Why Is Battery Balancing Important for Battery Packs?

Automotive power battery systems are composed of multiple individual cells connected in series to meet specific voltage and power requirements. In practical use, due to variations between individual cells, the capacity of the entire battery pack is limited by the capacity of the weakest cell. In a series-connected pack, although the current flowing through each cell is identical, differences in capacity lead to varying depths of discharge; high-capacity cells tend to be under-charged and under-discharged, while low-capacity cells are prone to over-charging and over-discharging. Consequently, high-capacity cells degrade slowly and last longer, whereas low-capacity cells degrade faster and have shorter lifespans. This disparity widens over time, eventually leading to premature failure of the entire battery pack due to the failure of the low-capacity cells.

The ACEY Battery Pack Tester can measure the capacity, cycle life, and charge-discharge performance of the battery pack.

The fundamental goal of balancing is to "shave peaks and fill valleys." This involves using external circuitry to regulate charge and discharge currents during charging, discharging, or idle periods to ensure consistent voltage across cells during operation, thereby extending the vehicle's driving range and the battery pack's service life.

The phenomenon where battery pack performance declines due to performance variations among individual cells is typically referred to as "cell mismatch" or "imbalance." In most cases, this imbalance stems from imperfections in manufacturing processes and testing methods rather than changes in the cells' inherent chemical properties. Even when cells are sorted and matched after production, imbalances can still occur—for instance, differing self-discharge rates can cause capacity imbalances during idle periods, while varying internal resistances can lead to the overcharging of specific cells during the charging process.
To achieve balancing control for individual cells, the balancer serves as a core component of the Battery Management System (BMS); without a balancer, the management system—even with access to measurement data—would be unable to take corrective action, rendering the concept of "management" meaningless.

I. Basic Model of Balancing Control Management

The balancing control model discussed in this chapter applies to battery packs with a series configuration, assuming a total of *n* cells within the pack. As illustrated by this model, the capacity and state of charge (SOC) of individual cells within the pack may vary at any given time, necessitating balancing control management for power battery packs. For electric vehicles, inconsistency among power battery cells is inevitable, primarily due to the following two factors:

• Inconsistencies arising from the battery manufacturing process.
• Inconsistencies caused by operating environments.

II. Classification of Balancing Control Management

There is a wide variety of battery balancing control methods, with new ones constantly emerging. Classifying these methods is challenging because different classification criteria yield different results. The table below summarizes common classification criteria and their corresponding types.

III. Significance of Balancing Control Management

1. Balancing control management helps increase the overall capacity of the battery pack.
Consider a battery pack composed of four cells (B1, B2, B3, B4). Assume their nominal capacities are identical (C1=C2=C3=C4=Cmax); however, due to factors such as varying self-discharge rates, their initial capacities differ prior to a charging cycle—specifically, assume B2 has the highest initial capacity and B3 the lowest. Since the cells are connected in series, the pack's remaining capacity decreases uniformly; over time, however, an imbalance develops. Balancing control allows for an increase in the overall usable capacity.

2. Balancing control management helps regulate the depth of charge and discharge for power battery cells.
First, balancing management enhances battery safety. Without charge balancing management, consider this scenario: among the four cells, one (B2) nears full charge (i.e., its State of Charge, or SOC, exceeds 95%). Due to electromotive force and internal resistance characteristics, its operating voltage rises rapidly, making it susceptible to overcharging.

Second, balancing management extends battery lifespan. Third, balancing management enables uniform battery aging, thereby improving State of Health (SOH) consistency. Inconsistency in SOH poses difficulties for the battery management system; it complicates the already challenging task of estimating the battery's remaining charge (or State of Charge, SOC) and hinders the implementation of optimized charging strategies for the battery pack. To enhance SOH consistency across the pack, the factors influencing battery aging should affect each individual cell as equally as possible; implementing balancing control to ensure consistent depth of discharge (DOD) across cells is an effective method for achieving uniform aging. Balancing control maintains consistent discharge depth during operation, which not only extends the overall cycle life of the battery pack—as previously mentioned—but also ensures uniform degradation, resulting in SOH values ​​that are relatively similar across all cells.

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