There's a common misconception about charging: keeping the phone fully charged is always best. While some people with obsessive-compulsive tendencies might find the drop in battery level from 100% to 99% unbearable, constantly keeping your phone charging is not recommended, especially after it's fully charged. The phone battery simply can't withstand that kind of abuse.
Lithium batteries are not a panacea.
Many elders would tell you when you got your first mobile phone that you should first completely discharge the battery, then charge it for 12 hours, and then completely discharge it again, repeating this process three times. You, in your naivety, probably did exactly as they said. However, smartphones now using lithium batteries no longer require such a complicated battery maintenance process.
Nickel-metal hydride (NiMH) batteries require multiple charge-discharge cycles because they have a strong memory effect. The repeated charging and discharging process is designed to maximize their charge capacity. However, modern mobile phones use lithium-ion batteries, making this complex memory effect unnecessary. So, will lithium-ion batteries always maintain their maximum charge capacity?
The answer is definitely no. Although lithium batteries are known for their durability, their maximum capacity will inevitably decrease after multiple charge-discharge cycles. This loss occurs gradually over time, and besides normal charging, overcharging is the main culprit.
How to understand the overcharge phenomenon?
Overcharging is easy to understand; it occurs when a phone continues to charge its battery even when it indicates a full charge. Continuing to charge a fully charged battery causes structural changes in the positive electrode material, resulting in capacity loss. The decomposition of this material releases oxygen, which reacts violently with the electrolyte, potentially leading to an explosion.
Numerous cases of mobile phones exploding while charging have occurred in real life. For example, Xinhua News Agency reported on March 4, 2015, that a 23-year-old girl was burned while using her phone while charging, and Shanxi Evening News reported on June 15, 2015, that an 18-year-old boy was electrocuted while using his phone while charging. The causes of these tragedies can be attributed to factors such as damaged batteries, chargers lacking IC protection, and quality issues with the power supply itself. When charging is complete but current continues to flow, because the battery's capacity has reached its limit, most of the current is converted from electrical energy into heat energy and dissipated, causing the battery to heat up.
Common sense tells us that the best way to store batteries is at room temperature and in a dry place, whether it's a nickel-metal hydride battery or a lithium battery. When overcharging occurs, the heat generated by the current will undoubtedly be released in large quantities, causing the electrolyte at the positive electrode of the lithium battery to react and consume the maximum capacity of the lithium battery. When the heat accumulates to a certain level, events such as fire and explosion may occur.
Faced with such a low level of safety, mobile phone manufacturers and charger manufacturers naturally won't sit idly by. After all, they're the ones who will suffer when users experience safety issues. Therefore, there are many things to consider when charging mobile phones, especially for the now mainstream USB-based phones and chargers.
They really go to great lengths to prevent overcharging.
In fact, most mobile phones nowadays have IC protection features. The charger itself also monitors voltage and current. To protect the battery and user safety, trickle charging is often used in mobile devices.
When charging via USB, disregarding cable losses, the USB charging port negotiates with the phone to match the appropriate current. This ensures that the phone doesn't suffer significant current loss due to high current input. This is especially important when the battery level is low, such as around 10%.
At this point, the battery is charged most efficiently, and the current reaches its maximum. As the capacitance increases, the current required by the phone gradually decreases. When the battery is at its last 10% to 20% charge, the phone may even request a small pulse current to charge it, thus protecting the battery. This charging method is called trickle charging and is currently the mainstream charging solution for mobile phones. It aims to reduce the loss of the battery's maximum capacity and extend its lifespan.
Besides the overcharge protection provided by the phone battery, another way to prevent overcharge is through the charging socket. For a socket to provide overcharge protection, it must first offer a stable voltage; greater voltage fluctuations will affect the current. Especially in my country, where voltage fluctuations are frequent at night, ensuring a stable output voltage from the USB port is crucial. Generally, the voltage of a USB charging port is 5V. Some USB charging ports that can intelligently distribute current will slightly reduce the voltage when the phone battery is close to full charge, thus reducing the input current and relieving battery stress.
These methods are very effective in suppressing overcharging of mobile phones, but ultimately it is because we did not unplug the power cord after the phone was fully charged, causing the phone to repeatedly discharge and charge, resulting in a reduction in the upper limit of battery capacity.
Overcharging is not a trivial matter.
For ordinary mobile phone users, overcharging is no small matter. The lifespan of a mobile phone battery directly affects how long you can use the phone. If you let your phone be overcharged, the battery will likely fail in less than a year.
If you can see that the back of the battery is slightly bulging, it's probably due to overcharging. More seriously, overcharging can even directly affect your safety; those cheap batteries in counterfeit phones might just explode. To fundamentally avoid overcharging, it seems the key is to control the charging time.
