How the battery in a mobile device is charged?

The principle of battery charging on a modern mobile device is a complex process controlled by several chips that work together to ensure safety, efficiency and extended battery life. Let’s look at the main components and stages of the charging process using the IPhone as an example.

First, let’s understand what’s involved in the circuit and further break down what’s wrong with the myth.

  • Power Management Controller (PMIC)

The main chip responsible for charging the battery is the power management controller, or PMIC (Power Management Integrated Circuit). It distributes power to all of the device’s systems, manages the charging and discharging of the battery, and controls that power is only applied when it is safe to do so. The PMIC is responsible for:

  • Voltage and current regulation;
  • Protection against overcharging and overheating;
  • Controlling that the battery charges at the optimum rate without overheating.

Apple devices use a proprietary PMIC that interacts with the iOS operating system, allowing the phone to regulate charge and protect the battery.

  • Battery Charging IC (Battery Charging IC)

The Battery Charging IC is a chip that directly controls the charging process of the battery. It controls the charging current and voltage to ensure that power is safely delivered to the battery. The Charging Controller IC adapts to the state of the battery, allowing it to:

  • Charge faster when the charge is low (at the beginning of charging);
  • Reduce current as it approaches 100% to prevent overheating and wear and tear.

The IC is also able to slow down the charging process if it detects that the device temperature is too high.

  • Battery Management System (BMS) microcontroller

Battery Management System is a system on the battery itself that monitors the battery status, charge, voltage, temperature and degradation level. The BMS transmits real-time data about the state of the battery to the iOS operating system, which decides how to optimise the charge. For example, when Optimised Battery Charging is activated, BMS adjusts the charging rate to avoid the battery always being at 100% at night.

  • Temperature sensors

Temperature sensors are located throughout the device, including the battery and power controller, and provide safety during charging. They allow the system to automatically reduce the charge current or stop the process completely if the device overheats. This is an important element as lithium-ion batteries are sensitive to elevated(!!!) temperatures which can shorten their life.

  • Processor (CPU) and Operating System (iOS)

The CPU and iOS play a supporting but critical role in charging control. The operating system processes data from the BMS and temperature sensors and controls the execution of functions such as Optimized Battery Charging*. iOS regulates the charging speed and, if necessary, activates protective measures such as stopping charging when overheating occurs.

How does the charging process work?

  • Initial phase (fast charging)
    When the charge level is low, the PMIC and Battery Charging IC supply an increased current to quickly charge the battery to approximately 50-80%. At this point, the Charge Controller IC adapts to the state of the battery, maintaining the optimum rate and preventing overheating.
  • Transition to constant current
    As 80% is reached, the charging rate slows down. The controller switches to constant current, where the current gradually decreases. This prevents overheating and increases battery life.
  • Full charge and overcharge control
    Once 100% is reached, the PMIC and Battery Charging IC controllers stop charging, protecting the battery from overcharging. If the device is plugged in, the PMIC continues to supply power directly to the phone, bypassing the battery. This minimises discharge cycles and slows degradation.

Why does the battery swell?

A battery, especially a lithium-ion or lithium-polymer battery, bloating is the result of chemical processes that occur inside the battery due to various factors. This phenomenon is dangerous and can lead to damage to the device or even fire. Let’s take a look at the main causes of battery implosion.

  • Overcharging and overheating

One of the main causes of battery ballooning is overcharging, where the battery receives more energy than it can safely hold. This leads to overheating and causes a chemical reaction inside the battery that releases gases. Some of these gases cannot escape, causing pressure build-up and resulting in ballooning.

Example: if the charger is of poor quality and does not regulate the voltage, it may overcharge the battery.

  • Deep discharge and cell degradation

Lithium-ion batteries do not tolerate deep discharges, especially if they occur frequently. When a battery is discharged below a certain level, it causes chemical changes that can irreversibly damage the cell structure. Over time, this leads to degradation and accumulation of by-products such as gases that cause ballooning.

  • High temperatures

High temperatures can accelerate decomposition processes in lithium-ion batteries. When heated, chemical reactions in the cells proceed faster, which can lead to the release of gases. Overheating can be caused by external factors (e.g. leaving the phone in the sun) or internal factors (e.g. using the device under heavy load while charging).

  • Physical damage

Mechanical damage, such as puncturing or squeezing the battery, can cause a short circuit inside the cells, resulting in heat and off-gassing. Physical damage can also destroy the insulation, making the battery unstable.

  • Battery aging and natural degradation

Over time, the chemistry and structure of lithium-ion batteries degrade even under normal use. During charging and discharging, unwanted by-products can accumulate in the cells and gradually lead to ballooning. While ballooning is not necessarily a sign of an aging battery, it is more likely to occur in end-of-life devices.

  • Substandard components or manufacturing defects

Batteries manufactured improperly or with substandard materials may have defects that accelerate degradation or cause a reaction under normal conditions. As a result, such batteries may explode much earlier than expected.

Why does a battery crystallise?

Lithium-ion battery crystallisation is a phenomenon in which solid crystalline structures called dendrites form inside the cells. Dendrites are thin metallic growths that can grow between the layers of the battery and cause serious problems, up to and including short circuits and battery failure. Let’s look at why this happens and how it affects the battery.

