Inconsistencies in individual lithium-ion battery cells, when transmitted to the lithium-ion battery pack, inevitably lead to a loss of capacity in the lithium-ion battery pack, and consequently a reduction in its lifespan.
The significance of lithium-ion battery balancing lies in using power electronics technology to maintain the voltage deviation of individual lithium-ion battery cells or the battery pack within a predetermined range. This ensures that each individual lithium-ion battery cell remains in the same state during normal use, preventing overcharging and over-discharging. Without balancing control, as charge and discharge cycles increase, the voltage of each individual lithium-ion battery cell will gradually diverge, significantly reducing its lifespan.
During the use of lithium-ion battery assemblies, inconsistencies can occur between individual cells due to factors such as self-discharge levels and component temperatures. These inconsistencies in individual lithium-ion battery cells, in turn, affect the charge and discharge characteristics of the entire lithium-ion battery pack. Studies have shown that a 20% capacity difference between individual lithium-ion battery cells can lead to a 40% capacity loss in the entire lithium-ion battery pack.
Inconsistencies in individual lithium-ion battery cells can worsen over time due to the influence of random factors such as temperature. Generally, when the ambient temperature exceeds the optimal operating temperature of a lithium-ion battery by 10°C, its lifespan will be reduced by half. Because automotive lithium-ion battery systems typically have a large number of cells connected in series (usually between 88 and 100) and a capacity of 20 to 60 kWh, temperature differences can occur due to the different placement of each battery cell within the series.
Even within the same lithium-ion battery pack, temperature differences can occur due to variations in location and heating of the lithium-ion batteries. These temperature differences can significantly negatively impact the lifespan of lithium-ion batteries, causing imbalances that reduce driving range and shorten cycle life. These issues prevent the entire battery system from being fully utilized, resulting in battery system losses. Mitigating these losses will greatly extend the battery system's lifespan.
The consistency between individual lithium-ion battery cells has the most direct and significant impact on the capacity of lithium-ion batteries. While battery capacity is a parameter that cannot be directly measured in a short time, there is a one-to-one correspondence between the capacity of a single lithium-ion battery cell and its open-circuit voltage. The voltage of a single lithium-ion battery cell can be measured online in real time, making it a favorable condition for assessing the consistency level of individual cells. In battery management systems, the voltage value of a single lithium-ion battery cell is used as a trigger condition for conditions such as discharge termination and charge termination.
A parameter in this position, where the voltage consistency of individual lithium-ion battery cells varies too much, directly limits the charging and discharging capacity of the lithium-ion battery pack. Therefore, using lithium-ion battery balancing methods to address the problem of excessive voltage differences among individual cells in an already operational lithium-ion battery pack is an effective measure to improve the capacity of the lithium-ion battery pack and extend its lifespan.
