How to Wire an Auxiliary Battery in a Trailer? [With Diagram]

Auxiliary batteries help to provide extra power supply to the vehicle’s electronics when needed. That’s why many people want to install an auxiliary battery in their RV.

Wiring multiple batteries in a trailer can be complex. Wrong wiring can cause catastrophic consequences like damaging the battery or even catching fire.

So, I’ll show you how to wire an auxiliary battery in a trailer.

If you are using a 12V auxiliary battery you can wire it to the trailer in parallel. By doing so, multiple auxiliary batteries can be connected to the trailer. Also, many people wire 6V instead of 12V batteries. Then, you should connect multiple 6v auxiliary batteries in series first to make it 12V.

Don’t get confused already, I’ll make it easy for you. Let’s get started with it.

Can I Wire an Auxiliary Battery In a Trailer?

You can easily wire single or multiple auxiliary batteries in a trailer. There are primarily 12V and 6V auxiliary batteries used for trailers. For wiring multiple 12V batteries you need to wire them in parallel circuits. The voltage doesn’t change In parallel circuits.

However, to wire single or multiple 6V batteries you need to wire them in series first. Then combine the 6V in parallel to make a 12V connection. 

How to Wire An Auxiliary Battery In A Trailer?

I’ll go through both single and multiple wiring procedures for the auxiliary batteries to the trailer. Before that let’s learn a bit about auxiliary battery requirements.

Understanding Requirements of Auxiliary Battery In a Trailer

Primarily there are two factors to consider while selecting the batteries for a trailer. These are, how many batteries you can wire and what voltages are recommended.

The Number of Auxiliary Batteries You Can Wire 

Normally most people wire one or two auxiliary batteries in their trailers. However, you can wire four, six, or even eight batteries to the trailer. If your trailer requires more power you can always opt for more auxiliary batteries at any time.

The Voltage Requirement – 12V / 6V

Now coming to voltage, 12V auxiliary batteries are ideal for trailers. But you can also use 6v batteries in series and combine them in parallel circuits to make a 12V circuit.

Either way, it’s possible to wire both 12V and 6v batteries. I’ll show you how.

Now let’s move on to the detailed discussion of the wiring methods.

Method 1: Parallel Wiring 12V Auxiliary Batteries In a Trailer

To wire a single 12V auxiliary battery to the trailer follow the steps carefully. Check the below diagram to make the process easily understandable. 

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Tools Required

I previously discussed what wire to use to wire 12V batteries. For now, we’ve used 6-gauge AWG wire which is the best for automotives.


  • Make sure to wear protective rubber gloves.
  • Wear safety goggles or glasses.
  • Keep away all sources of ignition like cigarettes, flames, and sparks.
  • Make sure the batteries don’t have any visible cracks or damages.
  • Make sure the batteries aren’t leaking any fluids.

Now follow the below steps to wire the auxiliary batteries to the trailer.

Step1: Prepare the Auxiliary Battery

To prepare the auxiliary battery for wiring, take one length of wire and hook it to the positive terminal of the battery. Now take another length of wire and hook it into the negative (-) terminal. Use pliers and wrenches to tighten the bolts.

Step 2: Connect the Wires

Now comes the tricky part. Take one length of wire and attach a 10 amp fuse to it. Now hook the wire to the positive terminal of the auxiliary battery. Do the same with the main battery and attach the battery’s negative terminal to any grounding medium.

Step 3: Connect The Battery Isolator

Remove the plastic caps from the battery terminals if it has any. Now take the positive cable of both the batteries and connect the hook to the positive terminals of the battery isolators. See the diagram if you have any confusion. 

Step 4: Connect the Alternator

Lastly, take another length of wire and connect the wire with the alternator. Make sure you connect the cable along with a 10 amp fuse attached. The other terminal of the alternator should be connected with a grounding medium.

Congratulations! You’ve successfully wired an auxiliary battery in the trailer. 

In the wiring, we use a battery isolator to prevent draining the main battery when idle. 

How to Wire Two 12V Batteries In Parallel?

Wiring more than one battery in a trailer is easy but more than two becomes critical. But don’t worry, it gets easier with diagrams. Let’s explore it.

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Step 1: Connect the Wires

First, you have to connect the positive terminal of the both main and auxiliary batteries together. Do the same with the negative terminals. Take the length of the wire and the negative terminal of both batteries. 

Step 2: Wire The Batteries to The Trailer

Now, hook one piece of wire to the positive terminal of the main battery. And, hook another wire to the negative (-) terminal of the battery. Then, connect the positive (+) cable to the positive terminal of the trailer power. 

Then, connect the negative cable to the negative terminal of the trailer power. And, it’s done. Before ignition follows the diagram again to confirm the wiring is done in the correct order. Now, check the batteries by powering up the trailer. 

How To Wire 3 12V Batteries In Parallel?

