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

Source: https://www.youtube.com/watch?v=kLS5Cg_yNdM
Recap page: https://rapidrecap.app/video/kLS5Cg_yNdM
Generated: 2025-11-10T19:06:57.366+00:00

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## Quick Overview

The two-year experiment on 40 phones conclusively demonstrates that fast charging does not cause significantly more battery degradation compared to slow charging, with the Fast-Charging Group losing 12.3% capacity versus 11.8% for the Slow-Charging Group after 500 cycles, and performance throttling only becomes noticeable when iPhone battery health drops below 85%.

**Key Points:**
- After 500 charge cycles, the Fast-Charging Group (using 120W charging) lost 12.3% of battery capacity, while the Slow-Charging Group (using 18W charging) lost 11.8%, showing minimal difference.
- The experiment involved groups of iPhones charged at 120W (Fast), 18W (Slow), 50% Fast Charging (80W-100W), and a Control Group (30%-80% range).
- Performance throttling (slowing down the device) only became significant when the iPhone battery health dropped to 85% or below, regardless of the charging method used.
- A separate test on Android phones showed the Slow-Charging Group lost 8.8% capacity, while the Fast-Charging Group lost 8.5% after 500 cycles, reinforcing the minimal impact of fast charging.
- Keeping the battery level between 30% and 80% offers limited protection against wear, and keeping a phone fully charged or nearly empty for extended periods causes almost no change in battery capacity over one week.
- In a separate performance test, the phone with 77% battery health shut down after 3 hours and 52 minutes of continuous heavy use, whereas a new battery lasted significantly longer.

![Screenshot at 0:35: The initial setup clearly divides six iPhone 12s into a Fast-Charging Group \(top row\) and a Slow-Charging Group \(bottom row\) for the long-term test.](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-00-35.png)

**Context:** The video documents a long-term experiment conducted over two years, testing the impact of different charging speeds—specifically fast charging versus slow charging—on the long-term health and capacity of smartphone batteries, primarily focusing on various iPhone 12 models and some Android devices. The experiment aimed to verify common user beliefs that fast charging prematurely kills battery life.

## Detailed Analysis

The video details a two-year experiment testing battery degradation across different charging methods on 40 phones, primarily iPhones, but also including Android devices. The main finding is that fast charging (using 120W chargers for the fast group) does not significantly damage battery health compared to slow charging (using 18W chargers). After 500 charge cycles, the Fast-Charging Group lost 12.3% capacity (starting at 3000mAh, ending at 2610mAh), while the Slow-Charging Group lost 11.8% capacity (ending at 2670mAh). A control group, charged only between 30% and 80%, showed the least degradation at 0.3% loss, confirming that limiting the charging range offers marginal benefits. Performance throttling, where the phone slows down due to battery issues, only occurred when iPhone battery health dropped below 85%. An Android subset test corroborated these findings, showing minimal capacity difference (8.5% loss for fast charging vs. 8.8% for slow charging). Furthermore, testing showed that keeping a phone fully charged or nearly empty for a week resulted in negligible capacity loss, contradicting the common belief that full charge cycles are essential for battery longevity. The video concludes that for current smartphone batteries, daily charging habits have a minimal impact compared to the inherent chemical aging process.

### Experimental Setup

- Divided 6 iPhone 12s into Fast-Charging Group (120W) and Slow-Charging Group (18W)
- Ran 500 charge cycles over three years (Sept 2020 to March 2022)
- Used custom software to automate charge/discharge between 5% and 100%.

### iPhone Battery Capacity Loss (500 Cycles)

- Fast-Charging Group showed 12.3% capacity loss (3000mAh -> 2610mAh)
- Slow-Charging Group showed 11.8% capacity loss (3000mAh -> 2670mAh)
- Control Group (30%-80% charge) lost only 0.3%.

### Performance Impact

- Throttling began only when iPhone battery health dropped below 85%
- Phones with 94% and 89% health showed virtually no performance difference between new and old batteries in Geekbench tests.

### Android Battery Degradation

- Slow-Charging Group lost 8.8% capacity after 500 cycles
- Fast-Charging Group lost 8.5% capacity, showing negligible difference compared to iPhones.

### Charging Habits Myths

- Keeping battery between 30% and 80% provided limited benefit
- Leaving a phone fully charged or nearly empty for one week showed almost no capacity change.

![Screenshot at 0:35: The initial division of iPhone 12 models into Fast-Charging \(top\) and Slow-Charging \(bottom\) groups for the long-term test.](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-00-35.png)
![Screenshot at 0:51: Custom hardware setup using relays and a monitoring device to automate the charging/discharging cycle for one of the test phones.](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-00-51.png)
![Screenshot at 1:14: Visual representation showing the capacity loss after 500 cycles: Fast charging resulted in 2500mAh remaining from 3000mAh \(orange\), while slow charging resulted in 2800mAh remaining \(blue/cyan\).](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-01-14.png)
![Screenshot at 3:03: Final comparison graph for iPhones after 500 cycles, showing Fast-Charging Group lost 12.3% capacity, Slow-Charging Group lost 11.8%, and the 30%-80% Control Group lost 0.3%.](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-03-03.png)
![Screenshot at 3:14: Android test results after 500 cycles, showing Slow-Charging Group lost 8.8% and Fast-Charging Group lost 8.5%, indicating similar minimal impact.](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-03-14.png)
![Screenshot at 4:42: Final power remaining comparison after 4 hours of continuous use, showing phones with higher battery health \(94% Red, 89% Blue\) retained significantly more power \(39%, 41%\) than those with lower health \(81% Green, 85% White\).](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-04-42.png)
![Screenshot at 5:34: Geekbench Multi-Core Score comparison showing that even with a cooling clip, phones with lower battery health \(e.g., iPhone X at 77%\) score significantly lower than newer batteries, but health differences above 85% show minimal score variation.](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-05-34.png)
![Screenshot at 6:18: Visual comparison of battery health percentages across four different iPhones after 330 charge cycles, showing the fast-charged phone at 86% health while others are at 92%.](https://ss.rapidrecap.app/screens/kLS5Cg_yNdM/00-06-18.png)