What is equalization charging for lithium-ion battery packs? In today's highly advanced technological world, various high-tech solutions have appeared in our lives, bringing us convenience. But are you familiar with equalization charging for lithium-ion battery packs, which may be a component of these high-tech solutions?
When lithium-ion battery packs are manufactured and stored for an extended period, the voltage of each cell in the pack becomes inconsistent due to differences in static power consumption across the protection board and varying self-discharge rates among individual cells. Balancing mechanisms in lithium-ion battery packs balance the voltage, enabling full charging and discharging of the battery pack and maximizing its efficiency.
With current lithium-ion battery pack manufacturing capabilities and processes, slight differences will inevitably exist between individual lithium-ion battery cells during the production process, a phenomenon known as inconsistency. These inconsistencies manifest primarily in aspects such as cell capacity, internal resistance, self-discharge rate, and charge/discharge efficiency. This inconsistency between individual lithium-ion battery cells, when transferred to the battery pack, inevitably leads to a loss of capacity and consequently a reduced lifespan.
Common lithium-ion battery pack equalization charging technologies include constant shunt resistor equalization charging, on-off shunt resistor equalization charging, average battery voltage equalization charging, switched capacitor equalization charging, buck converter equalization charging, and inductor equalization charging. A lithium-ion battery pack must be charged in series; each battery should be kept balanced during charging. Otherwise, the performance and lifespan of the entire battery pack will be affected during use.
The balance charging management system for lithium-ion battery packs is mainly divided into passive balancing and active balancing. In a series-connected battery pack, although the current flowing through each individual cell is the same, the depth of discharge will vary due to differences in capacity, and the total capacity is large. Shallow charging and shallow discharging, and overcharging and over-discharging will always occur when the capacity is low. This will result in slow degradation of large capacity cells and extended lifespan; while small capacity cells will experience accelerated degradation and shorten lifespan, and the difference between the two will become increasingly larger.
Parallel balancing circuits are added to each individual cell of a lithium-ion battery pack to achieve current sharing. In this mode, when a battery is fully charged first, the balancing device prevents it from overcharging and converts excess energy into heat, while continuing to charge the undercharged battery. This method is simple, but it results in energy loss and is not suitable for fast-charging systems.
Without charge balancing, smaller capacity batteries will be overcharged to some extent. Before any cell in the group is over-discharged, the group's output power will be slightly higher than that cell's minimum capacity. With charge balancing, smaller capacity batteries will not be overcharged, and their actual charge capacity will be less than the other cells. The power that can be released is less than that without equalization, resulting in shorter discharge times for individual cells and a greater likelihood of over-discharge.
Before charging, each battery is discharged to the same level under the same load, and then charged with a constant current to ensure a more accurate balance between the batteries. However, regarding battery packs, due to the physical differences between individuals, it is difficult to achieve a completely consistent ideal effect after each battery has been deeply discharged. Even if the same effect is achieved after discharge, new imbalances will appear during charging.
Using lithium-ion battery balancing technology can solve mismatch problems, thereby improving the performance of series-connected lithium-ion battery packs. Battery mismatch can be corrected by balancing the batteries during the initial adjustment process, and then compensation can be made simply during charging. Although the defect rate of some lithium-ion battery manufacturers may be very low, we still need to supply further quality assurance to prevent premature battery life.
After comprehensively analyzing the advantages and disadvantages of various methods, the following issues were identified as significant: 1) Equalization time is a common and serious problem, often lasting several hours. 2) Most existing equalization techniques are based on external battery voltage equalization. Due to differences in individual battery capacity, the charge and discharge voltage characteristics of each individual battery are inconsistent, especially in the later stages of charging when the individual battery voltage rises rapidly. Using the external battery voltage as a standard for battery pack consistency presents a problem, namely, the balancing standard is unstable. Furthermore, research shows that this method has no significant impact on the increase in usable capacity of the battery pack before and after equalization. 3) Poor practicality; the circuit design cannot consider the operating conditions of electric vehicles and cannot be changed as the number of series-connected battery packs increases. Modular expansion, etc., are not feasible.