Here is the diagram that shows how to wire three 12V batteries in a parallel circuit.

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Now let’s get into the wiring procedure. Follow the steps below carefully.

Step 1: Prepare the Batteries

Connect the positive and negative wires to prepare the batteries. Then, take a length of wire and hook it to the positive terminals of all three batteries. Then, hook another length of wire to the negative terminals of all three batteries.

Step 2: Connect The Trailer Power

Now hook another piece of wire to the positive terminal of the first battery. Then hook a negative cable to the negative terminal of the last battery. Connect the positive and negative cables accordingly to the trailer power terminals.

You’re done with the wiring, check the diagram again and confirm you’ve followed it correctly. Now, check the batteries by powering up the ignition.

How To Wire 4 12V Batteries In Parallel?

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Step 1: Prepare The Batteries For Connection

To wire four batteries together, first position them in a suitable place. Then have some wire and connect the positive terminal of the first two batteries. Then, do the same with the other two batteries. 

Step 2: Make a Parallel Connection

Now get another piece of wire and connect one part to the positive terminal of the second battery. Then, connect another part of the wire to the positive terminal of the third battery to make the connection parallel. If it sounds confusing follow the diagram.

Step 3: Connect to The Trailer Power

Lastly, connect the positive wire of the first battery to the trailer’s positive terminal. Then, connect the negative wire to the negative terminal of the trailer power.

Method 2: Parallel Wiring 6V Auxiliary Batteries In a Trailer

We’ll see how you can wire multiple auxiliary batteries in a trailer. Follow the below diagrams to make a series connection first. Then, make a parallel circuit out of it.

How To Wire Two 6V Batteries In Parallel?

Follow the below diagram to wire two 6V batteries in a trailer in parallel.

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Now follow the below steps thoroughly. If you get confused between the steps I recommend you check the diagram and follow it accordingly.

Step 1: Make a Series Connection

To make a series connection, connect one part of the wire to the positive terminal of the first battery. Then connect another part of the wire to the negative terminal of the second battery. Doing it will create a serious connection.

Step 2: Connect to The Trailer Power

Now connect the positive terminal of the first battery to the positive terminal of the trailer battery. Then, connect the negative cable to the negative terminal of the trailer power. It’s done, now check the batteries by powering on the ignition.

How to Wire Four  6v Batteries In Parallel?

Lastly, we have four 6V batteries to connect in a parallel circuit. Follow the below diagram.



Step 1: Connect The Negatives

First you have to make a series connection by joining two batteries in a series together. So, to do that take two batteries and hook a cable to the positive terminal. Then hook the other part of the wire to the negative terminal of the battery. 

Do the same with the other two batteries to make a series connection. Make sure the four batteries don’t connect to each other. Follow the diagram if you’re confused.

Step 2: Connect The Positives

Now that we have connected the negatives, follow the diagram and hook the positive wires. Connect the positive terminals together of the first two batteries. Then, connect the two negative terminals of the other two batteries.

Step 3: Connect With The Trailer

Lastly, connect the batteries to the trailer power. Hook the wire to the positive terminal of the first battery and connect it to the positive terminal of the trailer. Then, connect the last battery’s negative connection to the negative terminal of the trailer.

What Type of Auxiliary Battery Is Best for a Trailer?

There are primarily four types of batteries that are best for trailers. Many top brands produce these car batteries. Check the table below for more info.

BatteriesMaximum Cycle*Lifespan*Cost
Lead Acid1500 cycles3 to 5 yearsCheck Price
AGM500-700 cycles5 to 7 yearsCheck Price
Gel1000 cycles10 to 20 yearsCheck Price
Lithium3000-5000 cycles2 to 5 yearsCheck Price

Field with asterisks* refers to typical data.

So the lithium, gel, and lead acid batteries are good for trailers since they have more cycles. Note that we put asterisks both on the maximum cycle and lifespan of the battery. It’s because the data is typical.

A battery’s lifespan depends on maintenance and overall usage. Battery lifespan can be extended with proper maintenance. Now, check the pros and cons of all of these four batteries in the table below.

Lead Acid Battery
Available in various shapes and sizes for convenience.No maintenance required.Offers a good life cycle.Prevents overcharging, and slow/fast charging.Budget-friendly.Supports low self-discharge rate.Offers good performance even at low or high temperatures.Heavier in size compared to other batteries.Has a poor weight-to-energy ratio.Recurrent deep-cycle may reduce the battery life.Takes longer to charge due to the slow charging rate. Needs to be stored fully charged to prevent sulfation.Not environmentally friendly.

Now, let’s discuss the pros and cons of AGM batteries.

AGM Battery
No maintenance required.Can be installed in both series and parallel connections.Environmentally safe and spill-proof.Slow discharging rate and minimal gassing for using less ventilated areas.They can withstand even colder temperatures.Shockproof and vibration-resistant.More expensive. Need specific chargers because they can’t withstand overcharging.Doesn’t work with old battery testers. Reduced capacity over time.

