By Paulo Rodrigues
It’s 11:30 AM. You’ve just sat down for a fresh cup of coffee, you fire up your web browser, and BAM:
You’ve only been working for two and a half hours—and let’s be honest, not all of that was working—but your laptop’s battery is already about to run out.
So why is my laptop battery running down? And how long should a laptop battery last?
Even when a laptop is brand new, it’s possible to run its battery down very quickly. One culprit is often capacity: laptops use batteries that are relatively small. Consider the battery capacity (measured in watt-hours or Wh) of these products:
Tesla Model 3 LR: 75,000 Wh
EGO electric lawn mower: 420 Wh
4 Energizer D cells: 126 Wh
Apple MacBook Air: 50 Wh
Apple advertises the MacBook Air as having up to 12 hours of battery life, and it’s easy to meet that advertised number. However, 50 Wh of capacity is not a lot. A 60 watt light bulb would deplete a MacBook Air’s battery in just 50 minutes. Achieving 12 hours of battery life in a laptop requires managing power consumption very carefully.
Laptop screens, though bright, use far less power than an incandescent light bulb. In fact, the most power-hungry component of modern laptops is not the screen, but the CPU. The brain of the computer, your laptop’s CPU is responsible for general data processing. Your web browser, for example, uses it to convert chunks of raw digital data into beautifully rendered web pages, helping you to waste hours of your weekend when you probably should have gone outside. All this capability requires electrical power, and some laptop CPUs can use upwards of 45 watts.
If CPUs are so power-hungry, why do they advertise 12 hours of battery life?
Consider a remarkable bird, the Common Swift. Migrating between Europe and southern Africa, it has been observed to fly continuously for up to 10 months at a time. All birds need to sleep, so how does the Common Swift sleep while staying in the air? It uses an amazing process called unihemispheric slow-wave sleep, putting one half of its brain to sleep for a period of time, then switching sleep to the other half.
Your laptop’s CPU is basically a Common Swift brain. When your computer sits idle, or if you’re doing a low-key task (like writing a blog post about batteries), the CPU shuts off large sections of itself that aren’t needed. This happens many times every second to save power without noticeably slowing you down.
If you switch to something more energy-intensive, like exporting a video, the CPU wakes all the way up and runs at full power. Your laptop will feel warm to the touch and you will hear some noise as the fan works to keep the temperature below an egg-frying level. The battery will drain much more quickly in this condition. The trick is to keep your computer out of this state as much as possible.
I’m just editing documents, yet my battery runs down in barely an hour. How long should a laptop battery last?
Pay close attention to the warmth and the fan noise. It can indicate that your computer’s software is running an otherwise healthy battery down.
These common software issues can cause your CPU to draw excessive power:
A misbehaving program can call for more power than necessary.
A background process (like a backup or anti-virus program) may need extra power.
You have unnecessary programs and/or windows open.
We see #3 most often, and web browsers are the biggest culprit. All of those pretty videos, animations, and annoying ads can make quick work of a fresh battery. The effect is multiplied if you have many windows or tabs open. In everyday use, you may not realize that you have 30-50 tabs open, but closing out of them can save a lot of power.
Sometimes my computer shuts off when it says there’s still a percentage of battery life remaining. Is it broken?
To answer this question, it helps to explain how modern rechargeable batteries work. Believe it or not, batteries don’t directly store electricity.
Instead, batteries work by storing a sophisticated combination of reactive chemicals. When you unplug your laptop, it begins drawing electric current from the battery, starting a chemical reaction inside the battery that provides power. Over time, the chemicals react further and further until they can no longer release enough current to keep your laptop running. Remember how that volcano from science class eventually used up the vinegar and baking soda and stopped erupting? It’s just like that.
With rechargeable batteries, this reaction is reversible. When you plug your laptop back in, your charger supplies an electric current back to the battery. This undoes the chemical reaction over a few hours until the battery is fully charged again.
Determining the battery charge level is a bit of a challenge for your computer. A fuel gauge in a car, for example, directly measures how much gas remains in the tank, using a float attached to an electronic sensor. As you use up the gasoline, the float sinks and the sensor measures the change. With a battery, by contrast, you never “use up” the battery’s chemicals, you only change their composition. A float, if it could fit inside, would never move.
When your computer tells you the battery is 40% charged, it’s not measuring the amount of anything inside the battery. Instead, it’s making an educated guess of the charge level. Your laptop tracks the battery’s voltage and current flow over time, it considers how long the battery has run in the past, then it gives you its best guess of the charge level from those measurements.
Sometimes, the guess is conservative, and you’ll see your computer stay on at 0% for 10-15 minutes before finally going to sleep. In other cases—especially if the computer is under heavy load—the chemical reaction will taper off sooner than expected and your computer will power down suddenly.
If your laptop is powering off well before it should—like if it drops from 40% to 0%—you may have a defective battery.
Do I need to do anything special to make my battery last longer?
Most people don’t. With moderate use, your battery can last the life of your computer. Lithium-ion laptop batteries are now designed to retain 80% of their capacity for at least 1000 full discharge cycles. If you use your laptop 250 days a year and run its battery down to 50%—a half-cycle—every day, your laptop will be 8 years old when the battery finally reaches 1000 cycles.
One caveat: lithium-ion batteries can suffer some degradation if they’re kept charged at 100% for many days at a time. If you have a Mac system, consider updating to macOS Catalina version 10.15.5 or later. A new feature called Battery Health Management (BHM) will monitor your battery’s usage and condition and adjust the maximum charge below 100% if needed. You can still turn BHM off in System Preferences to use the battery’s full capacity. Some Windows PCs from companies like Lenovo and Asus allow you to limit the maximum charge as well; check with your manufacturer for more information.
There are a couple of other situations where special treatment will prolong your battery longevity:
If you use the battery heavily during the day, plugging in when possible will reduce the total number of cycles you put on the battery.
If you’re using your laptop outdoors and the temperature is above 95°F, wait to charge your laptop until you’ve brought it back indoors.
Still not sure why your laptop battery is running down? You may be dealing with a defective battery, or something else. Don’t worry though — TSP can help you resolve the problem. We can help your company manage the lifecycle of your computers and other hardware, and provide proactive monitoring of batteries. With our help, you’ll be alerted to any battery problems before they ruin your day. Contact us to learn more!
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