Analysis of the efficiency, performance, and operational capacity of a high-voltage battery in an autonomous photovoltaic system for water supply

The integration of lithium-ion batteries (LIBs) into photovoltaic (PV) systems—both grid-connected and stand-alone—is increasingly common. This study presents experimental results from a multipurpose PV water pumping system equipped with an 11.5 kWh, 230.4 V LIB. Independent tests were conducted to characterize the charging and discharging processes, as well as the efficiencies and performance of the system, under real operating conditions. LIB full charge was performed exclusively with PV energy. Two distinct phases were identified: a fast and efficient charge to state of charge ( SOC ) values of between 87% and 95%, with the PV array at the maximum power point (MPP); and a slow and less efficient phase reaching near full charge ( SOC ≈ 100%). In this second phase, energy curtailment occurred due to battery management system limitations. These caused deviation from the MPP and resulted in wasted energy. During the transition between charging phases, a sudden rise in LIB voltage and a drop in PV power were observed. The LIB was discharged at constant power using the variable speed drive (VSD) of the water pumping system with maximum efficiency observed at lower frequencies in VSD operation. Converter efficiencies showed optimal values (>94%) at power levels above 500 W in the DC/DC conversion during charging from the PV field. During discharge, the greatest efficiency in the DC/AC conversion occurred at the lowest frequency (37 Hz). Operating the VSD at low power is recommended to maximize energy use, even if the water flow lessens. Battery capacity estimation using various SOC windows ( W SOC ) revealed a high variability at W SOC = 1%, while the 40–80% SOC range yielded the most reliable results. Final charging stages should be avoided due to PV energy losses and capacity underestimation. Performance ratios confirmed significant energy losses at the end of charging, which suggests the need to increase AC consumption through diversion loads to maximize PV generation. These findings offer valuable insights for the design, modeling, and operation of PV systems with LIB storage.