How much battery is left on your phone? For many people in modern society, this is a “life or death” question.
The strong demand for phone charging has also led to many gimmicks in the market: wireless charging, double-speed charging, and high-capacity power banks that can last a week of use.
The dazzling variety can be overwhelming, but what is the correct way to charge your phone?
01
Does the phone need to be fully charged and fully drained?
There are some long-standing myths about charging: the phone should be charged only when the battery runs out, it should be charged fully every time to maintain the battery, and charging at any time will affect the battery life.
In fact, these theories are outdated.
Older rechargeable batteries, such as nickel-cadmium (NiCd) batteries, were made from nickel hydroxide and cadmium, and had a significant drawback: the memory effect.
Simply put, when the battery wasn’t fully discharged or fully charged multiple times, it would “remember” the battery’s charge level when the charging began and ended, treating those as the maximum and minimum levels, thus reducing the battery’s capacity.
But today, nickel-cadmium batteries and other similar types have been phased out due to heavy metal pollution, high weight, and low energy density, replaced by lithium-ion batteries.
Most smartphones now use lithium-ion batteries, which have several advantages over nickel-cadmium batteries: higher energy density (more charge is stored for the same size or weight), and very little memory effect—removing the charger before it’s fully charged has little impact.
The guidelines for lithium-ion batteries are actually the opposite of those for nickel-cadmium batteries: don’t wait until the battery is completely empty to charge, and don’t charge it to 100% all the time. Deep discharging and recharging are not necessary.
Lithium-ion battery life is linked to charging cycles. Every time the battery is used up to 100%, it completes one cycle. The more cycles it goes through, the faster its capacity will decline.
Research on 11 types of lithium-ion batteries indicates that after 250 cycles, the battery capacity begins to degrade noticeably. After 300-500 cycles, the capacity drops to about 80% of its original value.
What’s the science behind this?
Data shows that deep charging and discharging of lithium-ion batteries damages the material structure of the anode and cathode. Lithium-ion batteries work by moving lithium ions between these electrodes, and when the space for them to move becomes smaller, the battery’s capacity decreases.
So, rather than “full charge, full discharge,” the better habit for modern phones is “multiple small charges.”
Another issue that is often overlooked, yet also affects battery life, is the sequence of plugging in the charger.
02
Which should be plugged in first, the phone or the charger?
Should you plug the charger into the power source first and then connect it to the phone, or should you connect the charger to the phone before plugging it into the power source?
The answer is: plug the charger into the power source first.
What impact could such a small difference make?
Here’s the key point: the voltage of household electricity in China is 220V, but the voltage of phone chargers is usually under 5V. When charging, the charger steps down the high-voltage current to low-voltage direct current, which is then supplied to the phone. If you plug the charger into the phone first, then connect it to the power source, it’s as if you’re directly connecting the phone to the power, which might cause a surge voltage. However, by plugging the charger into the power source first, it’s already in a stable state, outputting a steady 5V, and surge voltage is less likely to occur.
A surge, also called a spike, is a momentary overvoltage that exceeds the normal working voltage. Sometimes the flash you see when plugging and unplugging a switch is a surge. While it lasts only a microsecond, over time it can cause significant damage to the phone battery.
When you plug the charger into the power source first, this issue is almost entirely avoided unless the current itself is unstable.
03
After charging, should you unplug the phone or the charger first?
The answer is: unplug the phone first. Because when you disconnect the charger, there’s a reverse surge of current, which can accelerate battery aging.
Though the theory seems complicated, remembering the correct order of plugging and unplugging is easy. Think of the charger, cable, and socket as a whole: when charging, the phone “approaches” it; when fully charged, the phone is “abandoned.” This approach is much safer.
04
Power banks are also affected by “charging cycles”
Many people carry power banks to avoid being stuck without a socket when they’re out. They’re indeed convenient—a small portable power source can keep your phone running, like a lifesaver.
However, there are many misconceptions and marketing tricks surrounding power banks.
Currently, many power banks claim to have a capacity of 20,000mAh. However, most phones, even flagship models, have battery capacities of less than 5,000mAh. Some older models only had a meager 2,000-3,000mAh. If you bought a 20,000mAh power bank, it seems like it could keep a family of three charged for two days, right?
But when you actually use it, you may find it can’t even fully charge your phone twice. Is this false advertising? Actually, it’s a marketing trick.
The capacity listed by manufacturers is often the total capacity of the individual cells in the power bank. Because some power is lost during the conversion process, the actual power available for your phone is always less than the advertised value.
To know how many charges you can really get from a power bank, check the “rated capacity” listed in the manual. This is the minimum discharge capacity determined by lab tests.
A media review showed that those 20,000mAh power banks vary in actual capacity by up to 4,000mAh, which is enough for most phones to charge fully once.
It’s important to note that power banks also use lithium-ion batteries, so their lifespan is affected by charging cycles. The same advice applies: “multiple small charges,” not “full charge, full discharge.”
05
How to charge correctly?
If the phone has a weak link, it’s probably the battery. However, in daily use, there are some charging methods that seem correct but actually aren’t.
- Can’t mix charging cables and chargers?
It’s standard to receive a charger with a new phone, but recently some manufacturers have stopped including chargers, citing environmental concerns, and you’re left to buy one separately. Many people choose cheaper options because charging devices are often seen as disposable—if you don’t watch the cat, the cable gets chewed up. This leads to many phones using mismatched chargers and cables soon after purchase.
This has sparked rumors that using mismatched chargers can cause battery shorts and reduce lifespan.
In reality, this claim underestimates the charger’s capabilities.
Chargers usually have an intelligent chip that detects the required voltage and current for your phone and adjusts accordingly. For example, if you buy a fast charger but your phone doesn’t support fast charging, the charger will automatically adjust to the appropriate current.
Some people complain that their fast charger doesn’t work; this is because the charger is adjusting correctly according to the phone’s needs.
However, when purchasing, it’s still important to choose a reliable brand to avoid counterfeit chargers. The components in counterfeit chargers are often of poor quality and may fail to detect the proper current and voltage, potentially damaging the phone.
- Don’t charge while using the phone?
Some people believe that using the phone while charging will shorten battery life, but the real issue is the temperature rise caused by charging and using the phone at the same time.
Everyone has experienced the phone heating up during high-performance games or fast charging.
A survey found that even if you don’t use the phone, an increase in temperature still accelerates the decrease in battery capacity.
So for battery safety, it’s best not to multitask while charging.
Additionally, be cautious not to buy excessively thick phone cases.
There are many flashy phone cases on the market that look good but aren’t suitable for the phone. Like computers and other devices, phones also need to dissipate heat. If the phone is wrapped tightly in insulating materials, it may look scratch-free on the outside, but the internals may be damaged.
To avoid high temperatures, also avoid placing the phone on a pillow or in direct sunlight before bed. This not only protects the phone but also keeps you safe.
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