Experimental studies of a long-term operation of different batteries used in pv system

Three photovoltaic (PV) systems with the identical solar panels, the same charge controllers, and similar loads, but with different used battery types: AGM deep-cycle, LiFePO4 and lead-carbon were installed in Riga, Latvia for a long-term monitoring and analysis of the operation efficiency and potential energy losses in batteries under real operation conditions. Results of one full year of monitoring are analysed in this paper resulting quantitative indicators during different seasons. During the wintertime with low energy production, the difference in energy losses for different battery types is relatively big. On the other hand, the performance analysis during summer months, when the received solar energy increases significantly, shows the narrowing gap in stored and used energy balance between different battery types. Energy losses for all the battery types strongly depend on the monthly discharged energy, growing up to more than 50% for months with negligible amount of sunlight, meaning very ineffective operation of batteries during this period. However, the amount of energy that is lost is reduced, regardless of the type of battery tested, if the battery has a higher charge. It can be inferred that lead batteries have significantly bigger energy losses in comparison to the other two types of batteries that were observed at all of the recorded charge levels, ranging from 2 to 10 kWh. At lower charge levels, the gap between the energy losses caused by deep-cycle batteries and those caused by lithium-ion batteries becomes more pronounced.