Dual battery systems; making sense of the options

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Jun 19, 2018
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Seattle, Wa USA
Hey.
This is more of a "for future reference" question for me. There are an awful lot of options for dual battery systems out there. From Mud vendors, the 2 that come to mind first are those by @sleeoffroad, and @LC4LIFE. It seems to me that it depends a lot on how much monitoring you want/need, and if you want dual or split charging. After that, I guess one needs to decide on budget; $90 very bare bones kit from amazon to sky's the limit.
My case; Immediate future, very basic/common stuff iPhone et al charging, winch, maybe a small led light bar or pair of small floods. My current battery is a 2017 dated Interstate that came with the truck and seems to be doing fine with my very limited driving of the cruiser to date. A 12v fridge/freezer sounds really nice, but unless a "deal I can't refuse" pops up that likely will not happen within the next calendar year.
For the near term, I guess I should quit being a goof and at least have a battery tender for my infrequent driving habits.
Is there an actual question in here you ask?
If I go with a simple dual battery system, will I have to re-invent the wheel down the road, or can things (more complicated charging or solar) be added in without a lot of fuss?
In most of my reading, it seems like the addition of a fridge/freezer is the main tipping point for adding a second battery or some type of solar maintainer to not get stranded.
If any of you know of a build thread that has a "simple" system, please let me know. I've seen a lot of really cool but really elaborate stuff, and not much basic stuff(with pictures).
Thanks.
 

LC4LIFE

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Based on what you mention your needs are (at least today), a good group 27 or a 31 shoehorned in should get you all the capacity you need, at least for now. You can always go to dual batteries if the need arises or you simply decide you want it.

I have had everything from a complex, in cab monitored, in cab combining capable system to what I have now. I absolutely love what I have now and it is VERY simple. No more monitoring, in-cab combining, etc, and none of the additional wiring that goes with it. I have really landed on a KISS approach to my dual battery setup.

Personally, my requirements are -

Minimal amount of connections, meaning less failure points.
Redundancy for starting
I want the ability to run some accessories with the truck off and have absolutely no risk of them draining the starting battery
I want to know if both batteries are charging
I want to know the voltage of the second battery
Easy expansion to the rear quarter panel for my on board air, subwoofer amplifier, accessory fuse block etc.

BTW - if you don't drive often, yes, get a tender on that battery.

My 2 cents.

David
 
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87warrior

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Junction City, Kansas
I installed a pretty basic dual battery system. I really wanted to isolate power used by accessories, like a fridge, when parked to a dedicated house battery. There are many possible routes to achieve a functional dual battery setup; this is how I did it.

Objective:
Install a simple and cost effective dual battery system

Goals:
System should operate automatically, manually or be switched off
Ability to self jump start
Keep it safe

I like to draw basic diagrams when doing electrical work, for home or vehicle. This helps keep me organized and greatly reduces the chance of me wiring in a short circuit or having and un-fused power source. Below is a basic diagram for this dual battery system.

Dual Bat Wire.png


This works as follows:

Voltmeters - I can view the battery voltage anytime. With the solenoid deactivated the voltmeters will show the resting voltage of each battery. With the solenoid active the voltmeters will display the voltage of the batteries combined. Obviously if the rig is running and the solenoid is active, the voltage displayed should be the charging voltage and the same.

Charge/Solenoid control
- AUTO, the solenoid is activated when it receives power from an ignition power source. This setting automatically ties the batteries together when the vehicle is running and isolates them when it is off.
- ON, the solenoid is activated when it receives power from the house battery. This allows a self jump start in case the starting battery has died.
- OFF, the solenoid will not activate and the batteries are isolated from each other.

The control center for the batteries is fitted to my overhead console. The primary switches replaced the useless sunroof controls in my 100 since the sunroof hasn't operated in years. The small digital voltmeters were cut into the console.

IMG_20190623_132519.jpg


The 2nd battery tray I am using is an old style Slee Offroad tray, which is designed for an Optima battery. Since I was trying to keep costs down, I found a Group47 AGM battery that fit the tray. As a bonus the terminals are recessed into the battery case.

IMG_20190420_162455.jpg


At both the starting battery and house battery, I installed Bussman fuse holders and 300A MEGA/AMG fuses to protect the vehicle and batteries from a massive short circuit in the event a cable breaks. I used 2AWG welding wire, tinned copper lugs, braided loom and adhesive backed heat shrink to build the battery cables.

