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Explore the science behind trickle charging in lithium-ion batteries and why it's crucial for reaching full battery capacity, preventing polarization, and extending battery life.
Lithium-ion batteries are widely used in modern electronics such as smartphones, laptops, and electric vehicles. Understanding how to properly charge these batteries is essential to ensure their longevity and optimal performance. In this article, we will focus on the importance of trickle charging in the final stages of the charging process and why it's necessary to fully charge lithium-ion batteries.
Common lithium-ion batteries are composed of several key components, including the anode, cathode, electrolyte, separator, current collector, casing, and external circuits. During the charging process, two primary types of conduction occur: electron conduction and ion conduction.
Electron conduction takes place within the active material, current collectors, and external circuits. Ion conduction occurs in the active material and electrolyte. The key difference is that ion conduction is much slower than electron conduction. This difference leads to a phenomenon known as polarization, where ions move slower than electrons, leading to a delay in the charging process.
The slow ion conduction is caused by the mass of lithium ions and the resistance within the battery materials. Additionally, barriers such as the solid electrolyte interphase (SEI) layer can further impede ion movement. Polarization becomes more pronounced when the battery is nearly full, slowing down the ion transfer significantly.
In battery testing experiments, such as those conducted on a ternary button-type half-battery, it has been observed that after charging the battery to 4.3V at 0.1C, the voltage drops to 4.24V after resting for two minutes. This indicates that the battery is not yet fully charged, even though the voltage has reached the preset target.
This voltage drop is more pronounced when charging at higher rates, such as 0.5C (which can be considered fast charging). The main reason behind this phenomenon is polarization. When charging fast, the ions inside the battery move slower, which causes a temporary voltage increase, but the actual battery capacity isn't fully reached.
At this point, the charging process requires a lower current to allow the battery to reach its full capacity. This is where trickle charging becomes essential.
Trickle charging involves applying a small, constant current or voltage to the battery after it has reached the target voltage. This helps overcome polarization, allowing the ions to move at a slower pace and fully charge the battery.
During fast charging, such as 2C or 5C, the speed of ion conduction significantly slows down, especially after the voltage reaches 3.8V. This is because the rate of lithium-ion transfer in the cathode decreases as the charging progresses, causing the battery to charge slower. In the final stages, the polarization becomes more severe, and charging can be limited to around 80% of the battery's capacity during fast charging.
Another important factor to consider during the charging process is the heat generated by the battery. Fast charging increases the internal resistance of the battery, leading to significant heat buildup. Excessive heat can degrade the battery’s performance and shorten its lifespan.
Trickle charging helps reduce heat buildup by applying a low current to the battery, allowing it to charge slowly and safely. This process helps prevent overheating, battery swelling, and thermal runaway, ensuring that the battery remains functional for a longer period.
In summary, trickle charging is an essential process for lithium-ion batteries during the final stages of charging. It ensures that the battery reaches full capacity, mitigates polarization, reduces heat buildup, and prevents long-term damage. As devices continue to support faster charging rates, incorporating trickle charging methods in the final stages of charging will become even more critical to preserving battery health and extending overall battery life.
Apple’s lithium-ion batteries allow for flexible charging at any time without needing to discharge them to 100% first. A charge cycle is completed when 100% of the battery’s capacity is used, but not necessarily in a single charge. For instance, if you use 75% of the battery's capacity one day, fully charge it overnight, and use 25% the next day, you complete one charge cycle over two days. Multiple days may be needed to complete a full cycle. While the battery’s capacity decreases slightly with each cycle, Apple’s lithium-ion batteries retain at least 80% of their original capacity after numerous charge cycles, with variations depending on the product.