Efficient battery management becomes increasingly important as demand for portable power continues to rise, especially since balanced cells help ensure safety, high performance, and a longer battery life. When cells are mismatched, the battery pack’s total capacity decreases, leading to the overcharging of some cells and undercharging of others—conditions that accelerate degradation and reduce overall efficiency. The challenge is how to maintain an equal voltage and charge among the individual cells.

Typically, it’s possible to achieve cell balancing through either passive or active methods. Passive balancing, the more common approach because of its simplicity and low cost, equalizes cell voltages by dissipating excess energy from higher-voltage cells through a resistor or FET networks. While effective, this process wastes energy as heat.

In contrast, active cell balancing redistributes excess energy from higher-voltage cells to lower-voltage ones, improving efficiency and extending battery life. Implementing active cell balancing involves an isolated, bidirectional power converter capable of both charging and discharging individual cells.

This Power Tip presents an active cell-balancing design based on a bidirectional flyback topology and outlines the control circuitry required to achieve a reliable, high-performance solution.

System architecture

In a modular battery system, each module contains multiple cells and a corresponding bidirectional converter (the left side of Figure 1). This arrangement enables any cell within Module 1 to charge or discharge any cell in another module, and vice versa. Each cell connects to an array of switches and control circuits that regulate individual charge and discharge cycles.

Figure 1 A modular battery system block diagram with multiple cells a bidirectional converter where any cell within Module 1 can charge/discharge any cell in another module. Each cell connects to an array of switches and control circuits that regulate individual charge/discharge cycles. Source: Texas Instruments

Bidirectional flyback reference design

The block diagram in Figure 2 illustrates the design of a bidirectional flyback converter for active cell balancing. One side of the converter connects to the bus voltage (18 V to 36 V), which could be the top of the battery cell stack, while the other side connects to a single battery cell (3.0 V to 4.2 V). Both the primary and secondary sides employ flyback controllers, allowing the circuit to operate bidirectionally, charging or discharging the cell as required.

Figure 2 A bidirectional flyback for active cell balancing reference design. Source: Texas Instruments

A single control signal defines the power-flow direction, ensuring that both flyback integrated circuits (ICs) never operate simultaneously. The design delivers up to 5 A of charge or discharge current, protecting the cell while maintaining efficiency above 80% in both directions (Figure 3).

Figure 3 Efficiency data for charging (left) and discharging (right). Source: Texas Instruments

Charge mode (power from Vbus to Vcell)

In charge mode, the control signal enables the charge controller, allowing Q1 to act as the primary FET. D1 is unused. On the secondary side, the discharge controller is disabled, and Q2 is unused. D2 serves as the output diode providing power to the cell. The secondary side implements constant-current and constant-voltage loops to charge the cell at 5 A until reaching the programmed voltage (3.0 V to 4.2 V) while keeping the discharge controller disabled.

Discharge mode (power from Vcell to Vbus)

Just the opposite happens in discharge mode; the control signal enables the discharge controller and disables the charge controller. Q2 is now the primary FET, and D2 is inactive. D1 serves as the output diode while Q1 is unused. The cell side enforces an input current limit to prevent discharge of the cell above 5 A. The Vbus side features a constant-voltage loop to ensure that the Vbus remains within its setpoint.

Auxiliary power and bias circuits

The design also integrates two auxiliary DC/DC converters to maintain control functionality under all operating conditions. On the bus side, a buck regulator generates 10 V to bias the flyback IC and the discrete control logic that determines the charge and discharge direction. On the cell side, a boost regulator steps the cell voltage up to 10 V to power its controller and ensure that the control circuit is operational even at low cell voltages.

Multimodule operation

Figure 4 illustrates how multiple battery modules interconnect through the reference design’s units. The architecture allows an overcharged cell from a higher-voltage module, shown at the top of the figure, to transfer energy to an undercharged cell in any other module. The modules do not need to be connected adjacently. Energy can flow between any combination of cells across the pack.

Figure 4 Interconnection of battery modules using TI’s reference design for bidirectional balancing. Source: Texas Instruments

Future improvements

For higher-power systems (20 W to 100 W), adopting synchronous rectification on the secondary and an active-clamp circuit on the primary will reduce losses and improve efficiency, thus enhancing performance.

For systems exceeding 100 W, consider alternative topologies such as forward or inductor-inductor-capacitor (LLC) converters. Regardless of topology, you must ensure stability across the wide-input and cell-voltage ranges characteristic of large battery systems.

Modern multicell battery systems.

The bidirectional flyback-based active cell balancing approach offers a compact, efficient, and scalable solution for modern multicell battery systems. By recycling energy between cells rather than dissipating this energy as heat, the design improves both energy efficiency and battery longevity. Through careful control-loop optimization and modular scalability, this architecture enables high-performance balancing in portable, automotive, and renewable energy applications.

Sarmad Abedin is currently a systems engineer with Texas Instruments, working in the power design services (PDS) team, working on both automotive and industrial power supplies. He has been designing power supplies for the past 14 years and has experience in both isolated and non-isolated power supply topologies. He graduated from Rochester Institute of Technology in 2011 with his bachelor’s degree.

Related Content

• Active balancing: How it works and what are its advantages
• Achieving cell balancing for lithium-ion batteries
• Lithium cell balancing: When is enough, enough?
• Product How-To: Active balancing solutions for series-connected batteries

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