CHARGING YOUR LAPTOP

Whether they're in our computers, cell phones, or cars, the only time we think about batteries is when they're almost dead and we need to find some place to charge them—and then we're not thinking nice things. Batteries are an old-school technology. We stuff them into gadgets that are always getting smaller, faster, and cheaper, but battery technology doesn't yield to Moore's Law. What we know about batteries today is pretty much what we knew about batteries back when ENIAC was invented. As a result, batteries remain a primary limiting factor in our machines; they're the reason we don't have better cars, why your smartphone won't play a two-hour movie, and why your otherwise perfectly functional three-year-old laptop is useless on a plane trip.

Our daily struggle with batteries has spawned a cottage industry of advice about their proper care and feeding. Some argue that the way to get the most juice out of your gadget is to charge it as often as you can. Others caution about the sin of overcharging; this school holds that batteries are happiest when they're run down to zero every once in a while. Dig deeper into this line of thinking and you find its proponents are most concerned about a battery-destroying phenomenon known as the "memory effect"—a worry that if you keep re-charging your battery before the juice goes down to zero, it will gradually lose capacity. A related annoyance is your gadget's battery meter; the more often you charge and recharge your iPod or your cell phone, the more inaccurate its fuel gauge seems to become.

To clear up these annoyances and conflicting theories, I called up Isidor Buchmann, the CEO of Cadex Electronics, a Canadian company that makes battery-testing equipment. Buchman also runs Battery University, a very helpful Web site for battery enthusiasts and engineers. I asked Buchmann how we can make sure that our batteries last a long time. "There is not too much to discuss," he began, and then launched into a conversation exploring the numerous frailties of batteries. The upshot is this happy factoid: No matter what you do, your battery will become a useless piece of junk—one day it will reach a point where it can no longer be charged, and then you'll have to recycle it. It will die if you use it often. It will die if you hardly ever use it. It will die if you charge it too much. It will die if you charge it too little. You can pull the battery out of your camera, stuff it under your mattress, and come back for it in five years. Guess what? Your battery will be dead. And when I say dead, I mean dead—not that it's run out of juice, but that it can no longer hold a charge.

That said, there are ways to prolong your batteries' lives. Here are some of Buchmann's tips:

Laptops: The typical lifespan of a lithium-ion laptop battery is about 18 months to 2 years, Buchmann says, but yours will last much longer if you don't punish it too much. The main stresses include undercharging, overcharging, and one that few of us consider: heat. Temperatures inside a laptop can reach more than 110 degrees Fahrenheit, which is hell for a battery.

Ideally, Buchmann says, you should try to keep your battery charged from 20 percent to 80 percent. Keep in mind that these are guidelines for ideal use—it's generally inconvenient to unplug your machine before it goes all the way to 100. But even if you're not on constant guard, be mindful of charging your machine constantly, well past when you know it's full. You also should be conscious of letting your battery run all the way to zero.

Try to keep your laptop as cool as possible. The best technique here is to charge up your battery when the computer is turned off. When your laptop is turned on and plugged in, you should pull the battery out of your computer. Yes, pull it out. "I know that's inconvenient," Buchmann says, "but keeping your laptop plugged in when the battery's fully charged—that combination is bad for your battery."

What if your battery can't be removed, as in Apple's latest laptops? Buchmann says that new laptop designs often place batteries in cooler parts of the machine, and they may also include some intelligence to limit the stress of a fully-charged battery—that is, the machines stop applying a charge when the batteries are just under full capacity. This prolongs the battery's lifespan. The trouble is, most laptop manufacturers keep their charging algorithms secret, so it's not possible to know which laptops use this battery-prolonging technique. (Bachmann assumes that Apple does so based on its battery-promoting advertisements.) But even if your computer is clever enough to stop charging the battery when it's full, you're still better off removing the battery if it is removable. This will help your battery beat the heat.

Phones, cameras, and other gadgets: In general, the rules are the same as for laptops: Keep the battery in the 20 percent to 80 percent range, and keep it cool. If you leave your phone on a hot car seat all afternoon, or if you run down your camera at an all-day trip to the zoo and then forget it in a sock drawer for six months—well, somewhere a little battery angel loses its wings.

The best way to store batteries that you won't be using for a long time—as in a camera, though this also applies to laptop batteries—is to charge them to the 40 percent level first, Bachmann says. Batteries "self deplete"—meaning they lose power even if they're not in use. Charging the battery a little bit before you put it away ensures that it doesn't get down to dangerously low levels while in storage.

The memory effect: Will your battery lose capacity if you don't let it go down to zero every once in a while? Not likely. The memory effect applies only to nickel-cadmium batteries, whereas most modern electronics use lithium-ion or the more advanced lithium-ion polymer. Not only are lithium batteries immune to the memory effect, they also don't require you to do anything special the first time you use them (like charge them up for 24 hours, as some gadget manuals say). Nicad batteries are still found in cordless phones, electric toothbrushes, and other cheap gadgets, but they're usually pretty inexpensive to replace.

How come my battery gauge is off? Making an accurate battery gauge is much more difficult than measuring how much gas you've got in your car. Since a battery's capacity is constantly decreasing, your gauge will likely get less accurate the longer you own your gadget. The length of a charge also depends on impossible-to-predict environmental factors like temperature. "The technology to measure batteries is just not that good," Buchmann says. "We can't do it. It's that simple."

Most battery meters in electronics work by monitoring electrical inflows and outflows—in other words, how long you charge and how long you use your device. They also try to guess how your battery may have become degraded over time. But each time the computer makes such a guess, it adds errors into the calculation. These errors build up over time, and eventually you notice your laptop dying even though the battery meter says you've got 40 minutes left.

To solve this, you should occasionally "calibrate" your charge meter by depleting your battery completely, then charging it up fully. This usually resets your machine's "flag" for your battery's capacity. Of course, running it down and then charging it up again puts more stress on your battery, accelerating its death. But there's no getting around that.