Why Your Phone Battery Dies When It Still Has Juice

Image: Alex Cranz/Gizmodo
Image: Alex Cranz/Gizmodo

You’re plugging along with 3 percent battery and that Snorlax is one block away. Then, less than a minute later your phone is dead, the Snorlax is gone, and you are miserable. There are a two reasons for this. One: you’re playing Pokémon Go and need to stop. Two: figuring out how your battery holds a charge is less science and more witchcraft.


See, your phone can’t know the precise charge of your battery. It’s impossible short of inserting a sensor into the battery itself. And since a typical lithium ion battery is essentially a tiny chemical bomb that we use to generate power, inserting a sensor into it is a bad idea. Instead, your devices anticipate how much charge is left with the help of a little math. Mark Smirniotis from Wirecutter has created a great analogy on the subject, likening a battery to a large barrel you can never see into.

It’s fascinating and breaks down the three primary reasons for battery fuckery:

  1. Your device has trouble reading a charge because batteries are mysterious, and we can only anticipate the charge with algorithms. Sometimes those predictions are just wrong.
  2. Your battery loses capacity as it gets older, making the math even more inaccurate.
  3. Crystals develop in the battery, further diminishing capacity and wrecking your battery meters ability to do its job.

Wirecutter does leave out one other point. As I learned that time I had to test the fastest charging phone, not all batteries shut off at 0 percent. Many phones like to reserve a little juice to protect data in the event of a sudden shutdown or maintain internal clocks. (This is more common in phones where the battery can’t be removed). Consequently, some phones will shut down at true full battery discharge, and others will shut down early. (Laptops have this bad habit as well).

So if your phone’s battery is running low, it’s always best to play it safe. Reach for a portable charger.




What’s weird is with my wife’s Note 4, it can shut off when she’s closer to 20%, only for it to show 0% when she tries to turn it back on. I’m still trying to figure out how her phone estimates that poorly on battery life.