Make sure you connect the "batteries" in series, that is +[]- +[]- +[]- +[]-
so that their voltages are summed. For example, if you are using copper and zinc strips for your nodes with one copper and one zinc in each lemon, then connect the copper strip of one lemon to the zinc strip of the next lemon, so that you have a copper strip and one end of the series and a zinc strip at the other end. The potential between these two extremes should be the sum of the individual voltages across each cell.
Hopefully you have a voltmeter to check your setup.
