Dual Battery Design for a Noob - Step by Step run through (hopefully)

[GerLC100]

Hey guys
didn't get too much attention in the power forum, so I decided to start a thread here for my very specific needs and targets.
I know there's a lot of more complex things to think about, but I would like to start sorting out the basics (which main components do I need) and once done digging into details (fuses, busbar, ...).
A lot of other threads here have already given me a direction, but sometimes I also have the feeling they're quite old and not using the "state of the art" components. If my setup has been described somewhere else, feel free to shoot me the link.


What I want:
- additional power for the winch and compressor
- jump start option
- run some aux lights on the roof rack
- fridge not planned as of now
- robust system in the engine bay
- no solar planned for now (driving my van for years w/o...)
- in general the whole system is only nice to have. I have never run into a situation where I really need aux lights, where I really needed to jump start. And for winching I would just need to slow down.
- price is somewhat a factor, but will not compromise due to price. Redarcs 1225 for $465 or 1240 for $540 and only reason for 1225 is price... I'd go for the 1240.

So I go through step by step what my thoughts are so that responses stay a bit organized.
1. Electronic part manufacturer
Somehow was tending to Victron, but couldn't find a whole lot of info that I, as an electronic noob, understand.
So I decided to stay with Redarc as I could find quite some info and everyone seems to be happy.

2. Battery choice
Yep, everybody talks lithium. But as my battery is a) in the engine bay and b) I don't have a whole lot of power draw I'll be sticking with AGM.
Idea is to leave the current standard battery in and add a Odyssey AGM in the Slee dual battery tray. Once the current starter battery is dead I could swap that as well and use Slee battery tray upgrade. But the current starter is just a year old.

3. My idea about the system setup
I buy an Odyssey AGM 35 and put it into the Slee Dual Battery bracket next to the compressor.
I get a Redarc battery isolator, which based on my understanding supplies the additional power for the winch/ compressor as well as being capable of jump starting --> SBI12 or SBI212?
I add CruiserPatches AGM Battery Voltage Boost Regulator to keep the AGM topped up.
I add a Redarc BCDC as charger and to combine my current standard starter battery with the AGM as well as for future option of solar --> 1225? 1240?
At this point I should already be able to have additional power for the winch and compressor.

Does that sounds reasonable?!?

Once this all makes sense in my head we can start talking fuses and so on. But for now I would like to concentrate on the main hardware.

 

[excessive]

I have a dual setup

• main battery is a big Interstate AGM , biggest that will fit in the stock location. I had to modify the factory hold-down a bit.
• house battery is a smaller ( i think27F) lead acid. I welded up a little tray for it to sit where the AHC was. i wired my fridge, cig lighter, and winch to the house battery
• i use a Blue Sea ML-ACR for the combination and remote switch.

I have had this setup for years , and it works great.

 

[87warrior]

I have a dual setup

• main battery is a big Interstate AGM , biggest that will fit in the stock location. I had to modify the factory hold-down a bit.
• house battery is a smaller ( i think27F) lead acid. I welded up a little tray for it to sit where the AHC was. i wired my fridge, cig lighter, and winch to the house battery
• i use a Blue Sea ML-ACR for the combination and remote switch.

I have had this setup for years , and it works great.

I have practically the same setup, except I use a basic 500A solenoid in place of the ACR. I did utilize an ACR switch so the solenoid can be switched via ignition-on and switched manually for easy jump starts.

 

[suprarx7nut]

I have a second battery setup with a DCDC charger that uses solar to trickle charge the aux battery and THEN the main battery. I don't have garage space for the 100 right now so it's been awesome. I can leave it undriven for weeks on end and the battery is topped off and ready to roll all the time.

You don't need solar, but I've found it hugely convenient.

Anyways, I'd at least consider lithium for the second battery and put it in the cabin. AGM is old news and their capacity per weight is rather pathetic as of 2024. The engine bay is hot, battery life is shortened and if you want power in the cabin you still need to go through all the work of routing cables.

Be aware that using the second batt to jump start is only so valuable. If the main battery is really low, connecting another 12V source that may also be only at 50% or less may not be enough to start unless you can disconnect the main battery - the main can act like a huge sink that makes your aux battery struggle to contribute enough energy to crank. Instead, I would just carry a beefy NOCO jump pack or similar. $100 gets you a very good and potent jump starter that can charge off a low main battery and provide a burst of high voltage jumping power.

Redarc is odd in that their chargers don't allow any trickle charging of the main battery off solar unless you rig up a totally separate voltage tripped circuit. I thought I was missing something so I submitted a tech request. Sure enough, they just don't support it. Renogy and perhaps Victron do allow that. Again, I know you said no solar, but at least consider it.

 

[GerLC100]

Ok guys. Some time has gone and had some other stuff to do. Now I want to start pulling all materials in.