Now, let’s move into the advantages and disadvantages of gel batteries.

Gel Battery
Don’t require any maintenance. Gel batteries usually don’t leak.Vibration and shock resistance.Costly compared to other battery types.Extremely slow charging rate which can be irritating in emergencies.Needs a voltage charger, not a taped charger.

Alright, lastly I have the lithium batteries. Let’s look at the advantages and the disadvantages. 

Lithium Battery
Has high energy density.Lithium-ion batteries are rechargeable.No need to completely discharge before charging. Has the most charging and discharging cycles.The battery has a slow discharging rate when stored idle.Has a short lifespan compared to other batteries.Comparatively expensive.Lithium batteries are sensitive to high temperatures.Completely discharging the battery will prevent recharging it again.Can catch fire or explode if the separator gets damaged.

Can I Wire Auxiliary Batteries In a Series In a Trailer?

No, you cannot. If you’re using 6v batteries you cannot wire them directly in parallel. It’s because the voltage must be the same throughout the circuit.

But it doesn’t happen in series circuits when you increase the amp hours. For that reason, we need to wire parallel circuits.

That’s why two 6v batteries need to be wired together to make a pair of one 12V battery. However, if you’re using 12V batteries you can easily wire them in parallel circuits.

We’ll talk about how you can wire 6v batteries in series in parallel. Also, how to wire single or multiple 12V batteries in a parallel connection. Let’s start with the 12V battery.

Can I Charge A Trailer Auxiliary Battery?

Many people wonder whether they can charge the auxiliary batteries or not. The answer is not all the battery is rechargeable. You cannot charge sealed lead acid batteries, AGM, and Gel batteries. Only lithium-ion batteries are rechargeable.

Apart from lithium batteries, all the other trailer auxiliary batteries have a specific lifespan. Once the batteries reach the lifespan you have to replace them. But lithium batteries can be charged and they have the highest life cycles. 

The only problem with lithium batteries is the cost. However, lithium batteries are the best option if money is not an issue. Now, let’s see how you can charge a 12V lithium battery properly.

Now let’s see how to wire a 12V battery to the charger.

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Image Source: Lithium Hub


Make sure you follow the below instructions while wiring the charger to the battery.

  • Always double-check the battery terminal to prevent wrong connections. Any sort of misconnection can cause severe consequences.
  • Connect the positive(+) cable to the positive terminal and negative(-) cable to the negative terminal. Use a 6-AWG cable recommended for maximum performance.
  • If you’re using a manual charger, make sure the battery and charger voltage is always the same. Smart chargers automatically adjust voltage based on battery size and turn off when fully charged.

Auxiliary Battery Maintenance Tips

You’ll be happy to know that most of the trailer auxiliary batteries don’t need any sort of maintenance. If you remember, in the beginning, I discussed different types of batteries for trailers.

Lead acid batteries are the most primitive ones and they come in two different forms. There are sealed lead acid batteries and unsealed lead acid batteries. Sealed lead acid batteries don’t need any maintenance, unlike unsealed ones. 

Similarly, AGM and Gel batteries don’t need any kind of maintenance. They perform slightly better than AGM batteries in high temperatures and slow discharging conditions.

Lithium batteries also have higher performance and no maintenance. They’re also rechargeable when low but cannot be recharged if completely drained. Even though lithium batteries don’t require any maintenance but always use a good charger.


How To Charge A Trailer Battery While Driving?

You can charge your trailer battery while driving using an alternator. You just have to connect the battery to the alternator with a seven-pin plug. But the major downside of using alternators is it drains the battery while idle. Use a DC-DC charger or a battery isolator to prevent battery drain.

Do You Need Two Batteries For Travel Trailer?

You may need one or two auxiliary batteries if you use power-hungry equipment. Else the started battery will be drained and stressed to meet all the power load of the vehicle. That’s why many people have multiple batteries in their trailers. You can wire up to 8 auxiliary batteries to a trailer.

Can I Use a Car Auxiliary Battery on My Trailer?

Unfortunately, you can’t use a car auxiliary battery on your trailer. It’s because trailers only support deep-cycle batteries. You can use any of the batteries we’ve mentioned. The best would be lithium-ion batteries. However, sealed lead acid batteries or AGM batteries are also good choices.

What Is the Best Auxiliary Battery?

Lithium batteries are the best auxiliary batteries for trailers having the highest charging cycles and longer lifespans. They are also rechargeable when needed. Though lithium batteries are pricey, they’re definitely worth it.


That was all about your query on how to wire an auxiliary battery in a trailer. I tried to cover the topic from every angle and provided easy-to-follow guidelines.

Hopefully, the content was helpful. Let me know if I’ve missed anything.