Rear Dual Battery Set-Up on 4Runner

 In 5th Gen Mods, Accessories, DIY, Install

Rear Dual Battery Set-Up on 5th Gen 4Runner

Rear Dual Battery Set-Up on 5th Gen 4Runner

I have a 2015 Toyota 4Runner Trail and I have done a few modifications that include adding a 2nd battery to power my compressor, lights, a water pump and a fridge in the near future.

It all started when I finished installing a draw system in the back of the 4Runner and I found that when I took out the original slide and other components behind the 2nd-row seats, I was able to fit a Group 34 battery between the 2nd-row seats and my new draw system.

Part list and Materials for This Dual Battery Set-Up

  • Blue Sea ML ACR 7622 – Price
  • Ctek Dual 250s – Price
  • Odyssey Extreme AGM Group 34 Battery
  • Blue Sea Systems Mini OLED DC voltmeter – Price
  • Blue Sea Systems m-Series On/Off Switch – Price
  • Blue Sea Systems e-Series On/Off Switch- Price
  • Blue Sea Systems lower switch panel – Check Price
    • With Fused Switch, USB ports, 12v Socket rand mini OLED voltmeter
  • 2x Blue Sea Systems ANL Fuse – Check Price
  • 2x Blue Sea Systems ANL Fuse Block – Check Price
  • Blue Sea Systems 2723 Dual Bus Plus – Price
  • Cables and terminations
    • 2 AWG, 10 AWG, and 14 AWG
  • 10 Ton Crimper from Harbor Freight
  • Shrink wrap tubing and a heat gun
  • Vertamax Pure Sine Wave 1500-Watt Inverter – Check Price

I have to start off by saying that I would do a lot of things differently if I had to do it all over again.  As I go over the installation, I will explain what I would change and the reasons for the changes.

When I started to plan out the dual battery system, I researched all that I could across the internet and found a ton of great sites using the following Google search:

Dual Battery setup

I found forums, web pages, youtube videos and product sites.  The good news is that there is a ton of information out there.  There were some conflicting points but with enough consensus, I was able to determine a few things I wanted.

From the start, I wanted a high capacity, deep cycle battery, not a clone of the starter battery in the 4Runner. The Odyssey Extreme AGM group 34 was pretty much a favorite and available at my local AutoZone.

As discussed in other articles, the battery offered a good compromise between weight and performance.  Next, I knew I wanted a way to separate the batteries so that the starter battery would not drain under the load of the additional stuff I was installing.  I wanted to keep the starter battery ready to do its primary job – start the 4Runner!

I also thought it would be great to combine the batteries and have the ability to offer a jump from the 2nd battery in case the main battery did get drained.  Several forum posts also suggested combining the batteries during heavy winching – I’ve not tried that, but it sounds good.

Blue Sea Magnetic Latch – Automatic Charging Relay (ML-ACR)

At this point, I decided to purchase a Blue Sea Magnetic Latch – Automatic Charging Relay (ML-ACR for short).  The model I chose came with a remote switch as well as a manual switch on the device itself.  As the name implies, Blue Sea Systems makes products for marine applications and they have a great reputation.  The switch looks solid and can survive under the hood if you had no other place to install it. Along with the switch I also purchased 300-amp fuses and a bus bar to help with the installation.

AGM Batteries and Charging

When I did my research on AGM batteries, several sources talked about how to keep AGM batteries charged as well as issues folks were having using just the standard alternator to charge two batteries.  I reached out to Odyssey technical support and they pointed out that when using an Odyssey battery by itself as an upgrade to the Panasonic starter battery, the standard alternator should be sufficient to keep the Odyssey charged.  Although, they also said having a charger to “top-off’ the battery was a good idea.

From what I found, AGM batteries work best and last longer when they charged using chargers that know about AGM batteries.  These specialized charges take the battery through several stages and provide a higher charging voltage (14.4-14.7 depending on outside temperature) than what is provided by our standard alternators.  There are several products out there and Ctek popped up as a reputable company offering different charges.

I looked at two different types of charges:

  • AC-to-DC Charges – standard charges (that use 110 volts AC in the US, 220v in other parts of the world) from a wall outlet and convert the voltage to DC.
  • DC-to-DC Charges – in-vehicle charges that take DC voltage provided by the alternator and provide a suitable charging voltage to the 2nd battery.

