Fast charging method
To maximize the speed of chemical reactions in batteries, shorten the time it takes for them to reach a full charge, and minimize or reduce polarization of the positive and negative plates, thereby improving battery efficiency, fast charging technology has developed rapidly in recent years. Several commonly used fast charging methods are introduced below. These methods are designed around the optimal charging curve, aiming to make the actual charging curve as close as possible to the optimal charging curve.
Pulse charging method
The pulse charging method first charges the battery with a pulse current, then stops charging for a period of time, and then charges the battery again with a pulse current, repeating this cycle, as shown in Figure 11-5. The charging pulse fully charges the battery, while the intervals allow time for the oxygen and hydrogen produced by the chemical reactions to recombine and be absorbed, naturally eliminating concentration polarization and ohmic polarization. This reduces the battery's internal pressure, allowing the next round of constant current charging to proceed more smoothly and enabling the battery to absorb more charge. The intermittent pulses provide the battery with sufficient reaction time, reducing gas evolution and improving the battery's charging current acceptance rate.
Pulse charging can improve battery charging and discharging efficiency, save charging time, and extend battery life, but certain conditions must be met.
Since lithium-ion batteries rely primarily on the reciprocating movement of lithium ions in the cathode, anode, and electrolyte during charging and discharging, increasing the ion movement rate and diffusion coefficient is essential to achieve the goal of saving charging time and improving charging efficiency. An inappropriate charging rate or distribution will not only fail to achieve the desired goal but will also accelerate battery aging. As shown in Figure 11-2, the higher the charging rate, the smaller the chargeable capacity.
Electrochemical characteristic analysis shows that the charging rate of lithium-ion batteries is mainly limited by the diffusion rate of lithium ions and the characteristics of the positive and negative electrode materials. Using equation (11-3), a lithium-ion diffusion equation can be established.
In the formula, CL represents the lithium-ion concentration; x represents the diffusion distance; t represents the diffusion time; and DLi represents the lithium-ion diffusion coefficient.
Studies have shown that lithium-ion batteries exhibit two rapid aging periods during cycle testing:
1) The formation period of the SEI film (Solid Electrolyte Interphase), a process that consumes a portion of the available lithium ions to form the SEI film on the electrode surface.
2) At the end of each charging cycle, the resistance to the migration of lithium ions in the liquid phase inside the battery is relatively small, while the diffusion coefficient in the solid phase is relatively small. Therefore, if the charging current is too large at the end of the charging cycle, a large number of lithium ions will concentrate on the electrode surface, which can easily lead to the formation of lithium metal and reduce the lithium ion content.
The formation of the SEI film has a significant impact on battery life. If a good SEI film cannot be formed, although the battery may have high charge/discharge efficiency and usable capacity in the initial charge/discharge stages, the capacity will decrease sharply with increasing cycle count, especially in low-current pulse charging modes. Therefore, lithium-ion loss in the first stage is unavoidable. However, the current during pulse charging should not be too high. Excessive current will cause uneven SEI film formation, rapid SEI film thickening, and a significant increase in resistance, reducing the number of usable ions and leading to capacity loss.
Pulse charging primarily utilizes charging pauses or reverse discharge to eliminate polarization during the charging process. Polarization is closely related to battery type, manufacturing process, and material properties, and its variations are complex. With continuous improvements in battery technology, polarization has been well controlled. Therefore, the advantages of conventional pulse charging modes are not as pronounced. It is necessary to combine relevant battery characteristic parameters for real-time monitoring and adjust the amplitude and cycle of pulse charging to keep the battery in optimal operating condition-the intelligent charging mode described later.
