If you want to increase power on an experimental DC circuit, you can add a second power supply connected in parallel. A parallel circuit allows electricity more than one path to travel, and when more than one power supply is connected to a component, they each provide half the current. For example, a battery rated at 60 amp-hours put on a circuit that draws one ampere will run for 60 hours. Two batteries will run for twice as long because each battery only carries half an ampere per hour. You can use two 9-volt batteries to build a simple parallel circuit with two power supplies to illustrate the concept.
Connect the first battery to the component load in a simple circuit using alligator clips. A wire should come from each terminal of the battery to the two contact points of the component. Leave one wire disconnected from the circuit to prevent it closing.
Cut and strip your wire as necessary.
Place the second battery near the first one. Connect the two batteries' positive terminals together.
Connect the negative terminals together.
Close the original circuit by reconnecting the wire.
This is a basic parallel DC circuit, but you can use the same principle to connect more complex circuits.
Inspect the two batteries to determine that they are the same voltage, and that they are both in good condition. Never work with leaky or damaged batteries. Always handle alligator clips by their insulated portions.