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Does Your Overland Rig need Dual Batteries? What You Need To Know

If you’ve been driving for a few years, you’re likely to have experienced a dead battery at least once. By now, you should know that leaving your lights on when the engine is off can drain your car battery pretty quickly.

Today’s overlanders love accessories. Whether you’re running a winch, air compressors, fridge, lights or even adding USB outlets, your electrical system can only handle so much.

Jimahajer Toyota Tacoma Dometic fridge

On top of your accessories, you also rely on the battery to start the engine. The last thing you want to happen when you’re off the grid is a dead battery

Why would I need two batteries?                                  

Whether you’re camping just outside of the city or trekking across the continent for weeks at a time, a dead battery can ruin any trip.

When your car is running, the alternator keeps the battery charged. If you’re parked at camp, running your fridge and camp lights in a stationary car with the engine off is a quick way to discharge your battery. This could leave you off the grid longer than you had planned.

Genesis off road optima dual battery Jeep Wrangler JK

One way to prevent this is by adding a second battery to power your accessories and make sure your starter battery is topped off when it's time to go home.

How does it work?

In a dual battery system, one battery (referred to the starter battery) is dedicated to starting the car. The other battery (called the house or auxiliary battery) is dedicated to powering all your accessories when the engine is off. If needed, the auxiliary battery can be used to jump start the starter battery.

Dual Battery Diagram

Very basic dual battery set up shown above.

The two batteries are wired in parallel (positive to positive and negative to negative) through a relay. The relay is wired to an ignition source and senses voltage to link batteries together when the engine is running. When the engine is turned off, it isolates them so that the starter battery does not get drained when you are running your accessories off of the house battery.

Bluesea ML-ACR dual battery relay battery isolator  

Depending on what relay or isolator you run it can work similarly as described below:

  • When your engine is off, the auxiliary battery will continue to power the accessories connected to it for as long as it the auxiliary battery has power.
  • When the main battery drops a certain voltage (i.e. 12.7 volts) the isolator separates the batteries to ensure your starter battery will have enough power to start the engine.
  • After the engine is started, the main battery will charge to 13.2 volts before the isolator allows the auxiliary battery to charge.
  • When both batteries reach a certain voltage (i.e. 13.2 volts), the isolator will connect the two batteries so that both batteries are being charged.

In the event that the starter battery is drained, some isolators have the ability to connect both batteries together like built-in jumper cables to jump-start a dead starter battery off of the auxiliary battery.

How can I put one in my truck?

Researching dual battery set-ups can get confusing and overwhelming. There are different ways to do it and many parts involved. In addition to a second car battery, you will also need mounts, wiring, fuses and most importantly, the smart relay or isolator.

Toyota Land Cruiser dual battery system

If you own a popular vehicle such as a Jeep Wrangler, Toyota 4Runner or Toyota Tacoma, there are several kits to choose from. These kits are designed to drop in and come with all the relay and wiring precut, ready to bolt in.

If you want to go the DIY route, you can easily piece together your own. Aside from mounting, the concept applies to most any vehicle.

What kind of batteries should I use? 

A dual battery set up can be run on a regular car battery or deep cycle battery, such as an Odyssey or Optima. Some dual battery set ups will allow you to mix-and-match. Meaning you don’t even need to run two of the same batteries in your dual battery system.

Optima battery line up red top yellow top blue top

Optima offers different types of deep cycle batteries. Their red top is advertised as “the ultimate starting battery” and features a strong starting burst. Yellow top batteries feature strong cranking and cycling power for vehicles running multiple accessories. Blue top is designed for environments that experience heavy vibrations (boating) and long stationary periods (RVs).


Electric generators such as Jackery or Goal Zeros can power fridges and charge phones, laptops and lights without draining your car battery.

Jackery 500 powering Dometic fridge outside of Jeep winter camping

For example, your car battery can power your fridge while you’re driving. When you reach basecamp, you can unplug the fridge from your car and power it through the electric generator.

Viking battery pack charging car battery

Regardless if you go dual battery or not, it is always a good idea to bring a battery pack. Some battery packs will not only charge your phone but also jump start a dead car battery. 

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