Tuesday, February 9, 2010

Thoughts on Batteries and Laptops

Battery life has two dimensions: (1) how much charge the battery can hold, and (2) how this capacity changes over time.

In my own experience, I wore out a 13" PowerBook G4 battery in 13 months, bought a replacement, and saw it wear out again in about 14 months just by keeping the machine plugged in and sitting on my desk most of the time. After the 2nd replacement battery wore out (held less than an hours charge), I got more interested in what was going on and made some changes. The most visible change was keeping my laptop on a cooling stand.

When fully charged, Li-ion batteries have a limited shelf life and lose around 20% of their capacity per year. High charge levels and elevated temperatures hasten permanent capacity loss.


I was stunned to learn this, but it matches my own experience. The combination of normal laptop running temperatures (warm), and a fully charged Li-Ion battery was less than ideal. Contrary to popular wisdom, keeping Li-ion batteries fully charged as much as possible shortens their life.

Where did this popular wisdom come from? First, it is vital not to allow Lithium Ion batteries to completely discharge as this can create a safety hazard and will shut down the battery permanently. For this reason, the power system is designed around the battery used and shuts down when the charge becomes dangerously low. Second, deep discharge cycles are also known to hasten capacity loss. Since there is no "memory effect" with Li-Ion, waiting until your battery is nearly discharged is undesirable. For a typical portable device, this means it's best to charge the battery early and often.

But not all portable devices are used as portables anymore. A glaring omission from typical laptop systems is a feature to maintain the battery at less than fully charged to extend its useful life. My laptop easily spends 90% of its waking hours plugged in. I simply don't need the battery to be fully charged most of the time. As laptops have come to replace desktops for many users, the design of the battery system has failed to keep up with how it is often used. The ideal charge level for storing a Li-ion battery is around 40%.

One of the challenges Toyota faced in designing the Prius was battery longevity. Part of the solution they adopted was to maintain the battery between 40-60% charge as much as possible. The Prius uses nickel metal hydride batteries, but the principle is similar. Batteries last longer if you treat them gently.

You might be tempted to remove your laptop battery completely, but this is not a good idea. Modern laptops use the battery to smooth out spikes in the power consumption. If you remove the battery, the machine will respond by dropping the CPU speed to bring the entire system within the upper limit of the power supply.

Apple's latest designs use integrated Li-polymer batteries, so removal is impractical anyway. I don't know how Li-polymer batteries compare exactly, but the principle should still be valid. It would be helpful to make a distinction between charging for maintaining battery capacity (useful lifetime), and charging for the longest run time.



iPhone 3GS Lithium-Ion Battery Life


  1. Peter:

    Do you agree with Apple's advice, as posted at this URL: http://www.apple.com/batteries/notebooks.html

    Here's what it says on that web page:
    "For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her MacBook Pro on the train, then plugs it in at the office to charge. This keeps the battery juices flowing. If on the other hand, you use a desktop computer at work, and save a notebook for infrequent travel, Apple recommends charging and discharging its battery at least once per month."

  2. Discharging the battery occasionally helps recalibrate the laptop's battery sensor to recognize how much charge the battery can hold and when it is approaching empty. It does seem reasonable to use the battery occasionally to keep things running smoothly, and I do use my laptop away from my desk from time-to-time.

    Never the less, the battery system should be designed to accommodate the user rather than impose awkward requirements. Where's the option to cycle the battery once a month, or maintain it at 40% charge if you are not planning to travel any time soon?

  3. Peter:
    Thanks for your comments about battery life.
    "Never the less, the battery system should be designed..."
    Sounds like a project for someone with your credentials?

  4. Perhaps, but I think it would need to be done from inside Apple. I don't know of any published APIs to manage the battery system at that level. Suggestions are always welcome.