An equalization method based on lithium-ion battery cell SOC and cell capacity

Abstract

The invention discloses a balancing method based on lithium ion battery cell SOC and cell capacity. The working SOC window of each cell unit in a battery system can be set flexibly through the setting of target SOC, and the method gives consideration to the balance judgment from the aspect of battery parameters, and avoids a balancing error while improving the balancing efficiency. Moreover, a target electric quantity difference is used for specifically judging whether the electric quantity difference between each battery cell or a battery string and a target battery cell is within an allowed range or not. The balancing can be considered to be started if the electric quantity difference between each battery cell or the battery string and the target battery cell is out of the range. Therefore, each battery cell unit in the battery system can be enabled to work in an optimal SOC window, thereby achieving the longer service life of the battery cell units and the higher balancing efficiency, and improving the economical performance of the battery system.

Description

technical field [0001] The invention relates to the field of battery technology, in particular to an equalization method based on the SOC and capacity of a lithium-ion battery cell. Background technique [0002] As an energy storage system, lithium-ion batteries are widely used in new energy vehicles and energy storage systems. Due to the limitation of the voltage and capacity of single lithium batteries, battery cells must be connected in series, in parallel, or through a certain series-parallel combination to form The battery system provides sufficient power and energy for the electric vehicle to meet its power and cruising range requirements; if there is no difference between the battery cells in practical applications, then the battery pack and battery cells of the electric vehicle will have a significant impact on the service life and safety. However, in practical applications, due to the inconsistency of the manufacturing process and the use process environment, there ...

AI Technical Summary

Problems solved by technology

[0005] When the equalization time is sufficient, the equalization based on the battery cell voltage can equalize the battery cells in the battery system to the same voltage level; and these battery cells appear to be at the same voltage level when they stand still, which seems to be balanced However, this method has not fundamentally considered the reasons for the differences in battery cells in actual use, and designed an equalization method from the perspective of battery cell differences; in the same battery system, different The initial capacity and initial internal resistance of the battery cells are different; at the same time, the self-discharge rate of each battery cell is also different; and in actual use, the working environment of each battery cell is also different, which means that in The capacity and internal resistance of battery cells in actual use will continue to change during the aging process; these factors and other factors cause differences in different battery cells in the same battery system, including battery cell capacity, internal resistance and self-discharge rate difference

[0006] In actual use, the main disadvantages of battery cell voltage equalization are: 1) Voltage-based equalization often takes the battery cells in the battery system at the same voltage level as the equalization target, but the same voltage level at a certain moment does not The electrochemical characteristics of the same battery cell, such as the same cell capacity and cell internal resistance; this means that this method only cares about the external voltage of the battery cell rather than the electrochemical characteristics of the lithium battery; 2) Equalization based on the battery cell voltage The battery cell that needs to be balanced is selected through the measured voltage, but the cell that needs to be balanced based on the voltage judgment may become a cell that does not need to be balanced in the next judgment, and the current judgment does not need to be balanced. The judgment may also become a cell that needs to be balanced; this means that this method is sometimes self-contradictory, and the balance judgment often cannot select the battery cell that really needs to be balanced; 3) The balance based on the voltage of the battery cell is often It is designed to balance when the battery system is in a charging or static state, because this method believes that a high-voltage battery cell in a discharging state means a high SOC (State Of Charge) or more remaining power, during the discharging process It is impossible to select the battery cells that need to be balanced; this means that this method limits the time that can be balanced in practical applications, and this method generally does not support full-time balancing of the battery system; 4) When the battery system is in the initial stage of charging, The voltage of some battery cells is often higher than that of other batteries, but the situation may change in the later stage of charging. The voltage of the battery cells with lower voltage may be higher in the later stage of charging. This is often caused by the inconsistent capacity of the battery cells. This means that equalization based on battery cell voltage is best performed at the end of charging or in a static state

This also limits the time that can be equalized in practical applications. At the same time, if the equalization is performed in a static state to increase the equalization time, it also means that the time and energy consumption to maintain the entire system work are increased; 5) When the battery system is in In the charging state, based on the balance of the battery cell voltage without considering the difference in the internal resistance of the battery cell, the terminal voltage of the battery cell can be approximately calculated as Vt = Vocv+I*Rdc (Vt represents the terminal voltage ; Vocv is the open circuit voltage of the battery cell in the current state; I is the charge and discharge current, the charge is a positive value, and the discharge is a negative value; Rdc is the DC internal resistance of the battery cell or battery string); this means that if the battery The DC internal resistance of the monomer is different, so in actual use, the terminal voltage of the battery cell or battery string not only depends on the open circuit voltage Vocv, but also depends on the DC internal resistance Rdc and the charge and discharge current I, which also means A battery cell with a higher voltage in the charging state may not have a higher voltage in the static state because of the existence of the charging current I and the internal resistance I of the battery; moreover, even in the same charge-discharge cycle, the battery cell The DC internal resistance Rdc of the battery is not a fixed value. In the same cycle, the DC internal resistance Rdc of the same battery cell at different SOCs is also different. The balance based on the battery cell voltage does not change the DC internal resistance of the battery cell. Taking this into consideration, this increases the probability of misjudging the need for equalization; 6) The equalization based on the battery cell voltage often takes the charging cut-off voltage or the resting voltage as the equalization target in the charging state or the resting state, and then, the battery cell The initial capacity of the cells is not the same, and in actual use, the difference in the capacity of the cells will become larger and larger; if all the cells in the battery system are equalized to the same voltage level, in the next discharge state, the cell with a smaller capacity will The battery cell often reaches the low voltage threshold faster, and at the same time, the battery cell with a larger capacity will not reach the low voltage threshold; this phenomenon is very common in actual use, which indicates that the battery cell with a larger capacity Body or battery strings often work in the SOC window [SOC_Low, SOC_High] in actual use, where SOC_High can often reach the high SOC threshold of the battery system (often 100%), but SOC_Low is often lower than the SOC of the battery system The limit value (often 0%) is high; however, in actual use, the optimal operating SOC window of the battery cell or battery string is not from the SOC high threshold value to a lower value; based on the battery cell voltage The equalization does not consider the difference in battery cell capacity, and does not consider the optimal operating SOC window of lithium battery cells, which means that the balance based on battery cell voltage does not consider the economics of the battery system, such as the life of the battery system , maintenance and recycling, etc.