40-80 rule: New tip for extending battery life

Extend the life of a lithium cell by not topping it off. The 40-80 rule can extend the life of your battery, but be careful to keep it out of the heat.

The rules for ensuring that your electronics enjoy a long and healthy life have changed. Years ago, smart gadget owners would fill up their rechargeable batteries to 100 percent and then run them dry. Regularly topping off and emptying out batteries kept them limber even in old age.

But most people should now ignore that rule. The pro tip applied to nickel-based batteries, which companies have largely phased out of phones, laptops, and rechargeable accessories. Today's lithium-ion batteries prefer a softer touch.

To keep your gadgets humming for many years to come, stop charging them from zero to 100 percent, says Cadex Electronics, which designs power-monitoring systems. Instead, keep the battery meter between 40 percent and 80 percent.

Research shows that living to the extremes can wear out lithium batteries. Each time you plug in a gadget, it holds onto a little less energy than it did the time before. That's why laptops that once lasted for hours on a single charge eventually demand regular refills. After a certain number of recharge "cycles," the battery becomes weaker and weaker.

The average lithium battery can go from empty to a full charge between 300 and 500 times before it expires, according to Battery University, an online clearinghouse for battery research. But using the full spectrum of a lithium battery actually causes stress, reducing its longevity. That same battery could have lasted through 1,200 to 1,500 fill-ups if it went from zero to a 50 percent charge. By filling it up only halfway, you triple or quadruple the number of recharges.

High temperatures can exacerbate the problem. Leaving a laptop in a room that's 100 degrees F. (or leaving it on and plugged in so that it heats up to 100 on its own) can permanently reduce the capacity of a battery by 35 percent over the course of a year.

Of course, for some people, keeping their batteries locked between 40 percent and 80 percent is no more practical than only driving after midnight, when traffic is lighter. That's OK. You won't ruin a battery by filling it up. Many smart phones are designed to last a full two years even with leaving them plugged in overnight. The 40-80 rule is simply a way to squeeze the most out of your electronics.

Some laptops from Lenovo, Samsung, Sony, and others come with an optional battery saver that stops charging once it hits a certain percentage. And the Belkin Conserve Socket ($9.99) will cut power to a device after a set amount of time, allowing people to charge devices overnight without maxing them out.

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[Editor's note: The original subhead misstated the benefits of only charging batteries half way. Doing so will extend the life of the battery.]