Setting up the Best Dual Battery System
Dual Battery Systems are one of the most talked about subjects in the 4WD world. There are many different views and competing products, so most 4WD vehicle owners are really confused about which way to go. In this article I lay out the main dual battery system options so that you can make an informed choice.
To make it easier to select a dual battery system that suits your budget and needs I am going to list my last option first and then work my way to the best. I have also put a rough indication of how much more expensive than a traditional dual battery system each option is.
My Last Preference: Traditional Dual Battery System
A traditional dual battery system uses a dual battery isolator to connect the start battery and auxiliary battery in the vehicle. The way this system works is that the alternator chargers the start battery and when it has enough charge in it the dual battery isolator connects the two batteries so that the alternator charge flows across the start battery and into the auxiliary battery. This system is less than ideal as the two batteries are usually of different types with one being a start battery designed for high cranking and one being a deep cycle battery designed to run fridges, etc. These systems are especially bad when you use a calcium or calcium composite deep cycle battery. These batteries need around 14.7-15 volts to charge correctly and with many alternators being around 14.3-14.5 volts and the voltage then having to flow over the start battery and through the isolator and wiring which also creates a voltage drop the auxiliary in many systems will only charge 65-80% in many cases. There are two problems with this. The first is if you don’t charge the battery to, or close to, 100% the battery life will tend to decrease. If this is your situation good advice is to charge your auxiliary battery on a regular basis with a good quality 240V powered battery charger. This will fully charge the battery and extend its working life. The second problem is you should only really draw your deep cycle battery down to around 50% charge, anymore than this on a regular basis will shorten your battery life. So if you battery is only charging to 75% you effectively only have 25% (75%-50% = 25%) usable battery capacity compared to a fully charged battery 100%-50% = 50% usable capacity or double the first example.
4th Best Option: Use a better battery (+$0-$100)
A traditional dual battery system can be improved by selecting an AGM or Marine start battery. These batteries take charge much better than calcium type batteries so you will get more usable capacity and a longer battery life. I have a theory that AGMs in particular because their ideal charge voltage is 14.4 compared to14.7 volts for traditional batteries tend to take charge off your start battery which may reduce the life of the start battery. I haven’t had the opportunity to test this to confirm, but it is something to keep in mind. The problem seems to be worse in vehicles with a lower alternator output voltage.
3rd Best Option: Use the Same Battery Types (+$250-$500)
If you use the same type of batteries as the start battery and auxiliary battery they will charge much better together and overcome a lot of problems with the traditional set up. The problem with this is that not many batteries are capable of starting a vehicle and doing a good job of deep cycling for long periods. The two types that can are the Spiral core batteries and AGMs. These two are designed for high cranking so can start the vehicle, and deep cycle. The AGM certainly deep cycles better than the Spiral in my battery tests but the spiral has better starting power. The spiral is also much more expensive than the AGM which is a factor. We have found the AGMs to start most vehicles pretty well, however this can be improved by setting your isolator up so that both batteries connect together while the vehicle starts giving you heaps of cranking power. To do this successfully you’ll also need to make sure the wire size between the two batteries is heavy enough to transfer the charge current without too much voltage drop.
Another option that I haven’t tested is to use two marine start batteries in the system. I’ve found them to be great deep cycle batteries, however don’t have the cranking power to start a diesel engine well, as they are designed for smaller marine engines, however if you connect the starting power of both batteries this should start the vehicle well (just a note of caution: I haven’t actually tried this one, it’s just theory until I get the chance) and they are no more expensive than a traditional deep cycle battery.
2nd Best Option: Use an independent Charge System (+$600-$800)
An independent charge system ensures the start battery is well charged and then cuts the charge to that battery and redirects it to the auxiliary battery. This is a good system because it gets around the issue of the alternator having to push the battery charge over the start battery to get to the auxiliary, especially where they are two different battery types. Even though this is a great system it still only redirects the alternators charge to the auxiliary battery, it cannot increase the alternators voltage and does not regulate the voltage and current like the next option does, this doesn’t seem to be a huge issue on AGM and Spiral batteries that take charge at lower voltages however. It is also more expensive than the next option which is a down side and generally more expensive to get installed.
