In the age of electrification, promising technologies like battery electric vehicles and electric aircrafts are coming into the forefront of societal advancements. However, one major hurdle in electrification is speeding up the vehicle battery charging time. Re-fueling conventional vehicles and aircrafts generally takes minutes, whereas electric vehicles and aircrafts can take hours. For airlines, this down time can be very expensive, and a long charging period has been shown to reduce the likelihood of consumers purchasing pure electric vehicles.
To combat this, battery engineers are exploring options to reduce the amount of time required to charge a battery. Unfortunately, fast charging Li-ion batteries can cause premature battery degradation by initiating lithium plating, so an aging cost for fast charging must also be considered. Because of this, the system manufacturer (i.e automotive OEM, aircraft OEM, power tool OEM, or consumer electronics OEM) has to strike a balance between decreasing time required to charge a battery and the expected life of the battery (which can have a great effect on brand perception).
There are various charging protocols that both improve the battery life and shorten the charging time, when compared to traditional charging protocols. The effects of these charging protocols vary from cell to cell and need to be tested individually to fully understand their effects on cell charging time and degradation rate.
Testing and optimizing charging protocols is extremely resource intensive because it intentionally degrades lithium-ion cells, which can be expensive and time consuming. GT-AutoLion helps reduce this cost by supplementing experimental tests with virtual tests.
What is Lithium Plating?
One of the key contributors to battery degradation that comes with fast charging is lithium plating. Lithium plating is the reduction of lithium ions into lithium solid. It is caused when the potential in the anode falls below zero volts and cycling lithium-ions (Li+) reacts with electrons (e–) to form lithium metal (Li+ + e– -> Li). This lithium metal is deposited into the anode, lowering the porosity of the anode. Because Lithium-ions are consumed in this reaction, it decreases the capacity of the cell. Additionally, because it lowers the porosity of the anode, it increases the resistance of the cell.
Lithium plating occurs most frequently when Li-ion cells are charged with very high currents, especially at low temperatures.
Figure 1, taken from a paper using GT-AutoLion, shows how GT-AutoLion can be used to match experimental data of capacity fade with its built-in model for lithium plating. With this model, GT-AutoLion allows engineers to virtually test various charging strategies and their effect on both charging time and cell degradation.
Example Charging Protocols
A charging protocol is an algorithm which defines the charging methodology of a cell. Each charging protocol has different implementation costs and unique implications on charging time and cell degradation. Figure 2 summarizes three of the most common charging protocols.
The most common charging protocol is a constant-current-constant-voltage (CCCV) charge. During a CCCV charge, the cell is charged with constant current until a certain max voltage is reached. After, the cell discharges while maintaining the voltage at the previous max voltage, as shown in Figure 2 (left). A CCCV protocol is considered to be the simplest, safest, and most widely-used protocol to implement.
In boost charging (BC), the cell is charged with a constant boost current that is significantly higher than the subsequent constant current charge. The cell then discharges while maintaining a constant voltage. The BC protocol is shown in Figure 2 (middle). Implementing a BC protocol can decrease charging time without potentially losing cycle life.
Pulse charging (PC) is another charging protocol that can also be used. During PC, the current alternates between a high current and a low current and the voltage increases until an upper cutoff voltage is reached, as shown in Figure 2 (right). Pulse charging can reduce resistance due to diffusion, which reduces charging time and aging and improves the cycle life of a cell.
Fast Charge Strategy Development in Real-World Aging Simulation
While various charging patterns can be studied experimentally, these experimental tests often are not reflective of the real use case a battery may see in a vehicle, aircraft, power tool, or consumer electronic. As presented in a previous blog, GT-AutoLion and GT-SUITE can be used together to predict how a Li-ion battery will degrade over time while considering any use case such as various load profiles, drive cycles, and weather conditions. These analyses can also be upgraded to test the effect of the charging protocol on real-world charging time and battery degradation.
With GT-AutoLion and GT-SUITE, system manufacturers better understand the tradeoff between reducing the time required to charge a battery and maximizing the life of a battery. This tradeoff is imperative to understand because it has a profound effect on customer satisfaction and brand perception.
Written By: Vivek Pisharodi[/vc_column_text][/vc_column][/vc_row]