Shadowing Practice: Is Fast Charging Killing the Battery? A 2-Year Test on 40 Phones - Learn English Speaking with Video
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If you bought a phone charger recently, it probably supports fast charging.
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And you probably still have some slow chargers lying around.
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I've heard people say that when you charge overnight, you should use the slow charger.
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Because fast charging supposedly hurts the battery more.
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There's also a saying that keeping your battery between 30 and 80% is the healthiest range.
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So, does fast charging actually damage your battery?
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What's really the best way to charge your phone?
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Does battery wear make your phone slower?
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And at what point should you actually consider replacing your battery?
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Today, let's run an experiment to find out the answers.
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First, we prepared 6 iPhone 12s and divided them into a fast charge group and a slow charge group.
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Then we opened them up and measured the battery capacity of each one, and installed our custom software called the Discharge Loop.
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Once the app starts, the phone begins running a program to drain its battery.
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When the battery drops to 5%, the app sends a signal to a relay connected to the charger, telling it to close the circuit and starts charging.
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Once the phone reaches 100%, the app tells the switch to open again and stop charging, then starts draining the battery again, repeating the cycle endlessly.
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Since each cycle charges and discharges about 95% of the capacity, we count that as 0.95% of a full cycle.
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After 500 cycles, we open the phones again and re-measure their battery capacities.
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If the fast charge group shows significantly more capacity loss than the slow charge group, then we can say fast charging does hurt the battery more.
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In addition, we prepare another group of iPhone 12s.
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These phones stream from 80% down to 30%, then fast charge back to 80%, Since each round only uses 50% of the battery, we count that as a half a cycle.
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If after 500 cycles, this group's battery loss is less than the full fast charge group,
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then it proves that keeping your phone between 30% and 80% really does help protect the battery.
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Finally, we had one more iPhone 12 as a control group.
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This phone didn't go through any charge cycles, we only measured its capacity at the beginning and end, to rule out the effects of time and other factors.
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On Android, we set up the same test groups, but for fast charging, we used the IQ7 with 120W, and for slow charging, we used 18W.
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To keep the experiment running smoothly, we built an anti-crash device.
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If our discharge app crashes, this device detects the screen color change and reopens the app automatically.
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All the phones used in this experiment were purchased through official stores.
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We didn't communicate with any brands, so the results are completely independent.
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Running a test like this takes about 6 months, so if you could give us a like, we'd really appreciate it.
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Before the test began, just to prove we were really doing this for a long time, we planted some plants.
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Time flies.
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The plans didn't survive, but the experiment did.
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After 167 days, all the phones had completed 500 charge cycles.
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The iPhone's slow charge group lost 11.8% of its battery capacity.
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The fast charge group lost 12.3%, just 0.5% more.
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On Android, the slow charge group dropped 8.8%, while the fast charge group dropped 8.5%, about 3.3% less.
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So over 500 cycles, roughly a year and a half of use.
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That difference is almost unnoticeable.
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In addition, the iPhone 50% fast charge group lost 4% less capacity than the full fast charge group.
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The Android 50% fast charge group lost 2.5% less.
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So keeping your phone's battery between 30% and 80% does help reduce wear, but the improvement is limited.
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There's also a common belief that leaving your phone plugged at 100% is bad for the battery.
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That idea actually makes sense.
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Apple even says, if you store it fully charged for an extended period of time, the battery may lose some capacity.
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To test this, we took 6 iPhone 12s, measured their capacities, and split them into 3 groups.
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One left at 1%, one at 50%, and one at 100% when plugged in.
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Then we puffed them off and left them for the week.
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Guess what, their battery capacities didn't change at all.
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So battery aging is a long-term process.
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The best way to charge your phone is simply however you like.
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Don't overthink it.
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Don't trade mental energy for a tiny bit of battery life.
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Now that we know how charging affects battery wear, let's look at how battery wear affects your actual phone experience.
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We found four iPhone 12s with battery health at 94, 89, 85, 81%, plus one iPhone X at 77%.
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We teamed up with our friends, our geekier one, to test their battery life and performance.
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After four hours of continuous use, The remaining battery levels were 39, 41, 32, and 26%.
