How are Li-ion Batteries charged?

This informal CPD article ‘How are Li-ion Batteries charged?’ was provided by Dr. Frank Richter CEO of Greenectra, renowned battery experts, who can help you with training, recruiting, and consulting to fully optimize your battery innovation capabilities.

Understanding the Charging Process of Lithium-Ion Batteries

Lithium-ion (Li-ion) batteries have become the key element of many portable electronics, electric vehicles, and energy storage systems. When talking about renewable energy, Li-ion batteries can, for example store the energy from photovoltaic systems. Their efficiency, energy density, and long lifespan (if correctly configured) make them ideal as a power source and ideal for energy storage in many applications. However, understanding how these batteries are charged is crucial for maximizing their performance, longevity, and ensuring safety.

Basics of Lithium-Ion Batteries

Components of Li-ion Batteries 

• Anode: Typically made of some carbon, such as graphite. The anode accepts the lithium ions during charge.
• Cathode: Typically made from a lithium metal oxide. The cathode releases lithium ions during charge.

During charge, the way how we use the terms anode and cathode is not in line with common electrochemical definitions. It is a slang that has manifested over the years.

• Electrolyte: A lithium salt in a solvent facilitating the movement of Li-ions between the anode and cathode.
• Separator: A porous membrane (often multiple layers) in between anode and cathode that prevents an internal short.

How Li-ion Batteries Work

Li-ion batteries operate through charge and discharge cycles. During discharge, Li-ions move inside the battery from the anode to the cathode through the electrolyte. Outside the battery, we can use an electric current, which is electrons moving from anode to cathode. During charging, this process is reversed, with Li-ions and electrons moving back to the anode.

Charging Phases of Lithium-Ion Batteries

Li-ion batteries are typically charged in 2 phases.

Constant Current (CC) Phase: The charging process begins with the constant current phase. During this phase, the charger supplies a constant current to the battery, gradually increasing its voltage. This phase continues until the battery reaches its upper voltage limit, which is for example around 4.2 V for NMC cells.

Constant Voltage (CV) Phase: Once the battery reaches its upper voltage limit, it is not fully charged yet, unless the CC phase was carried out at a super low current rate. In the CV phase the voltage is held constant at the upper voltage limit, while the current gradually decreases. The CV phase ends when the decreasing current reaches a certain predefined value.

An optional 3^rd charging phase: Li-ions are subject to a low self-discharge. In some cases, there can be a 3^rd charging phase in which a very small charging current counteracts the self-discharge, keeping the battery at 100% state of charge. However, keeping the battery at the upper voltage limit does increase the aging (capacity loss and power loss) of the battery.

Key Parameters in Charging

Voltage and Current Limits

Maintaining proper voltage and current limits is critical. Exceeding the voltage limit can lead right into thermal runaway and result in overheating, toxic gass production, fire, or even explosion. Current limits somehow balance charging speed and battery health, as higher currents can degrade the battery faster. Even with the best thermal management system, charging cannot be carried out at any given charging rate. Increasing the charging rate, especially at low temperatures can lead to lithium plating, which can create an internal short circuit and also lead right into thermal runaway.

State of Charge (SoC)

The state of charge indicates the battery's current charge level, typically expressed as a percentage from 0% (fully discharged) to 100% (fully charged).

Temperature Management

Temperature plays a significant role in charging efficiency and charging safety. Chargers and the devices containing the battery often include temperature sensors to monitor the battery's temperature. Especially at low temperatures, there is the risk of lithium plating, which was briefly described before. When we think about thermal management, we often think only about getting rid of the heat during operation. But thermal management can also mean increasing the battery’s temperature to allow for a higher charging rate.

The Battery Management System (BMS) and the Safety Mechanisms in Charging

Monitoring

A BMS is important to monitoring and manage the charging process. It calculates the state of charge and measures voltage, current, and temperature, ensuring the battery is charged safely and efficiently.

Overcharge Protection

The overcharge protection mechanism prevents the battery from being charged above the voltage maximum.

Over-current Protection

The over-current protection mechanism prevents the battery from being charged at a too high current rate.

Thermal Runaway Prevention

Thermal runaway is a heat-driven stepwise process where a battery overheats uncontrollably, leading to production of toxic gasses, fire or even explosion scenarios.

Overcharging and charging at a too high current can lead to thermal runaway. As a result, the overcharge and over-current protection mechanisms can be seen as thermal runaway prevention. Modern chargers and BMS include additional features to detect and prevent thermal runaway, such as temperature sensors and automatic shutdown mechanisms. But this does not mean that a thermal runaway can always be stopped.

Factors Affecting Charging Efficiency

Age of the Battery

As batteries age, their capacity decreases, and the internal resistance increases. This affects charging efficiency and the thermal management, because a higher internal resistance leads to more heat produced during the charging. This produced heat is a loss, decreasing efficiency, but it is also something that needs to be thermally managed. If the thermal management is not capable of handling a certain produced heat (to keep the battery at save charging conditions), the charging rate needs to be decreased and charging time increases.

Environmental Conditions

Temperature influences charging efficiency. The internal resistance of the battery is lower at higher temperatures (of course always keep it within reasonable limits) and the risk of an internal short due to lithium plating is also decreased at higher temperatures.

Charging rate

Heat production (and also as a result thermal losses) are increased at high charging rates, decreasing efficiency. The higher temperature improves the efficiency to some extent due to the decreased internal resistance.

Conclusion

Understanding the charging process of lithium-ion batteries is essential for maximizing their performance and ensuring safety. Safe charging rates are typically lower than safe discharging rates. At low temperatures, safe charging rates are lower than at higher temperatures.

We hope this article was helpful. For more information from Greenectra, please visit their CPD Member Directory page. Alternatively, you can go to the CPD Industry Hubs for more articles, courses and events relevant to your Continuing Professional Development requirements.

Legal Disclaimer

The information provided in this article is for general informational purposes only and represents a personal view. All information is provided in good faith; however, Greenectra OÜ makes no representation or warranty of any kind, express or implied, regarding the accuracy, adequacy, validity, reliability, availability, or completeness of any information.

Under no circumstances shall Greenectra OÜ be held liable for any loss or damage of any kind incurred as a result of the use of this information or reliance on any information provided. Your use of this information and your reliance on any information is solely at your own risk.