Reflex™ Fast Charging Method
The ReflexTM fast charging method is a patented technology in the United States, initially designed primarily for charging nickel-cadmium batteries. This charging method alleviates the memory effect problem of nickel-cadmium batteries, thus significantly reducing the fast charging time. Compared to pulse charging methods, the biggest feature of the ReflexTM fast charging method is the addition of a negative pulse. Its mechanism utilizes the "barrier" effect provided by the negative pulse to eliminate bubbles generated on the electrode surface during the reaction process, reducing the temperature rise and increase in internal resistance during battery charging. This allows electrical energy to be converted into chemical energy within the battery as fully as possible, helping to eliminate concentration polarization caused by slow diffusion, improving the utilization rate of active materials within the battery, and thus increasing the number of charge-discharge cycles.
As shown in Figure 11-6, one working cycle of the ReflexTM fast charging method includes three stages: a forward charging pulse, a reverse instantaneous discharge pulse, and a pause in charging for maintenance. The function of the forward charging pulse is to provide a positive amplitude pulse current to charge the battery; the function of the reverse instantaneous discharge pulse is to make the electrolyte ions diffuse more evenly to delay the polarization reaction inside the battery, thereby improving the charging efficiency and increasing the battery life; the function of the stop charging phase is to make the electrolyte ions diffuse more evenly and mitigate the polarization phenomenon, thereby improving the charging efficiency and extending the battery life.
Variable Current Intermittent Charging Method
The variable current intermittent charging method is based on constant current charging and pulse charging, as shown in Figure 11-7. Its characteristic is that the constant current charging section is replaced with a voltage-limited variable current intermittent charging section. In the initial stages of charging, the variable current intermittent charging method is used to ensure a higher charging current and obtain the majority of the charging amount.
During the later stages of charging, a constant voltage charging phase is used to obtain overcharge and restore the battery to a fully charged state. By intermittently stopping charging, the oxygen and hydrogen produced by the chemical reaction in the battery have time to recombine and be absorbed, naturally eliminating concentration polarization and ohmic polarization. This reduces the internal pressure of the battery, allowing the next round of constant current charging to proceed more smoothly and enabling the battery to absorb more electricity.
Based on the variable current intermittent charging method, a variable voltage intermittent charging method has been proposed, as shown in Figure 11-8. The difference between the variable voltage and variable current intermittent charging methods is that the first stage is not intermittent constant current, but intermittent constant voltage.
Comparing Figures 11-7 and 11-8, it can be seen that Figure 11-8 better reflects the optimal charging curve. In each constant voltage charging stage, due to the constant voltage charging, the charging current naturally decreases exponentially, consistent with the characteristic that the acceptable battery current gradually decreases during the charging process.
Variable Voltage, Variable Current Wave-Type Intermittent Positive and Negative Zero-Pulse Fast Charging Method
Combining the advantages of pulse charging, Reflex™ fast charging, variable current intermittent charging, and variable voltage intermittent charging, the variable voltage, variable current wave-type positive and negative zero-pulse intermittent fast charging method has been developed and applied. The control of the pulse charging circuit generally falls into two categories:
1) The amplitude of the pulse current is variable, while the frequency of the PWM (power PWM) signal is fixed.
2) The amplitude of the pulse current is constant, while the frequency of the PWM signal is adjustable.
Figure 11-9 employs a control mode different from these two: both the pulse current amplitude and the frequency of the PWM signal are fixed, while the PWM duty cycle is adjustable. By adding an intermittent charging/stopping phase, more charge can be gained in a shorter time, improving the battery's charging acceptance capability.
Each charging mode has its own advantages and application scope. However, with the widespread use of electric vehicles, the demand for charging speeds is increasing, leading to the emergence of smart charging. Smart charging primarily aims to fully charge the battery or reach a set capacity in a short time. This is achieved by adjusting the current value according to the battery's State of Charge (SOC) and State of Health (SOH), making the charging time comparable to that of refueling a traditional vehicle.
Smart charging has attracted widespread attention. Researchers worldwide are focusing significant resources on researching charging control strategies to improve charging rates while effectively ensuring battery lifespan. This, in turn, enhances the practicality and social acceptance of electric vehicles.
