Battery of Questions on Batteries

So I’ve had several people ask me about battery charging in laptops recenetly and rather than respond to another individually I decided that I’d respond to this one via a blog post. This will get the question answered and it’ll result in a new blog post on a blog that hasn’t been updated in…. a long time.

The complete set of questions which cover most people’s quandaries on the topic follow:

[I]f the battery only has a limited number of charges, it is best to let it run down completely every time you unplug it at all, even if you’re only driving home from work and the computer is sleeping, or even if you just move the laptop from one outlet to another across the room? Also, since it’s best to unplug phones as soon as they are done charging, is it also best to do that with laptops? Does the laptop run off battery or wall power when it’s plugged in?

So the first question here is effectively should you always complete discharge your battery before charging it up again. The answer is pretty much a no. Modern Lithium Ion batteries (which nearly all portable electronics with built in rechargeables use) are far more effective than older battery types. This is especially true for laptop batteries. Laptop batteries include hardware inside the battery pack to monitor current capacity and store various statistics so the computer can use the battery more effectively. Every once and a while it doesn’t hurt to let the battery completely discharge and recharge as this will give that circuitry a full charge cycle to judge the battery’s effectiveness off of, otherwise it’s not a necessity.

A number of people have heard that fully discharging and recharging batteries can extend their lifetime. This isn’t the case with lithium ion batteries in laptops. Lithium ion batteries’ lifetime are measured in charge cycles. A charge cycle is a complete discharge worth of use and a complete charge worth of charging. That can mean a charge cycle is a full discharge and charge or it could mean four quarter discharges and charges, or two half discharges and charges or any combination you can think of. Fully discharging your battery just because you happen to drive home with it asleep in the car is going to result in unnecessary charge cycles and thus a shorter battery lifetime. Most recent (although not the very most recent) Apple laptops are rated for 300 charge cycles. 300 charge cycles may not be a lot but for a frame of reference I use my laptop relatively heavily and my year and a half old battery only has 95 charge cycles on it. If you have an Apple you can find out for yourself by opening System Profiler and going to the Power section.

What happens when the battery hits 300 charge cycles? It’s just not as effective anymore. Again using Apples as an example after 300 charge cycles all but the newest Macbooks and Macbook Pros’ batteries should still be able to provide 80% capacity after 300 charge cycles. Unessesaraly discharing and recharging a laptop’s battery is simply going to result in the computer needing a new battery sooner.

The second question (Should laptops be unplugged after charging like cell phones?) is partially answered by the above answer but not quite. First off, cell phone batteries are typically designed to last far longer and can because cell phones need much less power than a laptop. The reason it’s recommended to unplug your phone after charging isn’t because it harms the phone (or the charger) it’s simply because it draws more power than is needed. Some (and many non-smart phones) cell phones and other smaller portable devices are actually designed in such a way that the battery is continuously used and the charger is simply recharging the battery even while you’re using it plugged into the wall.

This isn’t the case with laptops. Laptops will run off of the power adapter (third question handled!) while plugged into the wall and charge the battery if needed at the same time. Once the battery is charged the adapter simply powers the computer and the battery is left to be ready when you need to move away from the wall.

Questions ANSWERED!