A battery assembling device has two brackets and an assembling unit. The brackets abut each other and can be securely mounted around two battery units. The assembling unit has a quick-release housing and two clamps. The quick-release housing has two electrode holes. The clamps are respectively mounted in the electrode holes and each clamp has two opposite threaded inner surfaces. The battery units can be bundled by the brackets in advance to form regular battery modules. Because the assembling unit is easily and quickly connected with the battery modules, the assembling of a cell of an electrical vehicle is greatly convenient and fast.