Step 1: Let's talk battery first.
As mentioned I'm not planning Lithium. Although others do it I just don't prefer a lithium inside the cabin in a daily driver with the kiddos sitting in it. Also need the space for loading stuff.
Means I'm going AGM.

Right now I'm thinking Odyssey. I read quite a bit about them here as well as in my Van forum and think (although there are 100 alternatives) I can't go wrong.
I would swap the current main battery as well to stick to the same system. So my idea is a Group 31 AGM in the Slee Main Bracket and a Group 35 AGM in the Slee Dual Battery bracket.

As AGM might not get properly charged I'm ok swapping in the diode from Cruiser Patch or install a Redarc. But I think there are solutions for it and we get to that in a later step.

How does that sound?

 

[Mike NXP]

Sounds like you're on the right track but just to give you an alternative, it may be worth considering if you really need a second battery.

Winches only run temporarily and AUX lighting is getting a lot more efficient with LED technology. You should make sense of your total amp/wattage load and see if you need the dual battery.

I've got a single battery doing all of the work you are asking for..... Off this battery I have a 12k winch. I have several aux lights as well. I don't have a permanent air compressor but I do hook up my ARB single compressor to this as well. If I got a fridge I would cross into the dual battery world but that's not in my future.

My last battery was the same X2 brand. It lasted 6 years and I swapped it just because. Never died once and I would run AUX lighting for a while with the truck off for camp light..... I still use that battery in my garage for random tasks.


Upgrade your terminals for a clean integration ( ignore the sloppy winch power shut off switch ) and add an fuse panel for your AUX lighting. Throw a small NOCO jumper pack in the vehicle just incase and bobs your uncle.

 

[GerLC100]

Thanks and noted. I go dual Have jump started too many people that had lights on and similar too often. Have kids wanting to charge their RC batteries via USB...
and to be honest: beside money there is no downside. So: Decision is made

 

[suprarx7nut]

Ok guys. Some time has gone and had some other stuff to do. Now I want to start pulling all materials in.

Step 1: Let's talk battery first.
As mentioned I'm not planning Lithium. Although others do it I just don't prefer a lithium inside the cabin in a daily driver with the kiddos sitting in it. Also need the space for loading stuff.
Means I'm going AGM.

Right now I'm thinking Odyssey. I read quite a bit about them here as well as in my Van forum and think (although there are 100 alternatives) I can't go wrong.
I would swap the current main battery as well to stick to the same system. So my idea is a Group 31 AGM in the Slee Main Bracket and a Group 35 AGM in the Slee Dual Battery bracket.

As AGM might not get properly charged I'm ok swapping in the diode from Cruiser Patch or install a Redarc. But I think there are solutions for it and we get to that in a later step.

How does that sound?

Is there some perceived danger of lithium?

How are you planning to charge?

IMO, the battery itself is almost secondary. The isolation, redundancy and added function is the benefit of dual battery. That's the part I would focus on. I'd rather have a tiny battery on a proper circuit than a massive battery (especially the more damage prone AGMs) on a simple parallel node.

 

[GerLC100]

As mentioned in the title: I'm a battery and electronic noob

My understanding is that there is a fire risk with dangerous smoke coming from a lithium battery. To have all possible "threads" outside of the cabin is/ was my main intent. But perhaps that's also a "historic" statement...
As lithium shouldn't be mounted in the engine compartment, I chose AGM...

Charging:
I'd do what's needed. If charging an AGM means a Redarc 1240 or whatever I'd do it. Thought starting with the battery is a good idea, but please feel free to guide me through differently.
As mentioned I'm willing to spend 1.5k for the whole system, which is surely higher level.

 

[hoser]

The two common Lithium Batteries are Lithium Ion and Lithium Iron Phosphate (LiFePO4/LFP). Lithium Ion is very popular for E-bikes and E-Scooters and can be quite dangerous and can catch fire just by puncturing them. I would not charge them inside my vehicle.

Much safer (chemically) and more expensive are the Lithium Iron Phosphate (LFP/LiFePO4) and even considered safer than AGM batteries. I have no issues charging these inside my vehicle.

Yes, LFP batteries don't like being in extreme hot nor cold temperatures. Some have a little heating unit that activates when cold (i.e 35 degrees). Some manufactures will approve them for under the hood installation but at a reduced lifespan. But even the reduced lifespan will likely be longer than an AGM. It would be hard for me to make that choice. It's like throwing away money.

 

[suprarx7nut]

As mentioned in the title: I'm a battery and electronic noob

My understanding is that there is a fire risk with dangerous smoke coming from a lithium battery. To have all possible "threads" outside of the cabin is/ was my main intent. But perhaps that's also a "historic" statement...
As lithium shouldn't be mounted in the engine compartment, I chose AGM...