The Best Option: Use a DC Battery Charger (+$200-$500)
In 2011 the CTEK and Redarc DC Battery Chargers started to come onto the market and we have now gained experience with these products. The great thing about a DC Battery charger is that it draws power from your start battery and then increases the voltage and manages the current flow to charge your auxiliary battery the same way a 240V powered smart charger would. This set up in my book is ideal and is especially great when you are charging a battery in a camper or caravan where the wiring may be 6 to 8 metres from the vehicle battery. In this instance you install the DC Charger near the auxiliary battery so that the DC Charger can compensate for the voltage drop in the wiring.
So in my opinion and in my experience you should pay the extra money and get a DC Battery Charger. With our CTEK D250S Dual currently selling for $359 vs the Redarc Dual Battery Isolator for $149 you’re looking at an extra $210 but I believe you will get much more usable capacity out of our batteries and much longer battery life which should pay for the DC Charger in the long term.
The main issue we have with the CTEK DC Charger is that it takes up a fair bit of room under the bonnet, so we tend to use the Redarc BCDC1220 (20A) which sells for $470 and the BCDC1240 (40A) for around $670. The CTEK D250S Dual and Redarc BCDC1240 also have solar input so they act as a solar regulator too, saving you the expense of buying a separate regulator if required.
Thanks for the simplicity of this article. Would you explore further the installation of a DC-DC charger in a camper-trailer. My CT has dual batteries fitted & takes power from the tow vehicle (dual battery), on-roof solar cell) & 160 w portable solar cell (as needed) but I think it would benefit from the addition of DC-DC charger at the CT dual battery site. Apart from LED lights, it is required to charge 2 x 40L Engel fridges – one as a freezer and the other as fridge for a week or so at a time.
Hi Gordon, basically the situation is the same for camper expect the wiring from the vehicle battery is longer causing more voltage drop, which makes the need for a good system (DC Charger in my opinion) much more important. All the same rules apply as in the article. I am looking to do an article for campers and caravans involving generator, solar, battery chargers & dual batteries shortly, so please keep your eye out for it. Brett
Great Brett – I will eagerly await that one.
Hi Brett, thanks for the article, We also will be looking out for the article on the camper & solar ect.. We currently have a camper with a ctek charger running 2 x95 amp batteries & 190watt solar panels & our batteries run down over night, we are running a dometic fridge that uses 6amps per hour & a CPAC machine that uses 4 amps per hour. We were thinking maybe its our charger, but will now look out for your article to see any other suggestions you have.
sharon
Hi Sharon, I think your real issue is your fridge. The dometic fridge is I assume a 3 way fridge? These are really a gas fridge that use 12 volt to run it while driving only. If you are running the fridge off 12 volt you’ll be using heaps of power and hence your shortage of battery power. I’d suggest that if you can you run your fridge off gas as this will be much more efficent and take the pressure off your power system. Regards Brett.
G’day Sharon . We also run a Dometic fridge. ( Ours is 3 way) and we only have one 55 amp battery which is both alternator and solar charged to run it. It can be flat in about 2 hours if we forget to switch to gas. The gas lasts very well and is extremely efficient.
I have used an Optima starting battery on my previous GQ Patrol 4.2 d with aftermarket turbo and on my current GU Patrol with 6.5 Chev diesel and yes they are expensive however I am only on my second Optima because the first one lasted 11 years which is excellent for a starting battery.
My second battery is AGM with a Redarc isolater
Good reading. I’m with you Andy – I have an optima starting battery (deep cycle) in the patrol & a 120AH AGM deep cycle secondary through a redarc fitted with push button to connect both batteries should the need arise. I’ve never had an issue with the batteries charging using this set up. Simple is really the best.
