Many smartphone owners have learned the hard way that their battery display is not to be trusted. If you’ve ever had a phone conk out on you at the 20 percent battery mark, there are a few likely explanations as to why.

According to a recent story from The Wirecutter, the battery percentage in the corner of your screen is nothing more than an estimate. A number like 47 percent may sound incredibly specific (and you may plan your phone time around it accordingly), but the only way to know for sure if it's accurate would be to build a sensor into the phone’s lithium ion battery. Phone manufacturers don’t do this; instead they use algorithms that measure how much of the battery space is being used based on the amount of voltage it emits. The Wirecutter compares this phenomenon to measuring the water in a barrel by looking at how fast water trickles out from a hole in the side.

This works pretty well most of the time, but it’s not a perfect indicator of battery life. Factors like the type of battery and how much it's being used both affect the rate at which it’s drained. Measuring battery becomes even more of a guessing game as your phone gets older. Over the years, crystals form inside the battery that reduce its capacity, making it harder to calculate.

The best way to keep your phone from becoming a lifeless brick in your pocket is to never let it dip into the red zone in the first place. If you already carry around a portable charger and juice it up regularly, there are a few more steps you can take to keep your phone running longer. Activate low power mode when trying to conserve energy, and delete your power-guzzling Facebook app in favor of visiting the website through Safari. In terms of maintaining your battery’s lifespan in the long-term, avoid using your device in extreme cold or heat.

[h/t The Wirecutter]

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