Why does your phone charge fast at first, then slow down?
Because lithium-ion batteries charge in two stages. They take a strong, steady current up to roughly 70 to 80%, then the charger holds the voltage steady and the current tapers off to protect the battery.
Quick Answer
Your phone’s lithium-ion battery charges in two stages. In the first, the charger pushes a steady, strong current, and the battery fills fast. Once the battery nears its maximum voltage, usually somewhere around 70 to 80%, the charger switches to holding the voltage steady instead. In that second stage, the current drops little by little as the battery fills, so the last stretch takes the longest. The slowdown is on purpose: pushing high current into a nearly full battery creates heat and stress that wear it out faster. Apple, for example, says its lithium-ion batteries fast-charge to 80% and then switch to slower trickle charging. iPhones can fast-charge to about 50% in around 30 minutes with the right adapter. Many phones also hold at 80% overnight: Apple’s Optimized Battery Charging learns your routine and finishes charging just before you usually unplug.
By the Numbers
Key Facts
- Stage 1
- Constant current: fast charging up to roughly 70–80%
- Stage 2
- Constant voltage: the current tapers off as the battery fills
- Why it slows
- High current near full creates heat and stress that age the battery
- Overnight trick
- Many phones pause at 80% and finish before you wake up
The two stages of charging
| Stage 1: constant current | Stage 2: constant voltage | |
|---|---|---|
| Battery level | Empty to roughly 70–80% | Roughly 70–80% to full |
| What the charger does | Pushes a steady, strong current | Holds the voltage steady |
| Speed | Fast | Slows down as the current drops |
The exact switch point varies by phone, charger and battery temperature.
Two stages, by design
Battery University, a long-running reference on battery technology, describes lithium-ion charging as constant current followed by constant voltage. In the second stage, the current keeps falling as the battery saturates, and the charge is complete when it drops to a small fraction of the battery’s rating.
Why not charge fast the whole way?
Near full, a battery’s voltage is high, and forcing in more current would add heat and chemical stress. Both speed up wear, so phones slow down on purpose. Apple says fast charging to 80% and then trickle charging extends the life of the battery.
Why some phones stop at 80%
Keeping a battery at 100% for hours adds wear. Apple’s Optimized Battery Charging, on by default on iPhones, learns when you usually unplug, pauses at 80% overnight and finishes charging shortly before you wake up. Many Android phones have similar features.
Myth vs. Fact
MythYour charger is broken if charging slows near the end.
FactSlowing down near full is built in to protect the battery.
MythYou should always charge to 100%.
FactBatteries wear more slowly when they aren’t kept full for long periods, which is why many phones pause at 80%.
The Bottom Line
Phones charge fast at first and slow down near full on purpose. The second, gentler stage protects the battery from heat and stress so it lasts longer.