As a storage battery for micro-grid, lithium battery can improve the stability of photovoltaic power generation system and improve power quality, but the cost is high. The use of decommissioned lithium battery of electric vehicle for energy storage unit of photovoltaic/battery micro-grid can not only reduce investment costs, but also alleviate the pressure of large-scale batteries entering the recycling phase. Based on the working principle of lithium battery, the equivalent circuit model of lithium battery is constructed. Cascaded topology of energy storage converter and multiple DC/DC converters is investigated. The energy storage converter uses a double-closed-loop strategy of voltage outer loop and current inner loop to stabilize the voltage of the DC bus. The multiple bidirectional DC/DC converter adopts a double-closed-loop control strategy based on the state of charge (SOC) of the battery pack to stabilize the power fluctuation of the photovoltaic power generation system. An photovoltaic/battery micro-grid system based on decommissioned lithium battery energy storage is built to verify the effectiveness of the control strategy.