In modern power systems, the integration of high-proportion renewable energy sources demands robust and flexible energy storage solutions. Battery energy storage systems (BESS) have emerged as a critical component for enhancing power quality, stabilizing grid frequency, and improving overall system economics. Among various BESS architectures, the decentralized configuration, which connects multiple battery clusters through dedicated DC/DC converters to a common DC bus, offers significant advantages in mitigating circulating currents and improving modularity. However, existing DC/DC converters for such systems often suffer from high cost, low efficiency, and complex topologies, especially when handling full-rated power. To address these challenges, we propose a novel multi-port equalization converter (MEC) for decentralized battery energy storage systems. The MEC leverages half-bridge chopper circuits to achieve battery cluster power regulation and intermediate bus energy balancing without external power supplies or isolation transformers, thereby simplifying the design and reducing initial investment. In this paper, we present the detailed topology, operating principles, control strategies, parameter design, and experimental validation of the MEC through simulation and a scaled-down prototype.
1. Topology and Operating Principle of the MEC
The proposed MEC topology is illustrated schematically in the figure below. The converter consists of a shared intermediate DC bus capacitor and multiple half-bridge chopper circuits. One half-bridge on the PCS side serves as the input port, connecting the intermediate capacitor to the PCS DC bus. The remaining half-bridge choppers on the battery side act as output ports, linking the intermediate bus to N battery clusters. This configuration enables multi-port operation while maintaining a simple, non-isolated structure.
To understand the power flow, we first define the average duty cycle d for the battery-side choppers. Let vpc be the PCS DC bus voltage, vsc be the intermediate bus voltage, and vOCVj and rbat be the open-circuit voltage and internal resistance of the j-th battery cluster, respectively. The dynamic equations for each battery cluster current ij and the intermediate bus voltage vsc are given by:
$$ v_{pc} d_1 – v_{sc} d_{j1} = v_{OCVj} + r_{bat} i_j + L \frac{di_j}{dt} $$
$$ C_{sc} \frac{dv_{sc}}{dt} = d_1 i_{be} – \sum_{j=1}^{n} d_{j1} i_j $$
where d1 is the duty cycle of the PCS-side half-bridge, dj1 is the duty cycle of the j-th battery-side chopper, and Csc is the intermediate bus capacitance. The average current of all battery clusters is denoted as i, and the total BESS current as ibe.
By applying small-signal analysis around a steady-state operating point, we derive the transfer functions that govern the system dynamics. For the average current control loop, the plant model is:
$$ G_{av}(s) = \frac{\hat{i}(s)}{\hat{v}_{pc}(s)} = \frac{1}{Ls + r_{bat}} $$
For the individual battery cluster current control, the deviation current Δij = ij – i leads to a similar first-order model:
$$ G_{I, j}(s) = \frac{\hat{i}_j(s)}{\hat{d}_{j1}(s)} = \frac{V_{sc}}{Ls + r_{bat}} $$
Finally, the intermediate bus voltage dynamics are described by:
$$ G_{cs}(s) = \frac{\hat{v}_{sc}(s)}{\hat{d}_1(s)} = \frac{I_{be}}{s C_{sc}} $$
These models form the foundation for designing the control loops, which are detailed in the next section.
2. Control Strategy for the MEC
Our control strategy comprises three main loops: average current control, battery cluster current control, and intermediate bus voltage balance control. The average current control regulates the total BESS current ibe by adjusting the PCS-side duty cycle d1. A PI controller is employed to track the reference average current with high bandwidth. The battery cluster current control ensures equal current sharing among clusters despite differences in SOC or internal resistance. As derived in the small-signal model, the system has a redundant degree of freedom; we designate the first cluster as the balancing branch and control the remaining n-1 clusters independently while imposing the constraint that the sum of duty cycles equals zero. This decouples the cluster currents and allows independent PI regulators for each cluster. The intermediate bus voltage controller stabilizes vsc at a constant reference by adjusting d1. A hysteresis comparator is included to handle the sign change of the total current during bidirectional operation. The overall control block diagrams are conceptually shown in the following table summarizing the key transfer functions and controller types.
| Control Loop | Plant Model | Controller Type |
|---|---|---|
| Average Current | 1/(Ls + rbat) | PI |
| Battery Cluster Current | Vsc/(Ls + rbat) | PI |
| Intermediate Bus Voltage | Ibe/(s Csc) | PI with hysteresis |
3. Parameter Design of the MEC
Proper selection of key components is essential for the MEC to achieve the desired performance. The intermediate bus voltage vsc is chosen based on the battery SOC operating range (10%–90%). Using the open-circuit voltage characteristics of LiFePO4 cells, the required voltage is determined by the equation:
$$ v_{sc} = \frac{v_{OCV}^{10\%} – v_{OCV}^{90\%}}{2} $$
where vOCV10% and vOCV90% are the open-circuit voltages at the extreme SOC levels. For our prototype, we set vsc = 133 V for the full-scale system and 10 V for the scaled-down experiment.
