Electric energy storage device and electric tool system

An electric energy storage device comprises four energy units with a same rated voltage and a socket. The four energy units are divided into two energy modules each comprising with two energy units. Each energy module is provided with a positive electrode and a negative electrode. The socket includes four voltage output terminals connected to the positive and negative electrodes of the two energy modules and is provided with two control parts, which respectively control the two energy units in the two corresponding energy modules to switch between parallel state and series state. An electric tool system is also provided, comprising a plug that is connected to the socket, and different plugs can connect the four energy units in different connection states. The electric energy storage device with multiple output voltages can be matched with various electric tools with different rated voltages, and reduces the cost.