Whether you want to double the output of your batteries, or double their endurance, simple steps exist to do either. Connect together 9-volt batteries using one of two wiring methods: series or parallel. Connecting batteries in series doubles the output voltage while maintaining endurance, with output at 18 volts. Connecting batteries in parallel doubles endurance while maintaining the output voltage, with endurance increased from six to 12 hours. Choose the method that best suits your requirements.
Put the 9-volt batteries next to each other on a table. Use a knife to cut four strips of American Wire Gauge (AWG) 18 gauge wire to connect the two 9-volt batteries in parallel. Cut two wire strips 1 inch long and cut the other two the length required to connect between the 9-volt batteries and the electrical equipment.
Remove 1/4 inch of plastic from the end of the four wire strips using wire strippers or a small knife. This reveals the inner metal core, which attaches to the battery terminals.
Find the terminal polarity of the two 9-volt batteries. These 9-volt batteries are usually PP3 type: rectangular in shape and two terminals on top. The positive is a small circular male connector; the negative terminal is a larger hexagonal or octagonal female connector.
Twist the ends of the exposed metal wire from a short strip and a long wire strip together. Repeat the process for the one remaining short strip and the one remaining long strip. You now effectively have two long strips of wire with a twisted wire join in each about 1 inch from the end of each strip.
Attach the exposed end of one of the wires closest to the twisted join onto the positive terminal of the first 9-volt battery. Stick about 1/2 inch of the tape to one side of the battery so the rest of the tape faces towards the terminal. Put the wire on the terminal, lay the tape over the wire and terminal and press down on it and stick the remaining tape on the other side of the battery.
Attach the exposed twisted wire that's 1 inch from where you connected the opposite end to the positive terminal onto the positive terminal of the second 9-volt battery with tape. The remaining opposite end of the long strip of wire connects to your electrical equipment.
Repeat the process for the other long wire strip. Connect the exposed end of the shortest part of the strip to the negative terminal of the first battery and then connect the exposed twisted wire, that's 1 inch from the end of where you connected it to the first battery, onto the negative terminal of the second battery. Use tape as before. The remaining opposite long end connects to the electrical equipment. The voltage produced from the two batteries remains 9-volts, but endurance doubles.
Use a knife to cut three strips of AWG 18 gauge wire. Cut one wire strip one-inch long; the other two need to be long enough to connect between the batteries and the electrical equipment.
Strip 1/4 inch of plastic off the end of the three strips. Use wire strippers or a small knife.
Attach one end of the short wire strip to the positive terminal of the first battery. The positive is a small circular male connector and is labelled "+." Use a strip of insulating tape to hold the wire onto the terminal.
Attach the opposite end of the short wire strip to the negative terminal of the second battery using the same method. The negative terminal is larger than the positive terminal and is hexagonal or octagonal in shape. It is labelled "-."
Attach the end of one of the longer wire strips to the negative terminal of the first battery and then attach one end of the second longer wire strip to the positive terminal of the second battery. Use strips of insulating tape as before. The opposite ends of the two wires connect to the electrical equipment. The voltage produced from the two batteries is 18 volts.