Ctek Dual 250s DC-to-DC charger

I decided to purchase a Ctek Dual 250s DC-to-DC charger for a couple of reasons.  It offered the specialized AGM charging capability and had a built in Solar regulator in case I decide to add solar panels in the future.

It also has the capability to isolate the batteries and here is one place I would do things differently in the future. I already purchased the ML-ACR – not cheap –  and although the ML-ACR is a great product, I could have done without it since the Ctek Dual 250s also isolates the batteries.  The only downside to the Ctek Dual 250s is that it only pushes about 20 amps of current.

You cannot use the Ctek Dual 250s to combine the batteries on its own. Ctek sells a companion product called the Smartpass that boosts the current capacity to 100 amps.  I did wind up keeping the ML-ACR and Ctek and will explain the reasoning and set up below.

Ctek Dual 250S and Smartpass

Note: If you are thinking about the Ctek Dual 250s and Smartpass you should consider Ctek’s  latest products which are called the Dual 250SA and Smartpass 120.  These newer products are just now coming to the US and offer “smart alternator” support as well as the capability to combine the batteries for 10 secs for a possible jump assist at 350 amps.

Dual Battery Cabling and Connectors

The next major purchase was cabling. I decided to use 2 AWG cabling throughout and bought it in bulk using my own connectors.  I had to keep buying more cable since I underestimated the required lengths a few times.  I also decided to crimp all the connectors.  If I do this again I would plan out the lengths more precisely and use pre-cut and pre-crimped cable.  They may cost more, but I believe it would save a lot of time and possibly offer better connectivity with better-crimped connectors.

5th Gen Toyota 4Runner Dual Battery setup Install

From the starter battery to the rear battery

The installation began at the starter battery with a switch and 300-amp fuse:

The switch isolates the starter battery from the back as well as the Light Bar I installed on my front bumper (the extra red cable shown in the photo above). I ran 2 AWG cable for both Positive and Negative connections through the firewall – a real painful process given’ the size of the cable. Here too I was able to find several sources online to help with how to pass the cables. I continued to run the cabling through the rocker panels and was able to pass them behind the 2nd-row seats.

Center console cables to the rear battery

At this time, I also ran 10 AWG cable for positive and negative connections from the center console back to the 2nd Battery.  I added 14 AWG cable for the remote switch connections to control the ML-ACR remotely, I use the remote switch every time I get in the 4Runner.

Final setup of the front console right under the main screen

Bottom Row: Blue Sea Systems 4356 Below Deck Outlet with a Fused Switch, USB ports, 12v Socket rand mini OLED voltmeter. A great product that came pre-wired – all I had to do was provide the positive and negative connections from the 2nd battery.

Upper Row: I installed the remote switch for the Blue Sea ML-ACR and a 2nd voltmeter to monitor the start battery.  For that 2nd Voltmeter, I tapped the positive connection from the existing 12-volt cigarette lighter port to the right.  Luckily, the existing connection to the 12-volt port only energizes after I start the engine.

There is a great video on how to remove the center console on YouTube. You can fast forward to the 2-minute mark to get to the details.

I made a box for the AGM Battery as well as wall space for the Ctek, ML-ACR and buss bar.  When I installed the drawer system, I was able to tap into access points on the floor of the 4Runner.  There was plenty of room for the battery between the seats and drawers.

Installed battery with connections taken from the front of the cab

On top of the Battery Box I installed the Blue Sea Bus Bar with all the connections to the added stuff.

As I mentioned earlier, I decided to keep both the ML-ACR and Ctek Dual 250s.  I wired them in serial (I’ll provide a full wiring diagram at the end) with the ML-ACR acting as the front-line switch with that remote rocker up front in the console.  With this set up I can directly control when the batteries are connected and disconnected.  I know this is a bit overkill because I have found that the Ctek does a great job to isolate the batteries.  However, with one cabling change in the back, I am able to bypass the Ctek and connect the batteries directly if I had to.  Also, if the Ctek ever fails for whatever reason, the ML-ACR will also automatically disconnect the batteries before either one gets too drained.