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The iPhone X at 77% battery health shut down after 3 hours and 52 minutes, then replaced all their batteries and run the test again.
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This time, the remaining power increased by 6, 5, 12, and 16% and 24%.
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So once your battery health drops to around 85%, you'll start noticing shorter battery life.
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When it falls near 80%, it's time to replace the battery.
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Android doesn't show official battery health numbers at the time.
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But we tested three Android phones from our experiment.
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Before the experiment, their remaining power after the same test were 35, 33, and 35%.
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After new batteries increased 9, 10, and 8%, roughly an extra 90 minutes of use.
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Does battery wear affect performance?
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Not really.
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Performance stayed nearly the same before and after replacement.
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But phones with degraded batteries throttle earlier.
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For example, this iPhone with 80% battery health slows down at 11%.
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But with the new battery, it doesn't throttle until 5%.
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In games, that means the new battery lets you play a bit longer before it lags.
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That's the end of our experiment section.
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Some of you might be wondering, if you only use 20 phones and 6 months, why does the title say 40 phones, 2 years?
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That's because we tried this 3 times.
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The first one was back in October 2020 with just 4 phones, too fast, too slow.
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Back then, I didn't know how to automate anything.
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So I literally drained the batteries by leaving games running, charged them manually, then repeated them for months.
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From October 4th to March 3rd, I completed 330 cycles, but one fast-charge phone aged a lot.
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The other didn't at all, no clear results.
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So I tried again.
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This time, I bought four more phones for each group, added Android, and used a robotic arm and smart plugs to automate the whole process.
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From May 31st to August 3rd, 2021, I ran over 200 cycles, but four phones broke, and the software kept crashing, another failure.
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Then came the third try, the one you just watched, and it finally worked.
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Sometimes I wonder, does anyone still care about whether fast charging hurts battery?
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Is it really worth all this time and effort?
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Then I remember my first phone.
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I always waited until it dropped below 20% before charging, and only unplugged at 100% because I thought that was good for the battery.
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I did that for three years.
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Looking back, it didn't help the the phone, but it made me feel like I was taking care of something important, because that phone meant a lot to me.
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It was my window to the world, my escape from the boredom.
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So even if that kind of care didn't matter technically, it still mattered emotionally.
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If this video helps more people use their phones a little more freely, then it's all worth it.
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All the phones from this test have been given away to our viewers after getting brand new batteries.
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If there's anything else you'd like to have to test, let us know in the comments.
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this, we've also done a video about whether using your phone while charging hurts the battery.
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Go check it out.
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See you next time.
Why Practice Speaking with This Video?
This video is perfect for english speaking practice because it blends casual conversation with technical details, mirroring real-life communication. You’ll hear natural phrasing like “I’ve heard people say” and “Guess what,” which are great for everyday chats. Plus, the topic—battery charging—is relatable, making it easy to stay engaged. Try shadow speech here: repeat the speaker’s lines aloud, mimicking their pace and tone. It’s a fun way to build fluency while learning something useful!
Grammar & Expressions in Context
- “If you’ve bought a phone charger recently, it probably supports fast charging.” This mixed conditional (present perfect + present simple) is key for talking about past actions with current results. Practice it to describe your own tech purchases.
- “We prepared six iPhone 12s and divided them into a fast-charge group and a slow-charge group.” The structure “and divided them into…” is useful for explaining processes—great for work or school presentations.
- “Time flies. The plants didn’t survive, but the experiment did.” Short, contrasting clauses like this add rhythm to speech. Use “but” to highlight surprises, just like the speaker does here.
Common Pronunciation Traps
Watch out for words like “capacity” (kuh-PAS-uh-tee, not ka-PAS-i-tee) and “degraded” (di-GRAYD-ed, not dee-GRADE-ed). The speaker also uses casual contractions like “didn’t” and “it’s” which can trip up learners—focus on blending the sounds smoothly. For extra help, use a shadowing app to slow down the audio and match the pronunciation. With practice, you’ll master these tricky spots and sound more natural!
Whether you’re using shadowspeaks techniques or just repeating lines, this video is a goldmine for improving your English. Start shadowing today and see how quickly your speaking skills grow!
What is the Shadowing Technique?
Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.