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WHYWhy does…? · #15EP.174 · 24 s

Why does…? #15: Why does your phone charge slower when it's almost full?

Cover of the lesson: Why does…? #15: Why does your phone charge slower when it's almost full?Coming to YouTube soon
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// WHAT YOU WILL LEARN

Lithium-ion charging has two stages: fast constant current up to about 80 %, then constant voltage where the current slowly tapers off to protect the battery.

// LESSON NOTES

Why does charging crawl after 80 %? 🐢🔋

⚡ Stage 1: the charger pushes FULL current — fast, up to about 80 %.
🔴 Then the battery hits its voltage limit (about 4.2 V per cell).
📉 Stage 2: the voltage is held and the current slowly shrinks — less heat, less wear.

⏱️ The last 20 % often takes about half an hour!
💡 That's why many phones pause at 80 % and finish just before you wake up.

Read the voiceover (transcript)
  1. Why does charging slow down after eighty percent?
  2. Up to about eighty, the charger pushes full current. Fast!
  3. Then it hits its voltage limit, about four point two volts.
  4. So the current slowly shrinks, to stop heat and damage.
  5. Guess: how long is the last twenty percent?
  6. Often half an hour!
  7. That's why many phones pause at eighty!
  8. Next: why does a laptop charger have a box? Follow for more!

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