00100dPORTRAIT_00100_BURST20190804124905909_COVER.jpg


I tucked the solenoid against the firewall directly in front of the driver's seat. The solenoid engages with a solid audible 'click'. I made sure to cover all positive terminals in the whole system.

00100dPORTRAIT_00100_BURST20190804124844090_COVER.jpg


I could easily add a solar charge controller + panel to the house battery if desired. With my current use of driving daily and only relying on the battery overnight, I have no problem maintaining a charge.
 
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Joined
Jun 19, 2018
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Seattle, Wa USA
@87warrior ; Thank you. That is all good stuff. I like the overhead console work as well. I'm not sure I would do overhead USB, but that's just me. I like the cables down low where they can snag everything else I have laying around.
 
Joined
Aug 28, 2014
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Santa Cruz
Honestly, aside from the technology inside the actual unit, a Redarc BCDC1225d is about as simple as you can get for a dual battery setup.

Mount the unit, mount any type of auxiliary battery (SLA, AGM, TPPL, LiFePo4), run (3) 8awg wires with supplied inline fuses, run common ground paths (2awg to engine block, 8awg to body) for the aux battery. Done

Carry 10’ of 2awg with battery clamps if self jumping is needed.

You can plug solar directly into the BCDC. It has an MPPT controller built in and prioritizes solar when 2 sources of charge are present.

Beauty of the BCDC unit is you can keep your start battery in place, your auxiliary battery will recharge quicker and the BCDC pushes in the proper voltage and amps to get your battery up to full 100 capacity. Parallel charging thru the alternator just can’t do that.

In cab monitoring is an option but not needed. There are no switches to hit. Redarc is well proven in 4wd setups around the world and have stellar customer service and support.
 
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Herndon VA
Honestly, aside from the technology inside the actual unit, a Redarc BCDC1225d is about as simple as you can get for a dual battery setup.

Mount the unit, mount any type of auxiliary battery (SLA, AGM, TPPL, LiFePo4), run (3) 8awg wires with supplied inline fuses, run common ground paths (2awg to engine block, 8awg to body) for the aux battery. Done

Carry 10’ of 2awg with battery clamps if self jumping is needed.

You can plug solar directly into the BCDC. It has an MPPT controller built in and prioritizes solar when 2 sources of charge are present.

Beauty of the BCDC unit is you can keep your start battery in place, your auxiliary battery will recharge quicker and the BCDC pushes in the proper voltage and amps to get your battery up to full 100 capacity. Parallel charging thru the alternator just can’t do that.

In cab monitoring is an option but not needed. There are no switches to hit. Redarc is well proven in 4wd setups around the world and have stellar customer service and support.
I really like this idea.. working on a dual battery setup now... just a quick question... when the car is running, it will charge the aux battery from the starting battery? Just not sure what is meant by "Parallel charging through the alternator"... probably a newb question but any help would be much appreciated.
 
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Here is the clearest explanation of the difference between parallel charging (aka, voltage sensing relay, SBI, ACR units) and DC to DC charging (aka BCDC unit) in a dual battery setup in a 4wd. The 80 series alternator is not a temp sensing or smart alternator but its voltage output is typically 13.9 or less, aside from startup.
 

LC4LIFE

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I like the Redarc, but have yet to move that direction. I still prefer something like the BlueSea ACR or ML-ACR. I think the Redarc is a good solution for newer vehicles with smart alternators, and for a couple of other specific reasons, but I wouldn't quite say its simple. Maybe once I add a 200 to my fleet I will feel different. Even though Redarc is proven in the outback, there is still quite a bit of tech inside it, you cannot mount in the engine compartment and there is no combining for self jumping. I also haven't quite gotten into the camp of needing a "smart charger" for the second battery either. I completely understand the reasoning to eek every bit of capacity from a battery and you need a full charge for that, but I also believe that need is only for real heavy users, and there aren't many of those. Another viable, simple system, is something based on the BlueSea and a solar panel. You can have, smart charging via the solar, in cab self jumping, and you can mount the ML-ACR under the hood. A different approach from the Redarc, but I would argue, just as effective and less expensive as well. I have also seen quite a few troubleshooting threads around the inter-web related to the Redarc installation, start isolation, etc. Confusing installation, another no go for me.
 

