Active balancing: How it works and what are its advantages

The stability and safety of lithium batteries require treating them with careful consideration. If lithium-ion battery cells do not operate within a constrained state-of-charge (SOC) range, their capacity can be reduced. If they are pushed beyond their SOC limits, these batteries can be damaged, leading to unstable and unsafe behavior. Therefore, to ensure the safety, lifetime, and capacity of lithium-ion battery cells, their SOC must be carefully limited.

To maximize each battery cell’s useful capacity and life, degradation must be minimized while operating all cells across a full SOC range. Simply keeping cells within a constrained SOC without intervention will avoid degradation but slowly decrease the usable capacity by the amount of SOC mismatch. That is because charging or discharging must stop when one cell reaches the upper or lower SOC limit, even though the other cells have remaining capacity (Figure 1).

Figure 1 The useful capacity of a battery pack is decreased by mismatched SOC. Source: Monolithic Power Systems

Most battery management systems (BMS) today include passive balancing to periodically bring all cells in series to a common SOC value. Passive balancing does this by connecting a resistor across each individual cell as necessary to dissipate energy and lower the SOC of the cell.

As an alternative to passive balancing, active balancing uses power conversion to redistribute charge among the cells in a battery pack. This enables a higher balancing current, lower heat generation, faster balancing time, higher energy efficiency, and longer operating range.

This article describes a few common active balancing methods and explains how these methods work.

Cell balancing

Cells in a pack develop capacity variation over time, even if they are initially well-matched. For example, cells at different physical locations in a pack can experience different temperatures or pressures that effect capacity. In addition, slight manufacturing differences can be amplified over time and create differences in capacity. Understanding capacity differences is critical to understanding the source of SOC imbalance.

Changes in battery cell SOC are primarily dictated by cell capacity and the current in, or out of, a cell. For example, a 4-Ahr cell receiving 1 A for 1 hr will experience a 25% SOC change, while a similar 2-Ahr cell will experience a 50% SOC change.

Maintaining SOC balance requires adjusting each cell’s charge/discharge current according to its capacity. Cells that are connected in parallel automatically do this, since current will flow from high-SOC cells to low-SOC cells. In contrast, cells in series experience the same current between cells, which creates an imbalance if there are capacity differences. This is important since most battery packs have series cell connections, even if they also include parallel connections.

SOC adjustment is possible for both passive and active balancing.

Passive balancing reduces cell SOC by placing a resistive load across individual cells (most commonly using BJT or MOSFET transistors). But active balancing takes a switch-mode approach to redistribute energy between cells in a battery pack.

The added complexity and cost of implementation has traditionally limited active balancing to battery systems with higher power levels and/or large capacity cells, such as batteries in power stations, commercial energy storage systems (ESS), home ESS, and battery backup units. New solutions are now available with significantly lower cost and complexity, enabling a growing range of applications to leverage the advantages of active balancing.

Passive balancing is typically limited to 0.25 A of current, while active balancing can support up to 6 A. A higher balancing current allows faster balancing, which supports larger-capacity battery cells, such as those used in ESS. In addition, a higher balancing current supports systems operating on fast cycles where balancing must be completed quickly.

Passive balancing simply dissipates energy; active balancing, however, redistributes energy with a significant improvement in energy efficiency. Passive balancing is only practical during the charge cycle, since operation during discharge hastens energy depletion from the pack. Conversely, active balancing can be implemented during charging or discharging.

The ability to actively balance during discharge provides more balancing time and allows charge to be transferred from the strong cells to the weak cells, thereby extending battery pack runtime (Figure 2). In summary, active balancing is advantageous for applications that require faster balancing, limited thermal load, improved energy efficiency, and increased system runtime.

Figure 2 Active balancing equalizes the SOC during charge and discharge. Source: Monolithic Power Systems

Active balancing methods

Commonly used active balancing topologies include direct transformer-based, switch matrix plus transformer, and bidirectional buck-boost balancing.

1. Transformer-based (bidirectional flyback) active balancer

A bidirectional flyback converter allows charge to be transferred in both directions. The bidirectional flyback is designed to operate as a boundary mode flyback converter. Each battery cell in the stack requires a bidirectional flyback, including a flyback transformer (Figure 3).

Figure 3 A transformer-based bidirectional active balancer transfers charge in both directions and can use a 24-V rail. Source: Monolithic Power Systems

When using different transformer designs, there are several possible energy transfer paths. For example, energy can be transferred from one cell to a sub-group of cells within the battery stack. Energy can be transferred from any cell to the top of the battery stack—connected to the battery pack terminals—which requires a large, high-voltage flyback transformer. Energy can also be transferred to or from an auxiliary power rail, such as a 24-V system shown in Figure 3.

Many transformers are often required when using the transformer-based active balancing approach, which results in large, costly solutions for battery packs with a high string count.

2. Switch matrix plus transformer active balancer

The switch matrix plus transformer method uses an array of switches to connect a transformer to and from individual cells; this reduces the number of transformers to one. Within a switch matrix, there are two categories of switches: cell switches and polarity switches.

The cell switches are back-to-back MOSFETs connected directly to the battery cells. They can block the current flowing in both charge and discharge directions. Conversely, the polarity switches block the current flowing in one direction only, and they are connected directly to the secondary side of a single, bidirectional flyback converter or a bidirectional forward converter (Figure 4).

Figure 4 A switch matrix-based bidirectional DC/DC active balancer uses an array of switches. Source: Monolithic Power Systems

The primary side of the bidirectional flyback converter or the forward converter is connected to the battery pack or an auxiliary power rail. In this arrangement, every cell can exchange the energy (during charge or discharge) with the battery pack or an auxiliary power rail. As noted, the primary advantage of the switch matrix plus transformer is that only one transformer is required.

3. Bidirectional buck-boost active balancer

A buck-boost active balancer takes a simpler approach by leveraging commonly used buck and boost battery charger technology. Rather than moving charge to various locations along a battery stack or to a separate power rail, buck-boost active balancing moves charge to directly adjacent cells. This greatly simplifies the balancing circuitry and leverages the simultaneous operation of many balancers to distribute charge across the entire stack.

A 2-channel buck-boost balancer provides bidirectional charge movement between two adjacent cells by operating in buck-balance mode or boost-balance mode. By placing a 2-channel buck-boost balancer on every pair of cells, charge can be moved throughout an entire pack (Figure 5).

Figure 5 A bidirectional “buck” and “boost” active balancer moves charges to directly adjacent cells. Source: Monolithic Power Systems

Compared to the two previous active balancers, a 2-channel buck-boost active balancer follows a simple process:

• In buck-balancing mode, the active balancer transfers energy from the upper cell (CU) to the lower cell (CL).
• In boost-balancing mode, the active balancer transfers energy from the CL to the CU.

Among the three types of active balancers, the bidirectional buck-boost active balancer is the simplest and most reliable. Table 1 compares all three active balancing methods.

Table 1 The above data highlights capabilities of three active balancing methods. Source: Monolithic Power Systems

Why active balancing is more viable

With a growing demand for safer, more energy efficient, and longer lasting lithium-ion battery systems, there is a growing demand for better cell balancing. Passive balancing, which is limited to small currents that simply dissipates energy, is no longer sufficient to meet these demands.

As a result, active balancing solutions are increasingly being adopted for their high-current, fast cell balancing advantages. In particular, bidirectional buck-boost active balancers offer simplicity and reliability.

Kelly Kong is battery management application manager at Monolithic Power Systems.

Greg Zimmer is business development manager at Monolithic Power Systems.

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