An enclosed lead storage battery includes an enclosed container, a group of electrode plates including positive electrode plates and negative electrode plates, and separators, with the group of electrode plates and the separators being housed in the container. The positive electrode plates, negative electrode plates, and separators are layered side by side, with each of the positive electrode plates and negative electrode plates including a top side, a bottom side, and opposite lateral sides. At least one of the positive electrode plates and at least one of the negative electrode plates provide a grid having no frames at opposite lateral sides of the grid, such as an expanded grid. To prevent a short-circuit between the positive and negative electrode plates, the battery includes a porous mat disposed between the container and the opposite lateral sides of the grid.