Greg_B

 
 
 
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I have the Blue Sea ML-ACR in my 80 between two batteries (27 series DP) that are same make/model/purchase date. I have one battery isolated and all it does it provide starting and glow function. The other battery runs the vehicles systems as well as house duties (fridge, USB chargers, etc) 100 watts of solar is on the roof, through a Victron Solar Charge Controller. This is hooked up to the 10 gauge tinned wire run (from the vehicle/house battery) where its distributes at the Blue Sea fuse panel in the back quarter. As you know, the ML-ACR automatically latches at a set voltage (with enough solar or the vehicle running). Even with all this, due to the vehicle/house battery being isolated and run down at night, the batteries develop a different state of charge over time on a trip. I know this as when I return and tuck the 80 away I put it on a Noco Gen 2 Mini charger. Each battery is charged independently. I've visually monitored the red/green monitor light (indication of 80% charged) and the start battery always hits green shortly after the charger is plugged in. Depending on length of trip and cycles of the vehicle/house battery the indicator light comes on later for the stressed battery. In addition, putting a volt meter on each battery after the green light comes on has shown the battery that was cycled is usually in the absorbtion or optimization stage much longer then the other battery before going into float. This tells me that even though many days on a trip the batteries are latched for hours with either the vehicle running and/or with solar they are not getting charged (filled up) the same. I am adopting switching the batteries side to side every winter, so over time the cycling is somewhat balanced for each.

I like the ML-ACR system for the reasons mentioned above: I can combine or separate as I want or leave it on auto; I know I have a battery sitting in reserve that will get me going. But it's not ideal for the batteries...even though they are same type/size/brand.

If I was running different battery types I would be inclined to put my vehicle wiring back on the start battery and look at an independent DC-DC battery charger for house battery charging duties.

Someone posted in a thread a while back a video in which it was explained why a deep cycle battery does not fully charge when paralleled with the vehicle battery for charging. I've seen that exact scenario a week into a trip where a deep cycle would not power a fridge through the night any longer as it was not getting fully charged even when latched to the vehicle charging system for 8 hours a day.

hth's
gb
 
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May 24, 2015
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My idea of a dual battery set up is wiring them in parallel such that both negatives are connected via 1/0 wire and both positives are connected via 1/0 wire. Then I'd just hook everything up to a set of terminals as if it was 1 battery. That way the batteries can share their full potential at all times and they discharge and charge simultaneously. Zero amperage solenoid requirement as they're hard wired to each other. Running winch and etc. Will pull both batteries down simultaneously and both will charge simultaneously from the alternator. I'd use marine or military style terminals, and then I'd put a dedicated 9/16ths(or appropriate size) wrench strapped to the battery side at all times which takes place of a series of switches, wirings, and solenoid. Should I be in a situation of risk where I can't winch with the engine running, or the vehicle will be parked there for days on end getting drained off of, I would pop the hood and disconnect the neg off the second batt. I'd then swap my leads over to the fully charge battery, leave the discharged one disconnected unless I absolutely needed to get charge back into it before I got home to charge it by itself off a battery tender. That way if 1 battery is heavily discharged and 1 is full, they're not sitting there connected free-feeding amperage from the charged one into the discharged one at UN-controllable rates. Just my .02 cents. Whipping out a 9/16 and disconnecting a cable every now again wouldn't bother me and I'd have piece of mind my $500 dual battery investment isn't going to fail on me before the full service life of the individual batteries is up.
 

RobW0

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May 22, 2019
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I like the Redarc, but have yet to move that direction. I still prefer something like the BlueSea ACR or ML-ACR. I think the Redarc is a good solution for newer vehicles with smart alternators, and for a couple of other specific reasons, but I wouldn't quite say its simple. Maybe once I add a 200 to my fleet I will feel different. Even though Redarc is proven in the outback, there is still quite a bit of tech inside it, you cannot mount in the engine compartment and there is no combining for self jumping. I also haven't quite gotten into the camp of needing a "smart charger" for the second battery either. I completely understand the reasoning to eek every bit of capacity from a battery and you need a full charge for that, but I also believe that need is only for real heavy users, and there aren't many of those. Another viable, simple system, is something based on the BlueSea and a solar panel. You can have, smart charging via the solar, in cab self jumping, and you can mount the ML-ACR under the hood. A different approach from the Redarc, but I would argue, just as effective and less expensive as well. I have also seen quite a few troubleshooting threads around the inter-web related to the Redarc installation, start isolation, etc. Confusing installation, another no go for me.
Not all of your information is correct in your statements. The Redarc BCDC units can be mounted under the hood and with the addition of the SBI12 and a momentary switch in the cab you can connect the two batteries for self jumping. You are correct however that the Redarc system is most likely more expensive. The Redarc BCDC1225 system, I have in my 200, was and easy and very clean install.
 
