A small spot on the concrete floor with cinderblocks to make a table / shelf for the chargers and batteries should be fine, just as long as nothing flammable is within a distance that flames could reach it if the batteries caught fire in there (not by the wall or under something). I just keep mine on a shelf in the garage during the cooler months, but I bring them inside when it’s hot.
But batteries stored directly on the floor is also bad, so you’d want something to set them on. Somehow batteries can discharge into the ground if left on a concrete floor. An old metal ammo can on a cinderblock would be good enough IMO.