I had bought a new iPod within weeks of my coworker: the same generation player, running on the same lithium-ion (li-ion) cobalt oxide battery. She plugged it into her computer every day to get to her music. That seemed like an astonishing mistake—obviously, her iPod's battery would suffer, since it would cycle every day, multiple times during each 8, 10 or 12-hour workday. My player, which I ran down completely before each charge, would burn less cycles, and retain more power in the long run.

A year later, her iPod could barely squeak through a subway ride from Brooklyn to Midtown Manhattan. Mine lasted twice as long with equivalent settings. Soon I was spreading the gospel, advising everyone with a li-ion-powered gadget to follow my example, and keep them unplugged and free from rampant cycling. This was four years ago, when Apple was still shaking off the stigma of underwhelming iPod battery life. No one knew exactly why some batteries lasted, and others didn't. But, I thought, I had guessed right (and I had evidence!).

Forget for the moment that I was wrong. Battery life shouldn't be a guessing game. Extending the life of your laptop or cellphone batteries should be a by-the-numbers, idiot-proof process governed by the laws of physics, and by the chemistry bottled up in those energy-dense lithium-ion cells. It certainly shouldn't be a topic for debate, or fertile ground for conflicting rumors, urban legends, and obtuse manufacturer's guidelines.

And yet, consumer electronics companies offers no true consensus on how to hold off battery death, and the Internet only deepens the confusion. One article claims that li-ion packs should be drained on a weekly basis; another recommends to drain them once a month; others say they should never be drained. The few tips that go unchallenged are often bizarre and unfeasible. No one puts their gadgets in the fridge. No one removes the battery when their laptop is plugged in. And despite Apple's pleas, no one remembers to regularly charge the last-gen iPod they rarely use. Reconcile (or, more likely, ignore) the discrepancies in expert opinion, and string together the entire spectrum of arcane suggestions and anecdotal evidence, and the result is a portrait of li-ion as a fickle, fragile energy storage technology. But there are battery-extending truths in the mix of myths, speculation and red herrings.

Forget the Number of Cycles

Compared to the batteries that they largely replaced, the li-ion cobalt oxide cells used in laptops, cellphones and other compact electronics are complex in construction, and roiling with chemical reactions. Lead-acid cells might store less energy than li-ion, and suffer from longer charge times and battery memory, where a partial drain and subsequent recharge lowers the total capacity. But whether they're full or empty, there's very little activity. Li-ion cells, on the other hand, are in an almost perpetual state of decline. "The deeper the discharge for a li-ion cell, the more damage you do over time," said Tom Hartley, a professor of electrical engineering at the University of Akron, who works with NASA to help extend the life of its batteries. "The fuller the charge is, the more damage you do over time. It likes to sit in the middle of a state of charge."

Tests have shown that raising the voltage of some li-ion cells by a tenth of a volt, from 4.1 V to 4.2 V, will cut its lifespan in half over time. Another 0.1 V boost will cut it by a factor of 3. Little or no charge can result in corrosion in the cells, creating a resistance that impedes the transfer of electrons through the battery. This vulnerability to high and low charge states is far more important than a li-ion battery's total number of cycles. After all, what constitutes a full charge cycle varies by device and manufacturer. According to Isidor Buchmann, founder and CEO of British Columbia-based battery-testing firm Cadex Electronics, most published cycle counts for consumer electronics are based on repeatedly discharging the cells by 80 percent. NASA, on the other hand, typically sets the Hubble's batteries to discharge by some 10 percent, allowing for 100,000 cycles before replacement. Cycles, in other words, are usually a red herring, a comforting statistic that has little bearing on the matrix of factors that determine battery longevity.

Confusing the issue even further is the impact of temperature on li-ion—sub-freezing conditions can kill the battery the instant the device is turned on, while heat might simply drop the cells' capacity. The latter could make the practice of leaving a laptop plugged in during use (to avoid having to use charge cycles, or for convenience sake) an even worse idea. The always-full, actively churning battery is exposed to constant heat from nearby circuitry. The level of potential damage depends on how the laptop is configured, particularly how well it bleeds off the excess energy from a direct power source.

What Research Really Tells Us About Getting Better Battery Life

The list of li-ion killers, while still mired in hidden math and inherently impossible to prioritize, goes something like this: too much charge, too little charge, and extreme heat or cold. There are radical responses, of course, such as taking out your laptop's battery when it's plugged in. But hopefully you aren't using a Mac, with those easily-disconnected (for your safety) magnetic chargers. And obviously you remembered to charge the battery to 40 or 50 percent capacity before detaching it.

For the less-obsessive, or anyone with an iPod, iPhone or other device with built-in li-ion, extreme measures are not an option. The only decision is how often, and how much, to charge. "For phones, I always advise people to charge more often," Buchmann says. "But with a laptop, not enough testing has been done. It's sort of a toss-up." To date, no significant testing has been conducted to determine the least destructive charging regimen for li-ion-powered gadgets in general. Buchmann and others see heat as a negligible factor in phones and other small devices, but even that is more educated guesswork than anything else.

Here's what we do know: The safest life for lithium-ion is one of moderation. The only way to minimize all of the factors that eat away at capacity is to pretend that there's less effective capacity. Never charge the batteries to 100 percent, and never let them come close to empty. The narrower that band of charge and discharge, the better, since what amounts to the cells' metabolic activity slows at 40 to 60 percent capacity. Chevy takes the choice out the user's hands, keeping the Volt's battery within a range of 20 to 80 percent charge. Buchmann has no proof, but suspects that the higher cycle counts of Apple's newer built-in laptop batteries use a similar approach, cutting off the voltage to cells before they reach 4.2 V. Whether it's true or not, that appears to be the best strategy for the current generation of li-ion (particularly the cobalt oxide variety found in consumer electronics). Ignore cycle count, and focus on keeping gadgets as close to half-full as circumstances permit. Full batteries are fine for travel, dead batteries are an unavoidable setback, but for day-to-day operation, let the Goldilocks standard be your guide.

Here's one other thing we know: Leaving li-ion plugged in is a problem. Even when persistent heat is not an issue, a constant state of charge is tantamount to working your batteries to death. I was wrong about my coworker's iPod—it wasn't cycling up and down while connected to her computer, sent to an early grave by Apple's well-intentioned requirement that all dock connections supply power while transferring data. What sent her iPod to an early grave was too much living, a constant state of charge that burned the battery at both ends.

At least, that's my best guess.

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