Charging:
I'd do what's needed. If charging an AGM means a Redarc 1240 or whatever I'd do it. Thought starting with the battery is a good idea, but please feel free to guide me through differently.
As mentioned I'm willing to spend 1.5k for the whole system, which is surely higher level.

My short advice:

Get a LiPo battery, store it in the cabin.

Two ways to achieve that:
1. If you want solar, wire up a proper dual battery DC/DC charger with separate components: DC/DC charger. Battery (LiPo, not AGM). Solar. Renogy makes some effective and not-crazy-expensive options. RedArc makes simpler and more expensive options. Both are high quality.

2. If you don't care about solar or super fast charging from the alternator, go for a power bank. I think the EcoFlow line is great. They even have an "alternator charger" which gives you 800W charging just like a proper DC/DC charger. For well under $1000 you can have a massive battery AND the alternator charger AND a portable solar array. The best part? You can have it up and running in a few minutes and tackle the alternator wiring if you find that necessary. Otherwise, you just have a power dense and very safe device that gives you a ton of electrical power with very little effort that transfers into your next car, into your home during an outage, etc...

Why I'm saying all this:

AGM just doesn't make sense in 2024. It's heavy. It's damaged when discharged below 50%. It's relatively short-lived before it's just worn out and dead. It's not inherently safe. It still needs special charging consideration because the car isn't made to handle it. And finally... there's a better choice readily available.

AGM can emit hydrogen gas occasionally. That's why you can't (shouldn't) put them in the cabin and if you do, they need a formal battery box with a vent to the outside. That's also why it's advised that when you jump start a car, you connect ground last and you do it away from the battery - because the batteries can vent highly flammable gas and create an explosion/fire. LiPo doesn't do any of this.

Lithium batteries (LiPo at least) are generally regarded as safer at this point. Battery tech has turned annoyingly political thanks to EVs so there's plenty of misleading or outright false media coverage on battery fires. There's effectively a zero risk of lithium fire from any of the brand names today because they're all using LiPo.

RedArc makes only one battery. Guess what it is? LiPo.

Expedition Portal had a list of the most revolutionary gear from the last decade. Guess what made the list? LiPo batteries.

My 100 is currently set up with an outdated AGM setup. I'm likely upgrading it all to something like this in the near future:

EcoFlow 800W Alternator Charger | EcoFlow DELTA 2 + 800W Alternator Charger + Free 110W Solar Panel x 1 | EcoFlow

Please kindly note that the EcoFlow Alternator Charger and the portable power station will be shipped separately. Charge while you drive, and stay powered up on the road. 800W turbo alternator, 8× faster than traditional in-vehicle auxiliary power outlets. Top up 1kWh in only 1.3 hours. A 3-in-1...

us.ecoflow.com

 

[GerLC100]

hhmm.... now that brings it to another level... Thanks for the input!

Upgrading the main to a group 31 would provide much more capacity for safety, winching, air compressor, ...
With this kind of "mobile" solution I could also use it with the van... just mount exterior lights for evenings and run via the mobile battery...
With the alternator charging I could do charge while driving...

hhmm, need to give it a serious thought

 

[hoser]

@GerLC100 Why do you need extra battery capacity for an air compressor, even one with 30A draw? It's best to always fill tires while the engine is running. The tires will fill faster too. ~14 volts versus ~12 volts. On your next alternator change, go with the 2003+ alternator (130A vs 80/100A). Drops right in with just a change of the connector.

 

[Mike NXP]

Thanks and noted. I go dual Have jump started too many people that had lights on and similar too often. Have kids wanting to charge their RC batteries via USB...
and to be honest: beside money there is no downside. So: Decision is made

Fair enough , just figured I'd throw in a vote for single battery achieving all of the goals. Not much help in regards to a dual battery setup so carry on

 

[GerLC100]

Yep, engine running for sure. My very initial idea of the dual battery was to run everything but the "standard" off the aux. Means winch, compressor, add. lights, ...
Reading along I learned that bigger power drawing equipment like winch and compressor should be wired up to the main due to better charging.
My van has a huge 100ah+ capacity, so I always use the compressor for 4x 35's from 20 to 55psi w/o thoughts (and up to a few weeks back I never thought about draining my main when winching/ airing up...).

So yes, engine will always be running when winching/ airing up either way. And the Group 31 in the Slee bracket should provide enough capacity for those things.

have always focussed on a permanently mounted system and then wanted to copy/ paste it to my van. The mobile solution would be once the cost, but I could carry it in both vehicles as I never drive both at the same time.
Have always "denied thinking about it", but thanks to suprarx7nut's clear post it made me thinking. Especially the option of charging via the alternator while driving.

