There are certain properties in electricity which make it possible to wire two batteries together to increase the power reserve of the pair. By changing the wiring on those same two batteries, the power output can be doubled. Thus, you can wire two 12-volt batteries together and have a surplus reserve of 12-volt electrical power. Then you could rewire the same pair of 12-volt batteries and have 24-volt power available. This comes in handy with larger vehicles such as tractors that require large amounts of power on reserve. A pair of six-volt batteries can be wired together to create a 12-volt electrical system with plenty of cranking power on reserve.
Wire batteries in a series. Place the two batteries (12 volt) side-by-side. Use the wire cutters to cut a piece of wire that reaches from the negative post (-) of one battery to the positive post (+) of the other battery. With the wire strippers, remove a half inch of the wire's coating from each end of the wire and place terminal ends on each end. Secure the terminals with the crimping pliers. Attach the terminals to the battery posts and tighten with a wrench. Run a wire from the negative post of one battery to a ground. Run a wire from the other battery's positive post to the vehicle’s wiring harness. You will have 24-volt power.
Wire batteries in a parallel pattern. Place the two batteries (12 volt) beside each other. Use the wire cutter to cut a piece of wire that reaches from one battery's negative post to the other battery's negative post. Use the wire stripper to remove a half inch of coating from each end of the wire. Place terminal ends on each end of the wire and secure them with the crimping pliers. Connect the wire to the batteries' negative posts and tighten with a wrench. Cut a piece of wire with the wire cutter that reaches from one battery's positive post to the other battery's positive post. Prepare the wire as you did the ground wire and attach each end to the positive posts. With the batteries wired together, running a wire from the ground to a vehicle's chassis and a wire from the battery's positive post to the vehicle's wiring harness will provide 12-volt power with much reserve power storage capability.
Increase cranking power using batteries in a series. By wiring in series, two smaller six-volt batteries can be used to produce a greater amount of cranking power in a smaller package and often more cost effectively than wiring two similarly powered 12-volt batteries. By wiring two batteries in a parallel manner, a battery system is able to provide a great deal of prolonged energy when powering components such as high-powered stereo systems.
When wiring a pair of batteries together, make sure both batteries are identical. Mixing up batteries can cause electrical problems.
Never work on a battery system without disconnecting it from the vehicle's charging system. Removing the positive battery cable from the battery prior to working on it could save you from extensive damage from electrical shorts.