Things to Consider When Looking at A Dual Battery System

Most people look at installing a dual-battery system either because they want to run winches, portable refrigeration, additional lighting, and mobile phone chargers while camping and travelling in remote areas.

Vehicle batteries are charged by the vehicle’s alternator whilst the engine is running, so when the engine is shut down, the charging of the battery ceases. Using portable 12V appliances will draw power and reduce the power supply stored in the battery. This often results in the battery failing to start leaving you stranded. A dual-battery system solves this problem.

Electric winches can drain significant amps from the vehicle battery quicker than an alternator can replace them so a dual-battery system can be set up so you can run the winch off both batteries. You should also seriously consider the electrical appliances that you expect to run from the dual-battery system and calculate how much power source (in watt-hours) is required to run all the items plus how long you want to run the items between charge cycles (0-100%). 

It also helps when you factor in a safety margin of about 25-30%. If done correctly, this calculation will narrow down your options when deciding the capacity of the battery that you need.

There is so much more things to consider when looking to purchase a dual-battery system. Read on below so you can learn more about the dual-battery system and what’s the best choice for you.

What Are the Parts of a Dual-Battery System?

A dual-battery system consists of the two batteries with one set as a “primary battery” while the other is the “auxiliary battery”. The system is also composed of:

  • battery cradles
  • wiring for the discharging/charging system which connects into the vehicle’s electrical system; and
  • a battery isolator 

An optional visual read-out device can also be placed in the cab of the vehicle for you to check the charge of the battery. Some larger 4WD vehicles come with two starter batteries that are electronically paralleled for starting the vehicle. However, this should not be confused with a dual-battery system. Starter batteries can have CCA (Cold Cranking Amps) marked on them and they are called ‘shallow cycle’ batteries.

Important: The size of the cable linking the batteries in a dual battery system should be at least 13.5mm square.

What Are the Best Batteries for Dual-Battery Systems?

There are two main types of batteries. When you buy your vehicle from the manufacturer it will likely have a “shallow cycle” or single high-current discharge (HCD) battery. These HCD batteries or cranking batteries provide a brief high amp charge to start the engine. While the other type is called deep-cycle batteries

Deep-cycle batteries are preferred as the ‘auxiliary’ battery for a dual-battery system as they provide sustained power over long periods of time. They are also designed for repeat charge and discharge. These deep-cycle batteries can be larger in size so the one you purchase will obviously need to fit somewhere. But you can easily find space in the vehicle’s engine bay or behind the seat in the vehicle cab. However, deep-cell batteries cost more than HCDs.

Batteries also come in various sizes and weights. The number of internal lead plates can vary in number (i.e. 7, 9, 13, 15, 17 & 19) and thickness. So, the amp hour capacity also varies between each of them. Battery terminals can also range from standard, standard/wingnut to dual fit. 

Starting batteries have thinner plates and will accept a greater amount of charge more quickly. In contrast, deep-cycle batteries have thicker plates, so they only accept a small amount of faster charging before the resistance of the plates begins to reduce the amount of charge uptake. So, different batteries reach full charge at different voltages and with a dual battery system you need to consider if your vehicle alternator and battery isolator are capable of charging both batteries.

There are also wet cell and dry cell battery types that you can choose from. Wet cell batteries give off hydrogen gas, which is an explosive vapour, so you would not want to place this type of battery near any source of a likely spark. Extreme care should be taken when handling lead-acid batteries as any acid spill can cause serious burns.

Other battery types include:

  • Flooded Batteries. These types of batteries are also known as ‘wet cell’ as the lead plates are suspended in wet acid.  Caps are also found on the battery for you to check the electrolyte levels which require periodic topping up. Wet cell batteries should always be housed in a well-ventilated area and kept upright.
  • Gel-Cell Batteries. These batteries are called ‘gel’ batteries as the electrolyte is held in a jelly-like state. The batteries are also sealed and spill-proof. In addition, a three-stage smart charger is required to charge the batteries. The main selling point of gel-cell batteries is they hold their charge longer than ‘wet cell’ batteries but it will suffer permanent damage if overcharged. A rest period will be necessary after rapid charging to make sure the true state of charge is known.
  • Absorbed Glass Mat (AGM) Batteries. These have so far been the most compatible batteries for 4WD or RV use. Especially when the main source of charge is from the vehicle’s alternator as they can be recharged faster. AGM batteries have positive and negative plates that are separated by a glass matt while the acid is made of gel. Reports state that AGM batteries are less sensitive to charge rates than the gel-cell batteries. AGM batteries are also sealed, spill-proof, and maintenance-free.
  • Spiral Batteries. These are very similar to AGM batteries, but the lead plates are tightly compressed into spiral wound cells. Spiral batteries are smaller in size, lighter, sealed, and maintenance-free. The batteries are reported to have a high cranking performance, a longer shelf life (when not in use), a higher charge acceptance rate during recharging, and more likely to withstand vibrations. Some spiral batteries can also be installed in any position making for more flexible use.

Good sturdy battery trays or battery cradles are required to secure the battery in the vehicle. Battery tray joins should be MIG welded and powder-coated so that it will resist abrasion and corrosion.

What Are Isolators and What Do They Do?

Battery isolators do what the name suggests. They ‘isolate’ the battery used to start the vehicle leaving it to maintain its charge after you have turned off the vehicle’s engine. When you start a vehicle’s engine, there is a surge of power to the starter which triggers the alternator to start producing current. The isolator responds by providing power to the primary battery until it is fully recharged. When the primary battery is full, it switches the current over to charge the auxiliary battery. The 12V electrical items are then powered through the auxiliary battery.

