###### Abstract  
State-of-charge balance is vital for allowing multiple energy storage units (ESUs) to make the most of stored energy and ensure safe operation. Concerning scenarios wherein boost converters are used as the interfaces between ESUs and loads, this paper proposes a balancing strategy for realizing consistent state-of-charge (SoC) levels and equal currents among different ESUs. This strategy is valid for both parallel and series applications. Its advantages also include its high precision of SoC equalization without extra sensors and fast convergence. A common outer voltage loop is used to accomplish tight voltage regulation, while multiple inner current loops are utilized to achieve current control and SoC balance simultaneously. Firstly, by introducing SoC-based current distribution ratios (CDRs) to modify current references online, the currents are gradually adjusted to eliminate SoC deviations. Secondly, to expedite the balancing process, current saturations are further adopted. Thirdly, the influences of accelerating factor and current limits in CDR expressions are analyzed, and their selection guidelines are subsequently provided. Fourthly, the controller design, consisting of a dual loop, is illustrated to guarantee sufficient stability margins. Fifthly, an experimental platform consisting of three battery ESUs is developed to verify the proposed strategy. distributed energy storage unit (ESU); boost converter; state-of-charge (SoC) equalization; current balance; current saturation; fast convergence +
Footnote †: journal: _aciences_  
0000-0001-5000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-0000-0000-00000  
Balancing topologies mainly include the dissipative and non-dissipative types [3]. The former transfers excess energy to heat in power resistors. It is a simple approach, but it is only applicable to the charging process, and considerable energy losses are inevitable. The latter type adopts active balancing circuits to redistribute excess energy. Numerous balancing topologies with different virtues have been presented, such as multi-winding transformers [4], switched-capacitor circuits [5], LC resonant circuits [6] and non-inverting buck-boost circuits [7]. Favorable balancing results can be achieved, but the system costs increase. As for the control algorithms used to realize a consistent SoC, there are two main kinds, namely, the centralized and decentralized types. To accomplish centralized control, a central controller [8; 9] is employed to coordinate the sources, ESSs, and loads in real time. Nevertheless, potential single-point failures decrease system reliability [10]. Decentralized methods reduce dependence on communications and therefore enhance system reliability. As typical representatives of decentralized methods, plenty of SoC-based droop control schemes have been investigated. Due to virtual damping and inertia, the transient stability of dc-bus voltage was improved after a bidirectional dc-dc circuit was equivalently converted to an equivalent virtual dc machine in [11]. However, the line resistances and voltage drops were ignored, so the actual accuracy of SoC equalization deteriorated to some extent. In [12], SoC levels, line impedances, and virtual power ratings were all considered to achieve accurate power sharing. But five PI regulators were needed for each PCU, thus increasing implementation complexity. In [2], the droop coefficient was inversely proportional to the high-order term of the local SoC. Free from communications, power sharing and SoC balance were gradually obtained. Nevertheless, voltage deviation was inescapable, though it was constrained by choosing a proper exponent. Balancing speed is a vital index for balancing control. In [13], the adaptive virtual resistances were applied to flexibly adjust the power and status of ESUs for the rapid elimination of SoC errors. In [14], the relationship of the arctangent function is determined between droop coefficients and the individual SoC for faster SoC balancing.