To isolate and connect all the extra stuff, I installed another mechanical switch, shown below with additional USB and 12-volt ports – sorry, the exposure is not great, but the additional ports are just above the big red switch.

The switch is on the passenger side just behind the rear passenger door.  The Photo below is taken form the front of the 4Runner.

One final component I installed at this point is a 1500-watt pure sine wave inverter with its outlets pointing to the back of the 4Runner as shown below:

I don’t have a lot of data on the VertaMax inverter, so I can’t say how well it will perform.  It works as advertised but I have not put any load on it yet.  It had good reviews on Amazon so that’s why I picked it at the time.

As promised below is the full diagram – really a mix of hand drawings with my notes.  Please check out the Ctek and Blue Sea websites for full descriptions of their products and more precise wiring diagrams.

And this leads to the last thing I would change if I were to do it again: a better plan from the start.  I have to admit, most of this was trial and error. I wanted to save money, so I crimped the cable connectors myself.  I made the boxes and used what spare parts I had.

I wound up overspending on some parts, overkill on other parts and I must have wired the back three different ways before I came up with the final version pictured above.  Next time, I’ll plan the setup, get proper cable lengths and all cable connections specified – up front.  I know all wishful thinking and I wouldn’t have had half the fun I did doing it the hard way.

Glad to answer questions if I can…

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Sensei Dad (Gus)Colin HughesGusRogelio YanezAmjed Recent comment authors
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Aaron Anderson
Aaron Anderson

Will you still use an SPOD with the dual battery?


Heya! i just wanted to ask you why did you use a 300A fuse? , isn’t it pretty high for such a system?

Also could you please share more pictures of where you have installed the system if there is an updated pics of it?

i have ordered the parts and waiting for them to arrive soon


Heya Gus! thanks for the info!

I don’t believe the 2 AWG cables will be enough for the 300 Amps Loads on the long run, they should be above 250MM cables. Even if the fuses are 300Amps you will require cables that can handle above 300 Amps.

try to change the cables which are based for load purposes.

Eng. Amjed


Sorry .. typo error above 25MM

Rogelio Yanez
Rogelio Yanez

Hi, so with this setup is your house battery being charged by the alternator and the Ctek DC DC charger at the same time?


Heya Gus,

I am about to start with the project, but i was wondering about the CTEK unit, do you think there is an alternative for it, like for instance a Normal MTTP Charger ??

also for the lower panel unit, how did you install it (mount it)?


Hi Amjed! There are a few good quality alternatives to the CTEK I Installed. In my set up I went a little overboard with both the Blue Sea ML and the CTEK. I think the CTEK alone would have sufficed. Don’t forget the CTEK has an MTTP, a DC to DC charger and also seperates the batteries when the voltage is right. The only point the Blue Sea offered was a way for me to use the house battery to jump the main (plus the isolation and remote switch that I use all the time). The newer version of the… Read more »


Dear Gus,

thanks for the clarification, however, i do believe that the Blue Sea ML does the job in keeping the batteries isolated, for instance, according to your circuit, if I would require the battery to be enabled and felt that there is a drop in the circuit, keeping the switch on Auto mode will get the batteries charged.

frankly speaking, i wouldn’t need a solar charger XD, since now there are many alternatives for solar systems that are portable and can be used separately.

Do you think if i removed the CTEK charger will do the job?


Hey Amjed…

The CTEK offers DC to DC charging and at a voltage optimized for my AGM battery. From what I’ve gathered AGM batteries benefit from a higher charging voltage and the CTEK manages the charging process much like a wall charger. As long as you occasionally attach some sort of smart charger to the house battery you should be fine.




Does the 5th Gen have a smart alternator? Did you have to run a line from the C-tek to the ignition switch? Thanks.


Hey Sensi, its been a while since ive sent you a message. Well i have done the circuit and i didnt place the DC To DC charger, and it was right that if i left it like that the battery will die gradually. to a point my battery went down to 0 volts, so i have purchased an external charger from BOSCH so i can use it also on other cars till my remote system stopped to operate XD got BLUE Sea to send me a new ACR system with its switch but i got this message from them when… Read more »

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