Brett, it is good to get all this information but being female I rarely understand the electrical/mechanical side of things but I believe that I have followed your simple explanation of this subjectand will be asking my hubby questions to make sure i have the info right. He doesn’t travel with me but will catch up and stay a week or 2. What I really would like to know about is duel battery systems for the caravan or have I got it wrong and this is for both van & vehicle
Regards
Wilma
Hi Wilma, this article applies to both vehicles and caravans. In-fact the situation is made worse in vans because the long wiring between batteries causes extra voltage drop, so its even more important to have a good system. Wilma I know the technical stuff is more suited to guys in most cases, but my staff and I are happy to give you first hand assistance if you ever need it: (03)50242200. Regards, Brett
Thanks, Brett, for opening the topic and for your informative, user-oriented articles. I have found them very interesting food for thought.
A battery type that deserves a place in the dual battery consideration is the Lithium-Iron-Phosphate (LiFePO4) battery. See ‘Overlander’, June 2010, pp 130-132. There are apparently two types, one optimised for power and the other for energy, but both capable of doing a very good job in both cranking and deep cycle roles. The big disadvantage of them is the initial cost, but I understand that their longevity makes the cost become about 70% of the cost of equivalent capacity lead-acid batteries over time. If a price is put on reliability, then the equation is even more in favour of the LiFePO4 types. Even at failure point, I understand the situation is not catastrophic – total discard of the failed battery – because the failed cells can be individually replaced.
The LiFePO4 battery is capable of doing two roles as in Brett’s Third Best Option, meaning that the cranking and spare batteries should be able to each get the right charge, as long as the wiring between them has very low resistance.
The voltage of a nominally 12V LiFePO4 battery stays at 12V under engine cranking conditions, and not 8 – 8.5V as for lead-acid batteries. This means a much healthier spark and faster starting in petrol engine vehicles. The charging efficiency is rather greater too, resulting in less loading on the alternator and, therefore, better fuel efficiency in both petrol and diesel engine vehicles.
The form factor of LiFePO4 batteries can be any shape desired, so that they can be positioned under a rear seat if that suits. Whether these batteries behave well in the heated environment of the engine bay, I don’t know, but would like to find out. LiFePO4 batteries are also a fraction (about one-sixth) of the weight of the equivalent lead-acid battery.
I don’t have any personal experience of dual battery systems, but I have been wanting to install one for some time, and have been following the suggested options with interest. To connect two batteries together, I favour the semiconductor approach using MOSFETs, as advocated by ‘Silicon Chip’. See Jaycar Part No MB-3680 or Altronics N2104. These involve connecting the batteries together using a very low resistance semiconductor (the channel of the MOSFET), meaning that the disadvantages of having a diode drop or an effectively zero resistance of a relay, and theoretically infinite current possibilities, are avoided.
Reading Brett’s arguments for his favoured options, and having read about the LiFePO4 batteries, I can see that the best option, if one could afford it, would be two like LiFePO4 batteries as the dual battery system. As an interim measure, I wonder about keeping my existing lead-acid cranking battery and having a LiFePO4 battery for the ‘house’ loads, separated by a MOSFET switch. However, I don’t know enough about the chemistry of the LiFePO4 battery to know how its charging voltage compares with the lead-acid cranking battery, and hence whether either will suffer from being not fully charged.
LiFeTech (http://www.lifetechlithium.com/) offer a special charger for their batteries, but their web site is too general to give any guidance on charging LiFePO4 and normal lead-acid batteries in parallel. Is this a case for further investigation, Brett?
Cheers,
Rob
Hi Rob, thanks for the great comment. I’d love to investigate further but this may take some time to get to. Love to hear from customers with a Lithium battery in a dual battery system. They do exist out there but pretty rare at this point. Rob, when I email my newsletter list next I’ll see if we have any customers with real world experiences with this type of system.
