Why Is My Battery Estimate Never Accurate?

Laptops, tablets, and phones never seem to know exactly how many hours of power they have left. The estimate may jump from two hours to five hours before dropping back down to one hour. Even worse, the battery may suddenly die without warning.

Your Device Is Just Guessing Based on Your Current Usage

Your device’s battery drains faster when it does more demanding things. All your laptop can do is monitor how fast its battery has been draining over the last several minutes and make an informed guess. So even if you have 100% battery power left, you’ll see a different number of estimated hours depending on how hard your computer is currently working.

For example, a laptop might estimate ten hours of battery life if you dim its display and browse the web. Crank the display brightness to maximum and launch a demanding PC game, though, and the estimate might become two hours.

It’s just a prediction. If you see a low estimate, you can make your battery last longer by easing up on it. If you see a high estimate, you won’t get anywhere near that many hours if you start using your hardware heavily.

This estimate can also fluctuate based on the work your computer is doing in the background. For example, Windows might be installing updates–that will use more CPU resources, drain battery power more quickly, and lower the estimate. Even if you’re just browsing the web, some websites–especially ones with video ads–use more system resources than others.

In other words, your device makes an estimate based on the current rate of battery drain. It’s similar to why progress bars can be so inaccurate. It can’t predict the future, only guess based on current conditions.

If the Battery Dies Without Warning, It Doesn’t Know How Much Juice it Has

Batteries suffer wear and tear as you use them. This causes them to decrease in health. In other words, they won’t hold as much power.

The hardware that monitors how much capacity the battery has left may have an inaccurate estimate, which why a device might die when the operating system is still reporting 10% or even 20% battery power left.

Some devices are better at understanding how much power they have. Circuitry on the battery itself generally reports the battery’s capacity and health to the operating system. But, if you find your device dying when the operating system reports 20% or 10% battery life left, the battery may need recalibration.

You can recalibrate a battery by draining it down to empty before charging it back to full capacity. It won’t make the battery last any longer, but it will help the hardware understand how much capacity the battery has left and make more accurate estimates.

You can check the health and capacity of your battery on pretty much any device, too. On Windows, you can generate a battery health report that shows you the “design capacity” that your battery had when it arrived from the factory and the “full charge capacity” it currently has. The current full charge capacity will be lower than the original design capacity due to the battery deteriorating.

On a Mac, hold down the Option key and click the battery icon on the menu bar. You’ll see a “Condition:” line that gives you an estimate of the battery’s condition. If it’s normal, you’re good. If it’s “Replace Soon” or “Replace Now”, the battery has deteriorated so much that you’ll need to replace it.


Battery life estimates will never be completely accurate, but the percentage figure is more accurate than the time estimate. If the reported percentage seems wrong, recalibrate the battery so it understands how much power the battery can actually store.

Chris Hoffman is a technology writer and all-around computer geek. He's as at home using the Linux terminal as he is digging into the Windows registry. Connect with him on Google+.