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The Challenge: Recharge a Battery

How do we get this direct current electricity back into the battery?

We need to generate about 14v to charge the battery. If we produce much less we will never charge it and, more importantly, if we generate a lower voltage than the battery already has then electricity will begin to flow from the battery back into the generator. This will cause the battery to go dead and could cause a dangerous short circuit.

To avoid this, we incorporated a second circuit from the generator. In old fashioned car circuits this would be called a shunt circuit. When the current flowing through the shunt circuit reaches a certain level it creates a magnetic field that is strong enough to move a piece of iron, in our case a nail. This can act as a switch connecting the generator to the battery when the voltage is high enough. This is a basic solenoid.
Animation of circuit