What Gauge Wire to Connect Two 12v Batteries in Parallel?

When you’re trying to connect to batteries in parallel one of the more important points is choosing the right wire gauge. Using the correct wire gauge in any battery connection increases the system efficiency while minimizing risks. Using the wrong gauge wires to link two 12v batteries together can cause shock, damage, and failure of key components. 

So, what wire gauge should you use to connect two-12v batteries in parallel? Well, in my experience, you will need a wire whose gauge is in the range of 4 to 6. The main factor here is the amount of amperage generated by the two 12-volt batteries connected in parallel. The length of the wire used also plays a major role in the wire gauge determination. Linking of batteries in parallel does not affect the effective voltage; it only increases the amperage hence thicker wires are ideal for higher values of amps (amperage).

Read this guide to the end to know which various wire gauges to use in your 12-volt battery connection.

The standard wire gauge for linking two 12v batteries is 4 and 6 wire gauges. For most RVs and travel trailers, a 10-gauge wire is the best choice. 10-gauge wires are rated for up to 30 amperes. Generally, the type of wire gauge/thickness to use for any connection relies on the number of amps running through the wires.

Can you Connect Two 12v Batteries?

The straightforward answer to that question is yes, you can connect two 12v batteries. This type of connection is safe if you need more amps.

However, for a safe two 12v battery connection, you should use the correct wire gauge to hook on the batteries’ terminals. Connecting two 12-volt batteries in parallel increases the system amperage. In that sense, a thicker wire is needed to comfortably ferry the current, otherwise, you may end up damaging your circuit components or devices. (1)

To avoid a short circuit, do not go from the open positive to the open negative while making the connection. You might also harm your batteries if you proceed that way.

For a series connection (of two 12v batteries), the number of amps rating will not change but the voltage will increase. In that type of connection, drag your wires from the negative terminal of your 1st battery to the positive terminal on the 2nd battery. And then use the wires to connect the 1st terminal of the 1st battery to the negative terminal on the 2nd battery.

What Gauge Wire Should you Use to Connect Two 12v Batteries in Parallel?

wiring tools

Again, the number of amps flowing through the wire is the determining factor. Additionally, the length of the wire in a parallel connection also influences gauge wire choice. If the wire is long, the thickness should be increased – the wire length varies proportionally with its thickness.

As an illustration, a 4-feet wire with 30 amps flowing through it requires a ten gauge wire. But if the wire length is increased to about 15 feet, you will need an eight gauge wire. If the wire length is further increased to about 20 feet, you will require a six gauge wire in your connection – if the amount of amps is kept constant. If the length of the wire reaches 30 feet, you will need a four gauge wire to connect your batteries.

You need to tighten your connection and use the right wires. Also, make sure you strategically keep the wires so that they stay firm and undisturbed. If the wires are mishandled while the device is working, they may break loose or cause a shock if they come into contact with conducting surfaces. (2)

How to Wire 12-volt Batteries in a Parallel Connection

We will now carefully discuss how to wire a 12-volt battery in parallel correctly. You should know that the voltage will not increase, instead, the amperage will. Bearing that in mind, you, therefore, need to check the wire gauge you are to use.  

The 10-gauge wires are rated to up to 30 amps.  So, if your parallel connection will produce more than 30 amps you need a thicker wire gauge. Follow the procedure below to wire 12-volt batteries in parallel to double the capacity while maintaining voltage.

two batteries in parallel connection

  1. Align your batteries so that the positive terminals are on one end and the negative on the other end linearly.
  2. Use the multimeter to verify the voltage of each battery. Set the multimeter to the voltage setting and connect the probe leads to the battery terminal. Attach the red lead to the positive or red terminal and the black probe to the black terminal of the battery. Note the voltage values of the two batteries, one at a time.
  3. Grab the red 10-gauge wire and connect the two positive terminals of the two batteries. Take the negative cable and connect the other negative terminals of the two batteries.
  4. Fasten the connection of the two negative and positive terminal connections. That’s it; you have connected the two batteries in parallel.
  5. Now verify the voltage values of both batteries using a multimeter, and compare the readings. You will notice that the voltage value does not change.

In verifying the voltage of the two 12-volt batteries connected in series, I recommend going for a cross-connection. That is, connect the multimeter to the battery terminals diagonally. The cross-connection will make the two batteries share the entire battery bank hence giving consistent results.

12-volt Parallel Battery Cable

parallel battery cable wire

The wires for the 12 volts battery connection are widely available in the market. Note that the cost will not be very high and it will depend on the length – the longer the wire, the higher the cost. Also, note that some stores have a fixed amount of wire or the length that you can purchase.

Take a look at some of our related articles below.

(1) ferry – https://www.thrillist.com/travel/nation/9-of-the-world-s-most-epic-ferry-rides
(2) strategically – https://www.indeed.com/career-advice/career-development/strategic-thinker

Video References

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About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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