This circuit shows the nature of voltage sources simulated in series. As shown in the diagram, the voltage sources are simply added together to create, in this case, 3V at the peak.
Now think for a second here, what would happen if voltage sources are connected in parallel? Answer, not good things. Voltage sources will become a short circuit and damage the voltage source. Also with two voltage sources in parallel, there will be two voltages trying to set two different voltages which is not possible.
