A 9-volt battery is a common power source for portable radios, smoke detectors and various other electronic devices. Unlike other batteries designed for portable electronics, with positive and negative terminals on opposite sides of the battery, the 9-volt has both terminals on the top edge. If you need to double the voltage output to a device, you can connect 9V batteries together in series using wires with alligator clips. Connecting a single 9V battery to a device takes less than a minute.
Remove the battery cover from the back of the electronic device in order to install a fresh 9V battery.
Pull off the plastic or rubber cap on the old battery, taking care not to dislodge the two wires connected to the cap, which is the battery terminal for your electronic device.
Line up the positive and negative terminals on the battery with the "+" and "-" signs on the cap for your device and snap the cap onto the battery terminals. Insert the battery with connected cap into the battery compartment of your device and replace the cover.
Attach the alligator clips on each end of an electrical wire from the positive terminal of one 9V battery to the negative terminal of another. This is the first step in connecting two batteries in series in order to double the voltage output.
Connect another wire to the free terminal on each battery with the alligator clip on the wire end.
Attach the other end of each wire to the corresponding positive or negative terminal of your device or electronics project requiring 18 volts.
Do not connect batteries in series and then attach to any device normally operating on 9 volts. Connecting two batteries in series will double the voltage and likely burn out a standard 9-volt device. Series connections are useful for electronics projects requiring increased voltage.