The intermediate bus capacitance Csc is designed to limit the voltage ripple within an acceptable range. Considering the worst-case power exchange between clusters with extreme SOC differences, the maximum energy exchange time Δtmax is derived from the duty cycle difference. The capacitance value is then given by:
$$ C_{sc} = \frac{n}{2} \cdot \frac{I \cdot \Delta t_{\text{max}}}{\alpha \cdot v_{sc}} $$
where α is the allowed ripple factor (set to 2% in our design), and n is the number of battery clusters (n=3). The filter inductor L for each battery cluster is chosen to limit current ripple Δij to β·Irated, leading to:
$$ L \leq \frac{v_{sc} \cdot \Delta t_{\text{max}}}{\beta \cdot I_{\text{rated}}} $$
With β = 10% and a switching frequency of 10 kHz, we obtain L = 2 mH for the scaled-down prototype. A summary of the design parameters is provided in the table below.
| Parameter | Symbol | Full-Scale Value | Scaled-Down Value |
|---|---|---|---|
| Number of battery clusters | n | 3 | 3 |
| Battery cluster rated voltage | E | 1331 V | 100 V |
| Battery cluster rated current | I | 70 A | 6 A |
| Intermediate bus voltage | Vsc | 133 V | 10 V |
| Intermediate bus capacitance | Csc | 9 mF | 10.8 mF |
| Filter inductance | L | 1.9 mH | 2 mH |
| Switching frequency | fs | 10 kHz | 10 kHz |
| PCS DC bus voltage | Vpc | 630 V | 100 V |
4. Simulation and Experimental Validation
To verify the feasibility of the proposed MEC and its control strategy, we conducted both simulation studies in MATLAB/Simulink and experimental tests on a 1.8 kW scaled-down prototype. The simulation model replicated a 1725 kW BESS with three battery clusters (LF280K cells, 280 Ah) operating at 0.25C nominal. Two simulation cases were designed: (1) clusters with identical SOC but different internal resistances, and (2) clusters with SOC values of 55%, 60%, and 65% (10% maximum deviation). The test sequence comprised five operating stages: Stage 1 (0–0.8s): average current set to 0.5 p.u. with MEC inactive; Stage 2 (0.8–1.6s): activate MEC; Stage 3 (1.6–2.4s): increase average current to 1 p.u.; Stage 4 (2.4–3.2s): reverse average current to –1 p.u.; Stage 5 (3.2–4.0s): reduce to –0.2 p.u. (light load). The simulation results demonstrated that the MEC successfully equalized battery cluster currents in all stages. The maximum current imbalance before activation was 6.95 A (case 1) and 9.7 A (case 2), which was reduced to zero after MEC activation. The intermediate bus voltage was maintained at 133 V with a ripple of only 1.95% during the most severe transient (full power reversal). The total BESS current tracked the reference within 22 ms, meeting IEC 62898-2 requirements. The results confirm the effectiveness of the proposed average current control and cluster current control loops.
For experimental validation, we built a 1.8 kW prototype using three half-bridge modules (rated voltage 100 V, rated current 6 A per cluster) and a RTU-BOX206 controller. The PCS was rated at 1.8 kW with a 100 V DC bus. The experimental procedure followed the same five stages as the simulation. The recorded waveforms show that before MEC activation, the cluster currents naturally differed by up to 1.9 A (0.316 p.u.) due to manufacturing tolerances. After activation, the current difference was eliminated within 0.2 s. During the full-power reversal from +6 A to –6 A per cluster, the intermediate bus voltage remained stable at 10 V with negligible deviation. The PCS DC bus voltage stayed at 100 V throughout, and the grid-side power factor transitioned smoothly from importing to exporting power. These experimental results further validate the practical applicability of the MEC in decentralized battery energy storage systems, demonstrating its ability to independently regulate cluster currents while maintaining intermediate bus voltage equilibrium without any external auxiliary power source.
5. Conclusion
In this paper, we have presented a novel multi-port equalization converter specifically designed for decentralized battery energy storage systems. The proposed MEC topology eliminates the need for external power supplies and isolation transformers by directly utilizing a PCS-side half-bridge to manage the intermediate bus energy balance. Through detailed theoretical analysis, we derived the small-signal models for average current, cluster current, and bus voltage control, and designed corresponding PI-based controllers. The parameter design methodology for the intermediate bus capacitance and filter inductors was provided, ensuring low ripple and robust operation. Both simulation and experimental results on a 1.8 kW scaled-down prototype confirmed that the MEC can effectively equalize battery cluster currents under various operating conditions, including SOC imbalance and bidirectional power flow, while keeping the intermediate bus voltage stable. The MEC offers a cost-effective and efficient solution for large-scale battery energy storage systems, with significant potential for further integration into grid-support applications. Future work will focus on fault-tolerant operation under short-circuit conditions and optimization of control coordination with the upstream PCS.