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Telluride, CO
Its important to figure out your usage first. How much power(fridge etc.) will you use, and how often will you drive it (Solar or no solar).

Are you going to setup camp and not start the vehicle for 4 or 5 days? If no, then solar seems un-needed as any time you start the car, your aux battery charges.

As far as battery monitors go, coming from doing camper solar setups if you do your calculations correct you'll just kind of "know" if you have enough power or not. Voltage monitoring isn't a great depiction of State of Charge anyways. I would go with the simple setup.

In a truck camper I had I installed a simple relay that closed at something like ~13.4 volts. Car on? Batteries charging. Car off, batteries seperated. No buttons, no holes in the firewall, few fail points. I made a jumper so I could cross the leads in case I needed to jump start the vehicle from the camper. Very simple, and like.. 20 bucks.
 
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Herndon VA
Here is the clearest explanation of the difference between parallel charging (aka, voltage sensing relay, SBI, ACR units) and DC to DC charging (aka BCDC unit) in a dual battery setup in a 4wd. The 80 series alternator is not a temp sensing or smart alternator but its voltage output is typically 13.9 or less, aside from startup.
Where would you mount the BCDC charger? They have a cool radiator mount for right hand drive.. but they say it needs to be within 1M of the battery... just seeing if you had any thoughts
 

LC4LIFE

Supporting Vendor
 
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Colorado Springs, CO
Not all of your information is correct in your statements. The Redarc BCDC units can be mounted under the hood and with the addition of the SBI12 and a momentary switch in the cab you can connect the two batteries for self jumping. You are correct however that the Redarc system is most likely more expensive. The Redarc BCDC1225 system, I have in my 200, was and easy and very clean install.
Thanks, It does look to be not quite as black and white about underhood installation as I thought.

When I was considering that same piece, the BCDC1225, the instructions says to avoid direct engine heat. I also spoke to a US rep and he said the cooler, the better as they work more efficiently when cooler. I believe that is why many of the installation pictures they show, along with Aussies installations, have it behind the grille. I can only assume its much cooler there instead of inside the engine compartments. I do appreciate their technology and attention to properly charging batteries though and I also think they do a very good job with modern alternators like your 200.
 

LC4LIFE

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Where would you mount the BCDC charger? They have a cool radiator mount for right hand drive.. but they say it needs to be within 1M of the battery... just seeing if you had any thoughts
I bet @Shoredreamer has some ideas. I think he has installed some Redarc systems.

I am not sure where I would put it though. Maybe if it could fit right behind one of the headlights up front in my 80 it could work, but I am really not a fan of that. I also don't like the grille mounting even though they say it is okay.
 
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RobW0

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Where would you mount the BCDC charger? They have a cool radiator mount for right hand drive.. but they say it needs to be within 1M of the battery... just seeing if you had any thoughts
I mounted mine next to my Aux battery. I fabricated a plate from polycarbonate and mounted it on the side of the Slee Aux battery tray. I have been happy with the performance of the BCDC so far. Not familiar with under the hood of and 80.

IMG_0868.jpg



IMG_0874.jpg
 
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I mounted mine next to my Aux battery. I fabricated a plate from polycarbonate and mounted it on the side of the Slee Aux battery tray. I have been happy with the performance of the BCDC so far. Not familiar with under the hood of and 80.

View attachment 2148179


View attachment 2148181
I saw your setup when i looked again.. that is exactly what i will plan to do... seems like it fits well!
 
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Where would you mount the BCDC charger? They have a cool radiator mount for right hand drive.. but they say it needs to be within 1M of the battery... just seeing if you had any thoughts
Ideally with electrical install locations, you want it as clean, dry and cool as possible. The BCDC also wants to be mounted as close as possible to the auxiliary battery in order to reduce or eliminate voltage drop. in my testing and experimenting, I have mounted the BCDC in an 80 in 4 different locations with no charging issues. My personal one is behind the PS headlight and it stays very cool there. Others near and around the aux battery or the starting battery. Taking temps throughout the engine bay, I’ve found the hottest place to mount would be high up, closer to the firewall. I’m sure a bracket can be made to mount it in front of the radiator, but I’d prefer, at least on an FZJ80, not to block airflow thru the radiator. I wouldn’t be afraid to mount the BCDC 10’ away from the aux battery either. With the 25amp unit, I run 8awg, but if you have longer runs, Redarc suggests a 6awg cable.
 
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