## 1 Introduction  
With gradual the research and development of new energy sources, renewable energy sources, such as solar energy, wind energy, geothermal energy, and tidal energy, have been substantially developed. Photovoltaic solar energy is considered clean and safe and has secured policy support in many countries. According to the estimation made by the World Photovoltaic Industry Association, photovoltaic power generation will account for 5% of the global electricity production by 2020, and 40% by 2040 [1]. China's \"13 th Five-Year Plan\" for power development, released in 2016, states that significant effort should be made to develop new energy sources. Among them, the total capacity of photovoltaic generators is expected to reach 110 GW or more, with over 50% accounted for by distributed photovoltaic power generation, reaching more than 60 GW [2]. In distributed photovoltaic power generation, the residential areas of users are directly utilized to construct the photovoltaic power generation system and to perform off-grid or grid-connected power generation. This solves the problem of the large occupied area that characterizes ground photovoltaic power generation systems. However, owing to power fluctuations due to unstable photovoltaic power generation, the continuity and reliability of the power supply cannot be guaranteed. A direct grid connection can lead to a harmonic pollution problem, and can affect the stability of the overall power grid. Therefore, the cost of the grid connection increases substantially, which is one of the reasons for the \"photovoltaic power discard\" phenomenon [3; 4; 5]. In off-grid and grid-connected applications,fluctuations in photovoltaic power will lead to reliability and safety issues; thus, solutions to stabilize the fluctuation are necessary. The currently available technologies include energy storage, power reduction, and integration with other energy sources; among these technologies, energy storage is a relatively developed solution [6; 7; 8]. Employing large-capacity energy storage technology has become mandatory for the grid connection of distributed photovoltaic power generation, and is an important basis for the future construction of smart power grids.  
In order to achieve renewable targets, the potential of the energy storage system (ESS) combined with photovoltaic (PV) panels can be applied not only to the electricity sector, but also to electric vehicles, which results in sustainable mobility, as well as being able to heat and power systems [9]. The main revenue of ESS is derived from an increase in self-consumption energy, and self-sufficiency and grid independence provide the necessary drive for people to install ESSs. The net present value (NPV) and levelized cost of electricity (LCOE) are two indicators that address the economics of ESS in residential buildings [10].  
The introduction of energy storage technology to distributed photovoltaic power generation systems not only helps to suppress power fluctuations in photovoltaic power systems and to enhance the reliability and stability of the grid connection (thereby improving the power quality of the grid), but it can also help regulate the peak-to-valley balance problem of the power grid, maintain grid load stability, realize the self-sustaining and paid surplus (sent to the grid) of the photovoltaic power system, and increase the income of the user. This can promote the development of distributed photovoltaic power generation, solve the \"photovoltaic power discard\" problem, ensure local consumption of photovoltaic power, and improve the quality of the photovoltaic energy system [11; 12; 13]. This paper mainly reports on the charging and discharging control technology of lithium titanate batteries used in photovoltaic energy storage systems.