​

 

[GerLC100]

Side question in case going that route:
I assume running aux lights, 12V fridge and all that stuff off something like an EcoFlow is no big deal right?
So I could more or less wiring everything up with a connector/ panel inside the cabin, plug the ecoflow in when needed and done. Just couldn't run the lights w/o the ecoflow then...
In that case Lithium wouldn't be as bad to me as it very limited time in the vehicle and I don't need to watch it 24/7 365d/a...

Side question 2:
Would there be any easy system to cut of a winch in case it's draining the battery too strong? like a safety switch that makes sure I don't empty the battery...?!?

 

[Mike NXP]

Side question in case going that route:
I assume running aux lights, 12V fridge and all that stuff off something like an EcoFlow is no big deal right?
So I could more or less wiring everything up with a connector/ panel inside the cabin, plug the ecoflow in when needed and done. Just couldn't run the lights w/o the ecoflow then...
In that case Lithium wouldn't be as bad to me as it very limited time in the vehicle and I don't need to watch it 24/7 365d/a...

Side question 2:
Would there be any easy system to cut of a winch in case it's draining the battery too strong? like a safety switch that makes sure I don't empty the battery...?!?

#1 - I used to run some 12v magnetic lights off my power pack for camp light and it worked fine. Not proper driving AUX lights but worked good enough for camp light. The next day I would use the vehicle to re-charge the power pack while driving. I still sort of do this but most of my AUX light is hardwired to the battery now

#2 - Scroll up and you will see a typical power shut off switch for the winch. Although finding a better way to mount it would be preferred. Power is turned off full time and switched on when needed for the winch.

 

[GerLC100]

#2 - Scroll up and you will see a typical power shut off switch for the winch. Although finding a better way to mount it would be preferred. Power is turned off full time and switched on when needed for the winch.

I meant the other way around. As far as I know heavy winching could lead to battery drain... so my question was: can I have a safety switch that turns off the winch during operation when the battery gets to a critical points.
Or never anyone been to this point?!

 

[suprarx7nut]

Side question in case going that route:
I assume running aux lights, 12V fridge and all that stuff off something like an EcoFlow is no big deal right?
So I could more or less wiring everything up with a connector/ panel inside the cabin, plug the ecoflow in when needed and done. Just couldn't run the lights w/o the ecoflow then...
In that case Lithium wouldn't be as bad to me as it very limited time in the vehicle and I don't need to watch it 24/7 365d/a...

Side question 2:
Would there be any easy system to cut of a winch in case it's draining the battery too strong? like a safety switch that makes sure I don't empty the battery...?!?

Fridge is totally fine. Those are all meant to run off the small and weak car wiring so that's all good.

Aux lights can be a bit more demanding if you're lighting up camp like a movie set, haha. So depends what we're talking about. Take a look at the specs and see what you think. Those power banks generally offer ~100W on a circuit or two for DC, some smaller circuits for charging small devices, etc... Then they have HUGE AC output. Enough to run a blender, lol.

I meant the other way around. As far as I know heavy winching could lead to battery drain... so my question was: can I have a safety switch that turns off the winch during operation when the battery gets to a critical points.
Or never anyone been to this point?!

You could certainly run a sensing cutoff, but I wouldn't. I just have a Blue Sea switch that diverts power from the battery to the winch or the 2nd battery setup (or neither). Pic below. If the car's running you'll be charging it the whole time with the alternator. AND.... with that Alternator Charger from EcoFlow you've got a substantial backup charger to top off the main battery if needed. That's one of my annoyances with the conventional DC-DC chargers actually. Some allow trickle charging, some allow nothing at all. But none I know of offer significant fast charging of the main battery from the secondary. Something that's very handy for people like me that aren't daily-driving the car. The Eco-Flow charger lacks some features of the old DC-DC setups, but it's got a lot of new stuff that's intriguing.

 

[4Beast]

My 2cents is that you don't really need to make it overly complicated, especially for your use case. I run the super simple dual battery system, that is a solenoid connected to the ignition, so that if it is in acc or on, the two batteries are connected, if it is off, they are separate and everything runs off the "house" battery and the "start" battery is isolated. I use AGM batteries with 0 gauge connecting them and 0 gauge to the winch. I believe it is 8 gauge to the rear, including for the fridge. It is a relatively long run, so there would be power loss with lower gauges. I also use a voltage booster in the ALT fuse.

SupraRX7 is right that the lifepo4 batteries are state of the art, but it's ok to not go that way. You probably really don't need a dual battery at all, honestly, if you're not running a fridge or some kind of ongoing draw. The winch is super power hungry, but you're not using that for a really long time (hopefully) and the truck is running during use (although alternators are actually way less efficient than people realize). I always have a NOCO GB70 around for jump starting, so jumping from the second battery isn't really needed.

If I was doing it again, I would only change to add a true battery monitor/isolator (national luna etc).