Is Fast Charging Killing the Battery? A 2-Year Test on 40 Phones

Technological concepts & product/feature focus

• Fast charging vs slow charging battery wear: The video tests whether higher-wattage charging accelerates battery capacity loss over time.
• Charge-range guidance (30%–80%): Evaluates whether limiting charge to the 30%–80% window reduces wear compared to charging up to 100%.
• Leaving a phone at 100% while plugged in: Tests whether staying fully charged for a week affects battery capacity.
• Battery health and real-world performance: Measures how reduced battery capacity impacts runtime and throttling behavior.
• Android vs iPhone charging behavior: Compares outcomes across platforms and charging power levels.

Experiment / methodology (tutorial-style description of setup)

Main charging-cycle capacity test (iPhone 12)

• Six iPhone 12s split into:
  • Fast-charge group (fast charging via “Discharge Loop” + charger control logic)
  • Slow-charge group
• Phones are opened and battery capacity is measured at:
  • the start, and then after aging.
• A custom app (“Discharge Loop”) repeatedly:
  1. Drains battery until 5%
  2. Signals a relay to start charging
  3. Stops charging at 100%
  4. Repeats
• Counting cycles:
  • each cycle charges/discharges about 95% of a full cycle (treated as 0.95).
• After 500 cycles, capacity is re-measured.

30%–80% range test (iPhone 12)

• Another group cycles from 80% down to 30%, then charges back to 80%.
• Each “round” uses about 50% of battery → treated as half a cycle.
• After 500 cycles, battery loss is compared to the full fast-charge group to see if the “healthiest range” helps.

Control group (iPhone 12)

• One iPhone is not cycled—only measured at the beginning/end to rule out time/other effects.

Android equivalent test

• Uses similar groups.
• Fast charging: iQOO 7 at 120W
• Slow charging: 18W
• Same overall 500-cycle framework.

Reliability / automation

• Uses an anti-crash device that detects a screen-color change and auto-reopens the discharge app if it crashes.
• Ensures phones are sourced from official stores and claims no brand interaction.
• Total runtime for the successful test: about 167 days.

Key results / analysis

1) Fast charging vs slow charging (battery capacity loss)

iPhone 12 (500 cycles)

• Slow-charge: 11.8% capacity loss
• Fast-charge: 12.3% capacity loss
• Difference: about 0.5% more for fast charging

Android (500 cycles)

• Slow-charge: 8.8% capacity loss
• Fast-charge: 8.5% capacity loss
• Difference: about 0.3% less for fast charging

Conclusion implied: Over ~500 cycles (~1.5 years of use per the video), the fast vs slow difference is extremely small / almost unnoticeable.

2) Does charging between 30% and 80% help?

iPhone 12

• The 50% fast-charge group (80%↔30% window) lost 4% less capacity than the full fast-charge group.

Android

• The 50% fast-charge group lost 2.5% less capacity than the full fast-charge group.

Conclusion implied: The 30%–80% practice does reduce wear, but the benefit is limited.

3) Is keeping the battery at 100% harmful?

• Tested iPhones left plugged in for a week at:
  • 1%, 50%, and 100%
• Result: battery capacities didn’t change at all after one week.

Conclusion implied: Battery aging is a long-term process; a short period at 100% doesn’t show measurable capacity loss.

4) When to replace the battery (battery health → user impact)

• Tested iPhones with battery health levels:
  • 94%, 89%, 85%, 81% plus an iPhone X at 77%
• After 4 hours of continuous use, remaining battery levels were:
  • 39%, 41%, 32%, 26%
• iPhone X (77%) shut down after 3h 52m.
• After replacing batteries, remaining power increased by roughly:
  • +6%, +5%, +12%, +16% (and +24% for the iPhone X segment as described)
• Threshold guidance from observations:
  • Notice shorter battery life around ~85% battery health
  • Consider replacement around ~80% (where performance/runtime worsens enough to be noticeable)

5) Does battery wear affect performance?

• Performance generally stayed nearly the same before vs after replacement.
• But throttling happened earlier with degraded batteries:
  • Example iPhone: with 85% battery health it throttled at about 11%
  • With a new battery it throttled only near 5%
• In games, this means a new battery can allow a bit longer before lag.

Earlier test attempts (why “40 phones, 2 years”)

• The title refers to multiple runs:
  1. First run (Oct 2020): 4 phones (2 fast, 2 slow). Manual setup; ~430 cycles by March 2021. No clear result.
  2. Second run (May–Aug 2021): ~200+ cycles with automation using robotic arm + smart plugs, but phones/software broke/crashed.
  3. Third run: the successful test described above.

“Best way to charge” guidance (final takeaway)

• Don’t obsess over whether fast charging “kills” batteries—real-world measured wear difference is tiny.
• Use whatever charging method you prefer.
• If you want extra protection, limiting charge time in the 30%–80% range helps somewhat.
• Don’t over-interpret “100% plugged in overnight” as strongly harmful based on short-term effects.

Main speakers / sources

• Primary source: The video creators/hosts conducting the experiments (including a collaboration with Geekerwan).
• Secondary source: Geekerwan (teamed for battery life/performance testing).
• Tools/software referenced: The creators’ custom app/software named “Discharge Loop” and an external relay/anti-crash automation setup.