aged faster than everything around it. It has—and the reasons are partly chemistry, partly habit and partly economics.

²Nearly all phones, laptops and electric cars run on lithium-ion batteries. Inside each cell, lithium ions shuttle between two electrodes: toward one when the battery charges, back toward the other when it powers a device. In a perfect battery, that shuttle could run forever. In a real one, every trip costs a little. Side reactions slowly build a thin film on the surface of an electrode, and that film traps some of the lithium that would otherwise carry charge. Tiny cracks form in the electrode materials as they swell and shrink. Each change is small, but the changes add up, and the battery's capacity—the total charge it can hold—shrinks.

³Engineers usually describe that decline in terms of "charge cycles." One cycle means using an amount of energy equal to the battery's full capacity, whether all at once or in pieces; draining a battery from 100 percent to 50 percent on two different days adds up to one cycle. Many manufacturers design phone batteries to keep roughly 80 percent of their original capacity after several hundred full cycles. For someone who charges a phone once a day, several hundred cycles go by in a couple of years.

⁴But cycles are only part of the story. Two batteries with the same number of cycles can age very differently, as the chart shows. Heat is the biggest enemy: high temperatures speed up the side reactions that consume lithium. A phone left on a car dashboard in July, or charged under a pillow, ages faster than one kept cool. Keeping a battery at 100 percent for long periods also adds stress, as does draining it all the way to empty. Fast charging, which pushes current in quickly, produces extra heat.

⁵This is why many newer phones include settings that pause charging at 80 percent or that learn an owner's routine and finish charging just before the morning alarm. These features do not make a battery last forever, but they slow the clock. A battery that spends most of its life between roughly 20 and 80 percent, at moderate temperatures, typically lasts noticeably longer than one that is regularly overheated and topped off.

⁶Here is where chemistry meets economics. A tired battery does not mean a broken phone. In most cases the processor, screen and camera work as well as they did on the day of purchase; only the battery has worn. Replacing a battery usually costs a fraction of the price of a new phone. Yet many people replace the whole device, partly because battery replacement can seem difficult or expensive and partly because a phone that dies by lunch simply feels old. Battery replacement, once hard to find, is now offered by many repair shops and by some manufacturers themselves.

⁷That choice has consequences beyond one household's budget. Building a new phone requires mining and refining metals, and most of a phone's lifetime carbon footprint comes from manufacturing it, not from charging it. Keeping a phone in use for an extra year or two is one of the simplest ways to reduce that impact; a phone kept for four years instead of two spreads its manufacturing footprint over twice as much use. Some governments have taken note: the European Union, for example, has adopted rules that will require the batteries in many portable devices to be removable and replaceable by their owners.

⁸None of this means you should fret over every percentage point. Batteries are consumables, like tires or the soles of shoes; they are meant to wear out eventually. The goal is not to stop the wear but to understand it, so that when the afternoon-charger moment arrives, you know whether you need a new phone—or just a new battery.