November 25, 2025, 7:29 am | Read time: 4 minutes

The YouTube channels “HTX Studio” and “Geekerwan” conducted several charging experiments with iPhones and Android devices over months. The goal was to determine how different charging habits affect battery capacity in the long term and when replacing a battery is truly worthwhile. All smartphones were individually purchased to exclude manufacturer manipulation.

Fast Charging in Long-Term Tests

In the first experiment, nine iPhones were divided into three groups: One group was repeatedly charged from five to 100 percent with a conventional charger, the second with a fast charger under the same conditions. The third group also used a fast charger but was only charged between 30 and 80 percent. The charging process was automated, and the smartphones underwent a total of 500 full charging cycles–equivalent to about two years of normal use. The same experiment was conducted simultaneously with Android devices, which were either charged at 18 watts (normal charging) or 120 watts (fast charging).

After the test, the differences were manageable: The iPhones in the slow-charging group lost 11.8 percent capacity, the fast-charging group 12.3 percent, and the 30–80 percent group 8.3 percent. For the Android devices, the capacity loss was 8.8 percent (slow), 8.5 percent (fast), and 6 percent (30–80 percent).

The result: Fast charging hardly harms modern batteries more than normal charging. A slight advantage can be seen when keeping the battery in the mid-range–but the difference is minor.

Storage and Charge Level

A second experiment examined Apple’s official recommendation: If you don’t use an iPhone for an extended period, it should be stored at about 50 percent charge. Six iPhone 12 models were charged to one, 50, and 100 percent and left off for a week. The result: None of the groups showed measurable changes in capacity. In the short term, it doesn’t matter if the smartphone is turned off with an empty or full battery. However, whether a permanently empty or full battery can be damaged during very long storage remains unclear.

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When the Battery Should Really Be Replaced

Apple allows the so-called battery life to be displayed as a percentage. This number describes how long the device lasts compared to the original capacity of a new battery. An iPhone with a battery life of 90 percent can only store 90 percent of the energy it could when new–thus shortening the usage time before the device needs to be recharged. Currently, Android smartphones do not have a comparable, directly integrated system display, so a direct test was only possible with iPhones.

In the third experiment, the YouTubers wanted to find out at what point battery wear actually affects an iPhone’s performance. They used four iPhone 12 models with battery lives of 94, 89, 85, and 81 percent, and an iPhone X with 77 percent. All devices were tested under continuous load for four hours. The iPhones with more worn-out batteries lasted shorter–the iPhone X even shut down after 3 hours and 52 minutes. After replacing the batteries, the picture changed: All models lasted longer.

The analysis also showed: At a battery condition of about 85 percent, the device starts to slow down once the remaining charge falls below 10 percent. According to the test, replacement is recommended at 80 percent, as performance and runtime noticeably decline.

Many Battery Myths No Longer Hold True

The tests clearly show that fast charging hardly harms modern smartphones. The often-recommended 30–80 percent range offers only a small advantage but is not a necessity. A battery replacement is generally worthwhile once the capacity falls below 80 percent–then wear becomes noticeable in performance. Users can charge their smartphones as they wish–without feeling guilty about the battery.