Manual switches can also isolate batteries. However, with manual switches, the main downside is you will often forget to turn it on or off (depending on the sharpness of your memory). So, the more apt choice is an automatic battery management system. Automatic battery management systems link batteries for charging and isolate batteries for discharging. To avoid any damage to a vehicle’s onboard computer or electronic fuel injection (EFI) system, select an isolator that has built-in spike/surge protection.

The manufacturer of dual battery isolator Smart Start, Redarc Electronics, recommend fuses be fitted in the cable close to the positive terminals of each battery for safety and to reduce the risk of fire. 

What Are the Types of Battery Isolators?

There are so many types of isolators and here are some of them:

  • Diode Isolator. This type has battery circuits that are isolated from each other. They act as a one-way valve between the batteries. Due to the isolation of the current’s flow, each battery is an independent power source. These are not used much these days, but they can be found on older vehicles.
  • Solenoid Isolators. For a good basic ‘parallel’ charging dual battery system it is recommended that both batteries be identical (i.e. the age, size, capacity, and design of the battery). The batteries are set up to be charged in parallel and once the ignition is turned on the solenoid is automatically switched on and this allows the power to flow between the batteries. Once you turn off the ignition the solenoid opens thus isolating the main battery.
  • Electronic Isolators are the best of all dual battery charging systems. With electronic isolators, you can use dissimilar batteries as the electronics are specifically designed to recognise the difference and adjust the rate of charge. However, the main battery is always given priority.

There are dual battery specialists who advocate the electronic isolators as, in their opinion, people using basic parallel charging systems cannot tell if the main battery is flat or damaged. Consequently, the auxiliary battery will do most of the starting/cranking which depletes its charge leading to the depletion of both batteries.

Important: If you are a keen 4WD enthusiast, make sure the system you select is one that has dust and water resistance features.

What Are Smart Solenoids?

Smart Solenoid is an essentially smarter version of continuous duty solenoids. They have a voltage sensor that allows automatic operation. As a result, parallel charging does not commence until the main battery has reached a predetermined voltage (usually around 13.5V). 

After you reach this level, the batteries remain in parallel, after you turn off the ignition, the starting battery draws down to a predetermined voltage (around 12.8V). Then the solenoid contact opens, and the auxiliary battery is disconnected. This action prevents flattening of the starting battery.

What Are Voltage Sensing Relays?

Voltage Sensing Relays (VSRs) are not considered as a parallel charging system but they operate like a smart solenoid. However, it uses a relay (and not a solenoid) with the two batteries electrically separated. 

The auxiliary battery is disconnected from charging if the main battery voltage drops below a predetermined level. On the downside, the capacity for a large current draw can tend to be limited given their lighter construction.

Installation of A Dual Battery System

Installation of a dual battery system is best done by an expert. It requires connecting into the vehicle’s electrical system and this must consider the vehicle’s wiring.

 It can also be frustrating when you are anchoring the sturdy battery cradle to accommodate the weight of the deep cycle battery and finding the space for it in the vehicle’s engine bay. Care also needs to be taken with the installation of the isolator so that it will not be subjected to too much vibration or heat.

The type of wiring and cables for a dual-battery system also needs careful consideration to ensure a high charging rate to the auxiliary battery and to avoid voltage drop. Some of the brands/models that we highly recommend are:

  • Redarc Electronics;
  • ARRID Advanced dual-battery controller;
  • DBS Management System;
  • IBS Management System;
  • Piranha Off Road Products;
  • TJM; and
  • ARB.

If you are technically minded, dual battery management kits are available from the above suppliers and eBay.

How Can I Prolong Battery Life?

Battery life will be extended if you do not run the battery down more than 50% of their capacity. It would also be a good idea to know how many hours it will take for your vehicle’s alternator to charge the auxiliary battery. A standard vehicle alternator is not designed to recharge a deeply discharged battery so it could take up to 5-6 hours to charge an 85 Ah deep-cycle battery. The vehicle’s alternator can also be modified with a specialised charging alternator and a smart multistage regulator.

A few brand names for batteries are:

  • Century Batteries;
  • Trojan Batteries;
  • Exide Endurance Deep Cycle batteries;
  • Optima Batteries;
  • Federal Batteries Deep Cycle Series; and 
  • Concorde Lifeline AGM deep cycle batteries.

Related Questions

Do I Need Dual Batteries for A Fridge?

To keep the fridge running while the vehicle is parked, you need plenty of reserve battery power. So, a dual battery system is a must.

Does Idling Charge the Battery?

“Starting the engine draws 100 to 130 amps and idling the car for 15 minutes might put back three or four amps,” Brown-Harrison says. “If you’re idling only for 15 to 20 minutes, the battery never truly gets recharged. So, each time you start and leave it to idle, the charge will get lower and lower and lower.”

How Long Do Batteries Last?

While a battery that can make a car to start at the first turn of the key is a great thing, it doesn’t last forever. In fact, depending on where you live and how you drive, the condition of your charging system, and several other factors, a battery lasts about four years on average.

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James Mitchell

Hi, I’m Jimmy Mitchell and I love exploring this great country with my wife and two boys. I have a 2015 Sterling LX that is the Mitchell Family camping machine. Lets Getaway is the website where I share things about my trailer as I learn them, and help other camper owners to enjoy their RV even more.

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