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HI Brett I have an 80 series Land Cruiser fitted with a Redarc SB112 isolator kit connected to an AC Delco DCM27MF deep cycle battery I installed it early 2010 and am more than happy with it .My starting battery is a Besco 660 cca and I find them a good combination. The SB112 is an electronic unit and also has a circuit that allows me to close the contacts from via a circuit inside my cabin to use it as a starting battery if necessary .I have only had to do this once when the lights were left on by accident .
great stuff guys, Another way too get more charge to your start and then too the aux. is installing a (FUSED 100A.) Cable From the Pos. lug on the rear off your Alternator directly too the Pos. terminal on the Start Batt. In most cases the original wiring produces Voltage drop. I done this Mod to my GQ Patrol using 2B&S Cable. Leaving the original wire in-place. The fuse is there incase (rarely) a Diode in the Alternator gives up. And it wont over cook your battery. This gave me an extra 0.8-1 Volt. charge too my Aux. (deep cycle 120 Ah.) It now charges at 14.7 V while car is running, and drops as the regulator tells it too. I still agree to charge Batts’ at-least monthly on a 240V. charger. But the Mod ensures my deep cycle is at 100% when going to my destination, which is normally a 4-5 hour drive.
There is an issue with fitting an AGM battery under the bonnet as they dont like to get hot. I also modified my Prado (fit diode with fuse) to increase the charge voltage from 13.5 to 14.1v for the reasons you outlined in your post. You can mix batteries provided they are of the same construction (eg high cranking or CCA rating and marine battery). This makes sure you can get a full charge.
Perhaps you may like to have a read of my post on: http://www.pradopoint.com/showthread.php?23035-Conflicting-views-on-battery-type-for-dual-battery-set-up&p=323151&viewfull=1#post323151
Nifty…
Excellent article. I’m looking for answers to a small boat system ( camping tinny ) With a Mercury 3cylinder 40HP, Waeco 40litre, 120 watt solar panel and generator; I’m trying to balance between weight, efficiency and cost. Two batteries the same; (AGM or Marine) might do the job. I’m not sure I want a complicated dual charge system but would like to ensure that by mistake I do not flatten both batteries. A simple diode crosses my mind but I do not know enough about this.
Hi Trent, if you can give me a call on (03)50242200 next week when I’m back in the shop, I’ll go through your needs one on one. Brett
Hi Brett, excellent article very much appreciated by the “brand new to 4wding” category that I fit into.
I have a very specific setup that I would like to get your advice on. My hobby is diving and I have just purchased a new Prado 150 GXL Manual Diesel. I want to winch (Warn XD9000) gear and divers in and out of dive sites, so low load for 60 to 90 mins.
I am thinking of a Redarc BCDC1240 with two Optima Batteries.
Q1 – What batteries do I get for Primary and Auxillary?
Q2 – Where do I install the BCDC1240 in the vehicle?
Second part – I want to run laptops and up to 500W items from an inverter at the rear of the vehicle. I am thinking of a Redarc RA_R-12-1000S.
Q3 – Where do I install the RA_R-12-1000S in the vehicle?
Q4 – What is the optimal setup for wiring the inverter and dual battery system.
I will be getting an auto electrician to do the installation, but I would like your advice before I outlay the capital.
Thanks in advance.
Scubadude
Hi Mate, I’m back at the shop next week, sorry I’ve been on the road for the past week. If you’d like to give me a call I can talk to you one on one to make sure you get the right gear (03)50242200. Brett
thank u for your advice i got a duel batteries in a holden rodeo and i got the same size one to start with one in the back tray can i connect the caravan too if i extend the leeds ?thank u c.c
Hi Charlie, Yes you can connect the caravan battery to the auxiliary battery. Good idea to make sure they are the same battery type with good size cable between them, or look at a DC Charger to get the best results.