Simple question regarding dual battery set-up, diagram: 2016 4Runner


New member
Hey guys, I am looking to add an inexpensive, simple aux battery (35ah, to my 4Runner to help power my 12v fridge. I am just looking to add a little more capacity (ah) than my current starter battery alone provides (70ah). I already purchased a 100w Renogy solar set-up to keep things topped off for daytime hours, but after some overnight testing with my fridge, I am good for only about 14 hours overnight before my battery saver voltage cut-off engages at 12.27V (about 60% battery capacity... a relatively safe margin, no?).

Can someone have a look at my planned set-up here and recommend a good value (isolator or other?) between my starter battery and aux battery? I do not want to run strictly off my aux battery (not enough capacity) but would instead simply like to extend my total capacity wiring these in parallel. I assume it is not advisable to simply wire these in parallel without some kind of isolator between the in case of a faulty aux battery?

dual battery set-up.jpg

Thanks so much!


Well-known member
Sure, that’ll work, but its a lot of cost, complication and expense for 17 ahs (.5x35ah aux).

You could just replace your starter with a Northstar grp 27 95ah agm for $350 and be done. Or pay $500 for a 50 amp hour Battleborn with 45 usable ah and solar charge it. Not sure Id trust a HF battery.

Ive taken several long trips in the heat using my Northstar grp 27 in my 4runner and run my ARB 50qt from 4pm to 9am when it was 105° at 4 and still had 12.3 volts to start in the morning.

If you feel you have to have a dual system, I can vouch for IBS. Ran it for years in my Tacoma without a hitch. But I wouldn't put in a dual system again. LiFePo with a DCDC charger is way better IMO. Thats what Im now running in my camper.
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Agreed with the above. You are far better off to replace the starter battery with a lager unit. Less complexity and lower cost for sure. My Group 27 dropped into the Jeeps tray with relatively little modification and cost about $120. With 2 solar panels, it rarely even draws down into significant solar usage and the fridge runs 24/7 in a hot desert environment.

If you start talking about a much larger house battery, or significant loads beyond just a fridge, the calculation probably changes.


New member
2018 TRD Off-Road here. Smart planning removes user-introduced auxiliary powered devices from the starter battery. With a Dometic, even with circuit protection, you're introducing a limitation to your starter circuit. Since Dometic is "always on" you're continually drawing your battery down and exercising it; thus fatiguing the battery. Also, you will need to set your isolator in a way that allows solar charging to your starter battery and vise-versa via the alternator, but disconnected from the Aux battery to protect it when it's necessary. You're going to want a deep-cycle to not over tax the OEM (OEM-style) battery and the purpose of it. I drew it out, too, and did all the math to figure out total load plus a percentage allocated for growth. I ended up with two Group 35 batteries - one remains as the starter battery (with all OEM circuits included) and added a second Aux or "house" battery to run everything else. Should you have one battery and are upgrading to a second, the smart choice is to move all the "new" circuits to a house battery and keep the starter battery tuned to the OEM load the Toyota engineers figured out.

I trust Dometic, but I also like redundancy. Especially when it comes to starting a vehicle.

*full disclosure, I ended up purchasing and installing the Genesis kit as it met my needs. I had origionally designed a system with the second battery on the passenger side with an isolator, but succumed to laziness and actually appreciate the compact footprint of the product. My additions are separate from the starter circuit, and I'm able to jump-start myself should I need it. I have already jumped others on just the house battery. My solar is integrated into that circuit too. I've gone several days on solar/battery no problem and 2 days on just battery alone (temps were 90day/60s at night). YMMV, so design to your needs. Right now my system handles my compressor, fridge, limited lights, and I have plans to go full-stupid with a winch and more lights as well. I'm also considering adding a circuit to integrate the 115v outlet use via the aux battery as well, selectable with a dual-pole switch.

Primarily, the house battery is dedicated to my fridge. Secondary is all the wigits and doo-dads that I'll add. But nothing more to the starter battery.
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Expedition Leader
These slides are a bit dated as they do not discuss lithium iron options (short answer, use a B2B), but they do discuss your earlier questions about paralleling.


-- A 35Ah lead acid battery is simply not worth the effort.

-- One option is to install two of the largest, deep cycle batteries that will fit in parallel. There is a chance that your cooler could kill them, but not if you size everything correctly.

-- If you will stay all lead acid, the best option is the biggest deep cycle battery that will fit as your second battery with either an intelligent relay or a B2B. B2B is better if you have a newer, Euro 5/6 engine, or the US equivalent. (Usually identified as a "smart" alternator, etc.

-- B2B gives you the option of using a lithium iron battery, and, with a smaller vehicle, the weight savings alone are worth it.
In my GX470 , I ran a G34 starter and G35 house both Odyssey . These were separated via a Bluesea ACR. I mounted a Victron 75/15 MPPT under the hood, and carried a folding 100w Renogy panel. Nice thing about the ACR set up was that once the house battery was above 12.7(or close to that) via solar, the starter battery was being charged. With two AGMs under the hood, I hooked up a higher amp charger about once per week when at home . Even with the fuse that keeps the voltage higher, it doesn’t really follow the charging regiment as stated by the manufacturer. When camping I ran a 50qt ARB which saw lots of opening and closing with my thirst for adult beverages, ham radio, lighting at night and charging various phones and Bluetooth speakers.


New member
Thanks for the input guys. Agreed that the HFT 35ah battery is probably not worth the effort after all. I was hoping to simply "extend" my 24F-AGM capacity on the cheap with a simple parallel connection (as opposed to using the HFT battery as a stand alone unit), but yeah, I see that is not advisable. We'll see how the 24F-AGM I have in there now holds up to the next couple weeks we're camping along the Oregon coast. This is our first year with a fridge and we'll put out the solar set-up to help keep things topped off if parked for extended periods. Thanks for the heads up on the 27F-AGM... I'll ear mark that for our next battery. Any of the turn-key dual battery set-ups are too rich for my blood ($$$) but one of these days I'll probably pull the trigger on a Jackery 500.


Rendezvous Conspiracy
First off I would make the batteries the same, As for an isolator Blue Seas ACR is the bomb, Hook it up and forget it.. you can isolate the batteries, only charge one and you can connect both for a jump start. I would also fuse power in from the panel to controller and out from the battery with a breaker so it can be rest quickly. You could run the fridge off the load side of the controller if it has one. You really do not need the switch unless you want remote switching.1653095330837.png


Rendezvous Conspiracy
I have several threads on solar panel, controller and batteries I have running in my tundra and trailer. I have 2 Deka group 31
108ah in the truck and 2 in the trailer all the same and never failed me anywhere or any time. Batteries are not cheap, and neither is a tow from the back country if they come out at all to help you.


If you’d like an on the cheap option for isolating batteries, buy a golf cart relay and put it between the positive terminals on your batteries. Wire the small post up to ignition on and then it will be isolated when the truck is turned off and connected when on. There’s a thread here in expo somewhere about this. Like others have mentioned you’ll want to wire the fridge to the aux battery. The downsides to this setup is that if your aux battery is a deep cell it really won’t charge well with an alternator alone unless you drive a long way. If you’re using a standard lead acid battery you will shorten the life of it using it for the fridge. Standard batteries are made for big dumps of amperage, like starting the motor, not slow trickles. I solved all this by running dual yellow top d34 55ah batteries and a smart solenoid. It works great as long as I travel daily. They're well suited for this task and charge at the same rate off of the alternator. I’d need solar to stay in one place for long periods of time.

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