A single cell produces about 1.5 V. We are going to make 3 cells. Then rape a tape around them for fixing. keep in mind carbon has positive charge and magnesium has negative charge. For making series we will connect carbon and magnesium with wire and only leaving the first and last terminals. First and last terminals are our out put where carbon side has positive charge and the magnesium has negative charge. our total out put would be about 3.5 V. Where we can run a LED. if we increase an other cell then can charge a cell phone in very small ratio. Because these are very low amps. For real time charging cell phone we need big size of carbon and magnesium.