How To: Dual Battery Installation

Have you every gotten in your car, turned the key, and then heard…nothing? Well if you are around town you can get a jump from someone or call AAA. No big deal. But what if you are back in the woods, miles from the nearest paved road? That’s a problem.

The solution? Install a dual battery system!

What is a dual battery system? In most vehicles there is only a starter battery to provide enough power to start the engine. This works well as long the battery is in good condition and there are few accessories drawing power while the engine is off. In a dual battery system, a second battery is added that charges from the engine/alternator while the vehicle is running, but disconnects from the system when the vehicle is off. All accessories can now run off of the second battery leaving the starter battery fully charged to start the engine. If for some reason the starter battery does not have enough power to start the engine, the second battery can be connected to the starter system to help start the engine (basically giving yourself a jump!).

Dual Batteries 101 (Some background info, skip this if you just want to see my installation)

A difference in batteries
The original battery installed in a vehicle is a starter battery. These batteries are designed to deliver a lot of current very quickly to start your engine. The down side to this is a starter battery doesn’t last long delivering a small current over time, and the battery will be damaged by discharging the battery too much. The second battery in a typical dual battery system is what is called a Deep Cycle battery. These batteries are designed to deliver a smaller current for a longer time. Deep cycle batteries can be deeply discharged with no effects. This is ideal for using the battery to run lights, a radio, charge cell phones, etc while in camp.

Several ways to get there
The simplest way to install a dual battery system is to connect two batteries in parallel (+ to + and – to -). This lets both batteries charge together when the engine is running, but the batteries will also discharge together which will leave you with two dead batteries… that won’t work! So we need a way to connect the batteries when the engine is running, and disconnect them when the engine is off. I will list out some methods of accomplishing this and their pros and cons.

Manual high current switch between the starter battery and the second battery.
Pro: Simple installation, Low Cost, Effective
Con: Have to remember to flip switch every time you start the vehicle, and again when you shut down.

Battery Isolator. (A Large diode that allows current to flow one way only)
Pro: Simple installation, Moderate Cost, No moving parts, Automatic operation
Con: Voltage drop across the isolator results in lower battery charge, needs high current switch to jump batteries

Voltage Sensitive Relay. (A switch that is controlled automatically by sensing when the alternator is charging and when it stops)
Pro: Simple installation, Moderate Cost, Automatic operation
Con: Moving parts, needs high current switch to jump batteries

Intelligent Systems. (Hellroaring, National Luna)
Pro: Automatic operation, combine batteries with a small switch on dash, no high current switch required, install kits available
Con: Moderately complex installation, high cost

My Installation

I chose to go with the Voltage Sensitive Relay method. I’m using a BEP Marine 710-125A VSR (found one on clearance even!) and a BEP Marine 701 Master Battery Switch to control charging, isolation, and manual override operations. The battery I am using is a Die Hard Deep Cycle Group Size 29. With this installation, when the engine is on and the alternator and starter battery are at 13.7 volts the VSR senses the voltage and closes the circuit. This allows both starter and aux batteries to charge. When the engine shuts off and the starter battery voltage drops to 12.8 volts the VSR opens the circuit, isolating the two batteries. If for some reason my starter battery died, I can flip the manual switch to connect the batteries and start the engine from the second battery. The manual switch also offers a backup if the VSR fails, I can easily connect/isolate the batteries manually.

Side note on battery selection:
If you do much research on dual battery installations you will come across many people asking if they should use a sealed lead acid battery or an AGM (Absorbed Glass Mat) battery. There is concern over venting of hydrogen gas (explosive) and spilling of battery acid. In my opinion AGM is the best IF you have the budget and/or you are expecting to drive on terrain where a rollover is a real possiblity. The AGM battery is more powerful and can have a longer life than a sealed lead acid battery. For my situation I decided a sealed lead acid battery was sufficient. Venting of hydrogen gas at volume only occurs in an overcharging situation. For this to happen the voltage regulator on the alternator would have to fail. Also I generally have the fan on or the windows open which would quickly diffuse any gas buildup. As far as spilling acid in a rollover situation, I don’t plan on driving into extreme terrain and any other rollover is going to cause a lot more problems than battery acid leakage! It’s up to you to decide which way you want to go on this matter.

All of the wiring for the system is 4 gauge stranded copper wire. This is commonly used in car audio wiring, welding cables, and jumper cables. 4 gauge cable can safely carry around 135 – 150 amps (this actually varies according to the length of the cable but for distances less than 20 feet is a good estimate). I fused my cables with 100 amp fuses to be sure I’m well below the limits of the cables. Don’t overlook the fuses! Batteries have enough power to weld metal, don’t let that power get out of control!

“That’s a lot of words, show me some pictures!” Ok here is the circuit diagram of my system (created using CircuitLab):

In the diagram, everything from the starter battery to the left is the existing factory system. My system attaches to the starter battery and extends to the right and down. The great thing about this is it does not alter the factory system in any way. It can be  removed from the vehicle any time with no lasting effects.

Under the hood:

Here you see where my system starts. I used a set of 4 gauge jumper cables for my main wiring run because the wires are fused together making it easier to route through the vehicle than two separate cables. Note how I identified the polarity with different colored electrical tape. You can see the 100A fuse on the + cable shortly after leaving the battery. It is best to install fuses as close to the + battery terminals as possible. I’m using 100 amp AGU fuses which are commonly used in car audio installation. These are simple, low cost, and highly reliable. 12 volt circuit breakers are available but are more costly and possibly less reliable. All wiring connections are made using 4 gauge copper lugs.

Now let’s take a look at the main part of the installation.

Here you can see the auxillary battery box which is strapped down to the floor plate. Above that are the manual switch (red handle) and VSR. The three cables going into the manual switch are the main cable from the + post of the starter battery (Yellow), the cable from the + post of the auxiliary battery (red, not marked), and the distribution cable that goes to the fuse block (blue). The yellow marked cable is attached to one post of the switch, while the blue and unmarked red cable attach to the other post. The VSR mounted to the right of the manual switch has a cable going to eat post of the switch. For normal operation the manual switch remains open and the VSR automatically opens and closes the circuit between the manual switch posts. The small black wire coming out of the VSR is the ground wire. This is how the VSR determines the voltage on the main cable from the starter battery/alternator. Not show in this picture is the second cable from under the hood that attaches to the – post of the auxiliary battery. This ensures a good ground connection between all of the components in the system.

Finally we come to the fuse block (right side of picture):

The fuse block is where all components that run off of the auxiliary battery are attached. The ground cable attaches on the left side of the block, and the positive cable attaches on the right side. I am using a Blue Sea 5026 fuse block.


While there are many ways install a dual battery system, I am very happy with my setup. It has been performing perfectly now for about 4 months. The only other things I might add are voltage gauges for the starter and auxiliary battery. I hope this post is helpful to everyone!


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