Why iPhones Slow Down Charging at Around 80%

The process of charging your iPhone slows down as it reaches approximately 80% battery capacity. This behavior is not unique to iPhones; it is a common strategy adopted by many smartphone manufacturers to protect the battery's health and extend its overall lifespan.

Understanding the Battery Charging Process

Smartphone batteries, such as the lithium-ion batteries found in iPhones, are designed to charge quickly when the battery is low and then slow down as they near full capacity. This is due to the inherent characteristics of lithium-ion batteries, which allow for faster charging when the battery is low and naturally decelerate as it nears its full discharge capacity.

Protecting the Battery for Longevity

As the battery charges and reaches around 80%, the charging speed decreases. This is a protective measure to reduce the stress on the battery, which can help prevent overheating and extend the overall lifespan of the battery. This behavior is similar to what occurs in other lithium-ion batteries.

How Lithium-Ion Batteries Operate During Charging

Lithium-ion batteries have both current and voltage limits during charging. Too much current or too much voltage can be dangerous, even leading to the risk of starting a fire. Therefore, the charger must monitor and control these parameters to ensure safe operation.

Fast Charging and Voltage Management

When a lithium-ion battery is at a relatively low charge state, it can accept a high charging current, and its terminal voltage is low. Consequently, the charger feeds the battery the maximum safe current, while the voltage rises. This is the fastest charging period.

As the battery nears its maximum charge voltage (typically around 4.1 or 4.2 V), the charger circuitry switches to maintaining a constant voltage. At this point, the charge current naturally starts tapering off. It may take several more hours for the battery to reach a full charge as the current drops to almost zero while the voltage remains at its maximum. Then the charger will shut off entirely until the voltage drops below a certain threshold, at which point it will resume charging.

Comparison with Other Battery Types

While the charging behavior of lithium-ion batteries is similar to lead-acid batteries, there are some differences. For example, lead-acid batteries are typically fast-charged by a similar process, but they cannot be charged as quickly and are usually left in a "float" state at the end of the charge period instead of the charger shutting off completely.

Understanding the charging process and how it affects your iPhone's battery is crucial for maintaining optimal performance and longevity. By managing your charging habits, you can help ensure that your iPhone remains in top condition for as long as possible.