Why is the charging speed of smartphones slowing down as they approach full battery capacity?

As the phone’s battery capacity approaches 100%, the charging speed decreases and it usually takes longer to reach the charge level from 80% to 100%. The reason for this phenomenon is related to how lithium batteries work.

Three-stage lithium battery charging

Lithium batteries in three stages charge Each is designed to protect the battery in the most vulnerable situations possible. These three steps are:

  • Constant current precharge, also known as “drop charge”
  • Constant current setting mode
  • Constant voltage adjustment mode


The first step applies when the device battery is completely empty or its voltage is below 3.0 Volt Be. The battery cell should be reactivated slowly to protect against prolonged discharge of the battery.


The inactivating layer of the battery is a protective shield that forms as part of a typical chemical reaction. This layer may need to be recovered, and low-voltage drip charging allows the recovery process to take place. This step is usually done at about 10% of the maximum charging speed. This is the main reason for the long delay in turning on smartphones whose batteries are completely discharged.

For example, to turn on and start an iPhone with a completely discharged battery, a drop of voltage must be injected into the battery for a few minutes, in which case the empty battery icon will be displayed on the iPhone screen. In the pre-charging phase, the electric current is kept constant; While the voltage is transferred to the battery at a lower rate compared to the next stage of charging.

Constant current charging

When the battery reaches 3.0 volts, your smartphone will gradually charge faster. At this stage of charging, the current is set to a constant rate; While the voltage increases over time. This step is exactly when your device will charge as fast as possible and will take advantage of any fast charging feature available. This step will continue until the battery capacity reaches about 80% of speed and your device will receive the maximum electrical energy with ease.

Constant voltage charging

When the battery capacity reaches about 80%, the charge will switch to constant voltage mode. At this point, the voltage is kept constant to keep the battery at maximum charge; While the flow decreases slowly. This feature prevents overcharging and damage to the battery. Constant voltage charging also means that as you get closer to 100% charge, the charging speed of the device will decrease.


The process of decreasing the current intensity will continue until the battery is charged to its maximum fineness, and finally, when the battery capacity reaches 100%, the charging process of the device will stop completely. At this point, it’s best to unplug your smartphone.

How does this affect fast charging?

You may have noticed that some smartphone manufacturers refer to fast charging technology in their products; A feature that can charge your device’s battery up to 50 or 80% in a very short time. This is because fast charging is a conditional process. If you currently have a high battery life, fast charging technology is unlikely to have much of an impact on your device.

Rule 80-40 Battery charge

Fast charge can only be used during the constant current setting period. When you reach two modes of constant voltage regulation or the battery capacity reaches 80% or more, security measures will be taken to protect the battery cell from damage.

Overcharging the battery is not a good idea; Because at best, it may cause damage that will affect the battery’s ability to hold a charge. In the worst case, it will increase the temperature of the battery, which can lead to physical damage. Fortunately, modern smartphones manage the charging process automatically to prevent this from happening.

How do smartphones protect your battery?

Older rechargeable batteries had a problem called “memory effect”; Of course, this problem does not exist in lithium batteries. The problem with the memory effect is that the battery loses its capacity. Lithium batteries are not completely without problems and their capacity decreases over time and each time the charge cycle is repeated.

Optimal charge of my iPhone

The charge cycle does not simply mean a capacity increase from 0 to 100%; Rather, it indicates cumulative battery wear. For example, a charge of 50 to 100% on two consecutive days uses a full charge cycle. Some smartphone manufacturers offer a feature called “optimal charging” that helps prevent premature battery wear.

Optimized charging works by charging the device up to 80% and waiting. By learning your habits and routine of using the phone, the device will schedule the last stage of charging with the time when you most likely remove the charger (for example, in the morning). The iOS 13 operating system has added this feature to all iPhones, and OnePlus introduced a similar feature for its smartphones in January 2020.

Keep your battery in perfect health

There are a few things you can do to help keep your battery healthy. The first point is to avoid draining it completely if possible. Lithium batteries use shallow discharge; Where the 80-40 rule comes into play. Of course, this rule is sometimes known as 70-40.

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This logic dictates that for the health of the battery, it is better not to let its capacity be reduced to less than 40% and also not to charge it more than 80%. Lithium cells have the best performance and storage in this case, and reaching the battery level to more or less than these numbers can negatively affect the battery health of your device.

Unfortunately, this law cannot always be enforced; Unless you always pay attention to the smartphone and its charge percentage. Apple’s Shortcuts app allows users to create automations to automate certain tasks. In this way, you can always be informed about the charging status of the device by using this application.

To ensure optimal charging is enabled, you can go to Settings> Battery> Battery Health on iPhones. Of course, it takes a few weeks to learn how to use your phone, and after that, your battery will be managed intelligently.

If you do not have the option to use optimal charging with software management, the best rule of thumb is to charge your device more carefully. To do this, be careful not to connect the phone to the charger overnight. In fact, it is better to charge your phone daily and at shorter intervals.

This requires conscious effort, and may be a normal change to achieve the best possible result. If you are one of those people who change your smartphone every few years, this is a great way to avoid reducing the battery life of your device.

These features are not just for smartphones

Other devices that use lithium batteries, including tablets, laptops, game controls, wearables, and even powerbanks, can all benefit from a higher charge. Many of these devices do not have special charging modes; But all of them benefit from shallow discharge cycles and the 80-40 rule.

Other than the methods mentioned in this article, what other methods do you know to increase battery life?

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