1. Crystallisation mechanism and dendrite formation

Crystallisation, or dendrite formation, occurs due to the growth of metallic growths, mainly lithium, during charging and discharging. This phenomenon is observed under the following conditions:

  • High charging currents: During rapid charging and discharging, lithium ions can travel too fast, depositing unevenly on the battery anode. This creates conditions for lithium to be deposited in the form of dendrites.
  • Deep discharge: When a battery is heavily discharged, the internal cell voltages change, which can cause lithium ions to be unevenly distributed. This favours their accumulation in certain areas, which also leads to the formation of dendrites.
  • Low temperatures: At low temperatures, lithium ions begin to move slowly in the electrolyte. This can lead to their precipitation in the form of crystals, as they do not have time to be properly distributed on the anode surface.

2. How dendrites damage the battery

  • Short circuit: Dendrites growing towards the cathode can penetrate the separator, the thin layer of material that separates the anode and cathode. If the dendrites reach the cathode, it causes a short circuit. In some cases, this can cause the battery to overheat, smoke and even catch fire.
  • Reduced capacity and efficiency: The formation of dendrites takes up space in the cell, which reduces the available capacity of the battery. Crystallisation also reduces the battery’s ability to hold a charge as some of the active material is bound up in unused crystals.

Why crystallisation occurs over time?

Dendrites most often occur in batteries that are subjected to significant wear and tear — constant deep discharging, rapid charging and operation in unfavourable conditions (high or low temperatures). Over time, such batteries lose their original properties and lithium crystalline deposits accumulate in them, accelerating degradation and leading to failure.

Why does the battery become soft as a ‘rag’?

When a battery becomes soft as a rag, it is usually the result of significant degradation and depletion of the active materials within the battery. This effect is most commonly seen in lithium polymer batteries because they are encased in a flexible polymer pack that loses its elasticity over time.

The main causes of this phenomenon are as follows:

  • Decomposition and depletion of active materials

During the charging and discharging process, ions move between the anode and cathode inside the battery. Over time, these chemical processes wear down the active materials, causing them to degrade and decrease in quantity. When most of the active material degrades, the battery loses its ability to hold energy and the electrode structure becomes more friable. This causes the battery case to lose its shape and become softer.

  • Loss of internal pressure

Lithium polymer batteries contain a liquid or gel-like electrolyte that maintains the internal structure of the battery under pressure. However, as the battery degrades, the electrolyte breaks down or evaporates, which reduces the pressure inside the pack. As a result, the battery becomes softer as its walls are no longer supported by internal pressure.

  • Decrease in mechanical integrity

Over time, due to numerous charging and discharging cycles, the anode and cathode structure physically deteriorates. The electrodes begin to break down and turn into small particles, which weakens their strength and elasticity. This makes the entire battery pack less stable and softer to the touch.

  • Outgassing and loss of solidity

During normal battery operation, gases may be released due to chemical reactions. In newer batteries, these gases are held under high pressure or vented to the outside. But in older, degraded batteries, the gas mixture can escape from the active materials, causing the battery to become not only soft, but sometimes slightly bloated. At the same time, the battery structure itself loses elasticity and the outer shell begins to resemble a soft pouch.

  • Decrease in capacity and loss of density

When a battery loses a significant portion of its capacity, its internal materials no longer hold energy and therefore lose density. This also makes the battery less rigid, and it becomes ‘rag-like’, especially if the battery is severely discharged and cannot regain its shape even when fully charged.

  • Breakdown of the polymer shell

Lithium polymer batteries are encapsulated in a soft polymer shell that wears over time and loses its elasticity. Mechanical stresses, such as from expansion and contraction of the battery during charging and discharging, can weaken the polymer structure. This causes the outer pack to become soft and lose its shape.

Why does this happen more often with lithium polymer batteries?

Lithium polymer batteries have a flexible shell that is easier to deform and does not maintain the shape of the battery the way the rigid metal casing in lithium-ion batteries does. This is why lithium polymer batteries are more prone to develop a soft ‘raggy’ texture after severe degradation.

 

What is the Optimized Battery Charging feature in iPhone?

The Optimized Battery Charging feature in iPhone is designed to reduce battery wear and tear and extend battery life. It is based on preventing the battery from staying fully charged (100%) for long periods of time, which accelerates battery degradation. Here’s how this feature works.

How Optimised Battery Charging works?

  1. Behavioural pattern analysis
    Optimized Battery Charging uses machine learning to analyse your daily routine. For example, if you usually charge your phone at night and wake up at a certain time, the iPhone ‘remembers’ this and adapts to that schedule.
  2. Stop charging at 80%
    When Optimised Battery Charging is on, iPhone charges to 80% and then stops. This reduces the amount of time the battery is at its maximum charge level, preventing unnecessary wear and tear.
  3. Charge to 100% before use
    As you approach your expected wake-up time or the time when you normally take your phone off charge, the feature activates a top-up to 100%. This way, the phone is ready for full use when you need it, but avoids unnecessary time at maximum charge.
  4. Using location and time data
    iPhone can use data about your location and daily schedule to determine when the feature is needed. For example, if you are away from home or travelling, iPhone may not apply Optimised Battery Charging because your charging schedule may be different from usual.
  5. Monitor battery temperature
    If your phone detects that the battery temperature is high, it can further limit charging to avoid overheating, which also helps extend battery life.

Why is it good for the battery?

The lithium-ion batteries used in iPhones are more susceptible to degradation when left in a fully charged state for long periods of time. Optimised Battery Charging helps minimise the impact of this, allowing the battery to wear out more slowly and retain its capacity for longer.