## 6 Conclusions  
In this work a semi-active hybrid energy storage system (HESS) formed by a battery, a supercapacitor, and controlled Sepic/Zeta converter is proposed. The HESS is able to reduce the battery degradation since the supercapacitor is forced to absorb or deliver the high-frequency current perturbations present in the DC bus. Considering that batteries and supercapacitors can exhibit wide voltage variations in charging or discharging modes, it is demonstrated that the proposed control for the Sepic/Zeta converter is suitable to operate in a wide operating range, which means under different relationships between the supercapacitor and battery voltages. On the other hand, the proposed control strategy allows fast tracking of the high-frequency bus current components due to the incorporation of a feed-forward loop. In this way, fast changes in battery power are avoided, which can reduce the degradation of the batteries in the long run. In addition, the adaptive gain-scheduling control strategy ensures the desired performance under different voltage relationships between the battery and the supercapacitor (buck, boost, Unitary Gain). Moreover, the charging, discharging, and stand-by modes of the battery are correctly managed by the HESS. An important consequence of the supercapacitor voltage regulation is that unwanted state-of-charge conditions can be avoided; in this way, the capacitor voltage can be defined in such a way that the HESS always has the same capacity to deliver or absorb energy. The evaluation of this adaptability was performed with variations up to 67% of the nominal range, obtaining much better RMSE values for the reference tracking: between 60% and 76% lower in comparison with the classical LQG controller, and between 66% and 88% lower in comparison with the classical PI controller. The experimental results confirm the advantages of the proposed adaptive control system over classical PI and LQG strategies. In this context, the proposed feed-forward control loop increases the speed of the reference tracking, and thus considerable attenuation in high-frequency variations of the battery current is provided. These experiments were performed for a wide variation range (67% of the nominal value), achieving a much better absorption of the high-frequency components in comparison with the classical LQG (up to 68% lower RMSE) and PI (up to 84% lower RMSE) controllers. In conclusion, the proposed solution supports the battery operation by filtering the high-frequency components of the load current to prevent damage. In addition, the operation of this solution is transparent to the energy management system interacting with the battery, and hence no modification to the microgrid management system is needed. Finally, the experimental results demonstrate the viability of implementing the proposed control strategy using commercial and low-cost processing platforms. An additional advantage of the proposed open architecture based on the Zeta/Sepic converter is the possibility to replace the battery or the supercapacitor without replacing the other one; hence, it reduces the disposal problem in comparison with commercial closed architectures where the complete HESS device must be replaced in the case of a single ESS failure. A future work will study the replacement frequency of both the battery and supercapacitor based on the operation time of the HESS, also analyzing the disposal or repurposing options for both ESSs. Finally, the modularity of the proposed architecture enables us to combine multiple batteries, supercapacitors, and Zeta/Sepic converters to provide the power level needed in any application, only requiring us to divide the current reference (provided by the HP filter) by the number of the supercapacitors. This improvement will be developed in a future work to increase the power range of the target applications. All authors have contributed to the complete manuscript. All authors have read and agreed to the published version of the manuscript.