Hooking Up Two Batteries in Your Vehicle

An extra battery increases your vehicle's electrical supply.

There are two main reasons to add an extra battery to a vehicle. First, two batteries may make it easier to start the engine in cold weather. Second, an auxiliary battery can power extra appliances without draining the starter battery.

Cranking Power

Most vehicles are fitted with a single lead-acid battery. The battery powers the starter motor that cranks the engine. In cold weather, a battery may not deliver sufficient amps to power the motor. A second battery, hooked up in parallel, doubles the amps available.

Auxiliary Power

Audio systems, extra lights and trailer accessories such as kettles can drain a vehicle battery. A separate battery powering only the additional equipment can protect the cranking battery from accidental draining.

Parallel and Series

Two batteries linked in parallel have their positive terminals linked to one wire and their negative terminals linked to another wire. Their combined voltage remains the same, but the current they deliver doubles. Two batteries linked in series have the positive terminal of the first attached to the negative terminal of the second battery. The current remains the same, but the voltage doubles.

References

• AA1Car.com: Battery Testing
• University of Illinois: Batteries in Different Orders
• Battery University: Serial and Parallel Battery Configurations