In Charge: Dual-Battery Setups

Being prepared is a vital part of any successful off-road adventure. The farther off road you get, the more important it becomes. For example, a dead battery is already an inconvenience when you have to call someone for a quick jumpstart. Yet, it can be an outright disaster if you’re suddenly stranded deep in the woods.

Don’t think it can’t happen to you—even if you never forget to turn your headlights off, or never leave a cabin light on. You never know what unexpected accident can leave you with a drained battery—stuff happens when you’re out on the trail.

Then, there’s the fact that, for many overlanders, added battery power is needed in the first place—whether it’s for truck winches and campsite lighting, or for mini-fridges and media systems. It’s that double advantage— providing a backup for the primary battery and powering accessories—that makes dual-battery setups so popular.

Battery Background

If you’re unfamiliar with dual-battery setups, they’re exactly what they sound like. They involve wiring a second battery into your vehicle’s pre-existing electrical system. To understand the benefits of that to determine the best battery for dual-battery setups, though, let’s start with what an automotive battery does.

Now, without going too deep into the details, the primary function of your truck’s battery is to power the starter—the mechanism that gets a cold engine spinning and the spark plugs sparking. The running engine then powers the alternator, which provides most of the electricity for your truck’s electrical accessories. The alternator also recharges the battery; typically, the battery itself only provides accessory power when the engine is off.

Power demands, from fridges to winches, make dual-battery setups essential among adventurists.

The thing is, the electrical demands of trail-friendly trucks and SUVs are hardly typical. Consider the effect of a common off-road tool, like an electric winch. These are usually connected to the battery, so even if you use one with the engine on, the battery is being drained. You’re simply counting on the alternator to recharge it. But keep in mind, thanks to those pesky laws of physics, it can take more than a few minutes of charging to make up for a few minutes of winching. It’s an energy debt that can build quickly as your electrical use outstrips your ability to recharge the battery—especially if you’re also using electricity at the campsite.

Two is Better than One

At its most basic, an automotive dual-battery system, with the batteries connected in a parallel circuit, essentially doubles the amount of electricity you have available. Think of it as comparable to adding a larger gas tank. It’s certainly helpful, but it doesn’t really get to the core of the problem. While a parallel circuit can, for instance, keep the lights on longer without the engine running, one battery after another will each eventually go flat if there is a constant energy drain.

That’s why many off-road enthusiasts prefer dual-battery setups that can temporarily take one of the batteries offline—or isolate it from a truck’s main electrical system.

Manual Management

Are you one of those people who likes to celebrate their self-reliance when venturing off the beaten track, where there’s no one to count on but yourself? If so, a manually switched dual-battery charging system is likely the right choice.

In the least complicated setup, the installation includes a fairly conventional on/off switch. When your truck’s running, you flip the switch that connects the second battery to the system, allowing it to be charged by the alternator.

At your destination, you throw the switch the other way, disconnecting battery number two from the system and using it for power at the campsite. That way, your tools and toys aren’t using up electricity from the same battery you’ll need to start your vehicle with. Indeed, even if the second battery is entirely discharged, your primary battery remains unaffected.

A “next-level” approach comes with four-way manual switching, which gives owners more control over the system. With this approach, you can completely manage which battery is doing what job, and there’s also an advantage for when you don’t need the full strength of a dual-battery charging system. Many of the simpler switching systems are designed so that only the second battery can be used for bigger power draws, like refrigerators. Conversely, many four-way setups let you use the same battery for both starting your truck and powering those accessories. As a result, you can leave the second battery at home when you don’t really need it, yet still run those other features in less demanding scenarios.

The drawback for both systems: If you forget to properly switch between batteries, you can forget about starting your truck the next day.

Automatic Alternative

A good way to minimize the risk of human error with a manual isolator is to incorporate solenoids as part of your installation. You can think of these as smart switches that will automatically take your second battery in or out of your truck’s main electrical system, based on a variety of different factors.

Again, starting with the simplest type, you can wire a dual-battery charging system so that the second battery is switched on for charging as soon as the engine starts cranking. The solenoid then isolates it from the system when the engine is turned off. With this sort of switch in place, the accessories powered by battery number two can’t begin drawing electricity from battery number one, which could drain both batteries in the process.

However, there can be a concern if your primary battery ends up in a lower charge state than the secondary one, and you then try to start your truck. As you turn the key, and the secondary battery connects to the circuit, the starter battery could end up drawing power from the secondary one—instead of from the alternator. But, because your accessories battery will usually be of a different type than the starter battery—as discussed below—it may get drained without being able to help the primary unit kick over the engine. The outcome could be your worst-case scenario: two dead batteries.

Opting for voltage-sensitive relays is a wise solution. They don’t connect the secondary battery to the system when you turn the ignition key, but rather, wait until the engine is running and has first fully charged the primary battery. Once you’ve set up camp, and the engine’s off, both batteries will be able to provide power until the voltage in your primary unit falls below a certain level. At that point, the relay acts in the other direction to isolate the batteries again, preventing both from completely discharging.

What You Need

Yes, you’ll need two batteries for the best dual-battery setup, and you’ll need two different kinds of batteries. The primary unit should remain a typical starter battery. These are specifically engineered to deliver short bursts of current—sufficient to turn over your truck’s starter—without using up too much of their overall capacity. Accessories, on the other hand, are likely to draw on an energy source for an extended period of time, using up most or all of a battery’s charge at one go. For that purpose, you need a deep-cycle battery. The cost is about the same as a conventional truck battery, but you’ll be able to put the deep-cycle battery through a constant round of discharge/recharge that would quickly ruin a starter battery.

Batteries can be placed anywhere in the engine bay where there is room.

Your second battery should also find a home in the engine bay, where the least amount of extra wiring is necessary. Some trucks actually come with a dedicated location for holding a second battery, and many reliable aftermarket suppliers offer dual-battery boxes or trays for this task.

These can allow both batteries to be mounted right in the same holder, and right in the same location as the original battery. If you can’t fit a second battery underhood, you may have to get creative and consider mounting to the truck frame, or putting it in the bed.

Can You Do it Yourself?

When deciding whether to add a dual-battery setup to your rig, this may be the tipping point: Installing one can be among the easiest modifications you ever make. Many aftermarket companies have kits that include all the wiring, solenoids, switches, and mounting hardware; and if you have a particularly popular truck or SUV, you may be able to find a package custom-designed for your exact vehicle. Notably, these off-the-shelf solutions can also include frame mounts and other resources engineered to help with common installation challenges.

A few tools, a few hours, and a few hundred dollars can get the job done in ideal conditions. Just remember: You may need the dual-battery setup when conditions aren’t so ideal, which means that—no matter who installs it—you should verify that it works properly, in town, and not figure it out on the trail.

Editor’s note: A version of this article first appeared in the May/June 2017 print issue of Tread Magazine.