Estimation of state of charge of lead-acid battery used in solar photovoltaic system

7-19An accurate estimation of State of charge (SOC) of the lead-acid battery is of paramount importance for the efficient and reliable operation of solar photovoltaic (SPV) sytem. There are mainly four methods used for estimating SOC of the battery, viz. chemical, voltage, current integration and kalman filtering. In this present study, the SOC as indicated by the solar power conditioning unit (SPCU) was taken as reference and at every 5% SOC reduction, the other parameters such as- i) specific gravity of electrolyte, ii) battery terminal voltage, iii) (Ampere Hour) Ah and iv) energy deliverd to the resistive load were recorded. Based on this recorded values the SOC was predicted. The standard deviation (S.D.) of the difference of predicted SOC to the reference SOC was calculated based on specific gravity, Voltage, Ah and energy. The SD obtained was 6.17, 5.67, 0.33, 0.75 respectively. The specific gravity value for the battery electrolyte decreases with the decrease in the battery SOC%, the maximum value of SG at 100% SOC was 1.23 and the minimum at 20% SOC was 1.14. The terminal voltage was also got reduced with the reduction in SOC, from 24.85V at 100% SOC to 22.4V at 20% SOC. The energy stored by the battery during charging was 3.65 units and the energy delivered from the battery to the load was 3.245 units.  The efficiency of solar panel, lead-acid battery and the combined SPV system was 12.79%, 88.9% and 9.68% respectively. It was found that the SOC of the lead-acid battery would be more accurate when it is estimated based on current integration i.e., Ah, the SOC estimation based on energy is also acceptable since the SD for both is less than 1. Hence, through this investigation we can say that SOC prediction based on Ah or kWh measurement is more appropriate than specific gravity and voltage methods