dual battery,do it yourself

Submitted: Monday, Jul 14, 2003 at 23:20


g'day folks,i have been looking at all the dual systems for sale and reading all the archives .ive noticed a lot of people say a simple solanoid setup is best and cheapest and does the same thing.where would i find the info and instructions to perform this feat legerlos
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AnswerID: 24950   Submitted: Monday, Jul 14, 2003 at 23:39

Matt (W.A.) replied:

I have a mate in the same Boat, he Works for Dicksmiths and apparently they sell the kits to perform the said operation Try them, Also try Jaycar they also have the same type of Kit so I believe? About $40 for the kit and the cost of a battery tray/Battery

Hope this HelpsKeep It On The Rough Stuff

Matt (W.A.)
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FollowupID: 16842   Submitted: Tuesday, Jul 15, 2003 at 00:29

Ray M (Vic) posted:

I've built two of those kits. The mosfet protection diodes allow current flow from the aux to the main battery so isolation is not complete. If the main battery is in bad condition it can drain the aux battery.Hooroo
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FollowupID: 16843   Submitted: Tuesday, Jul 15, 2003 at 00:38

joc45 posted:

And if you try to crank the motor in this condition, the start current will be too much for the protection diodes and destroy the FETs.
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FollowupID: 16859   Submitted: Tuesday, Jul 15, 2003 at 10:09

baza posted:

If you use a deep cycle battery, this should not be an issue as the unit can handle 100 amps and a deep cycle would struggle to produce that much. It is also only for a short period of time - whilst the starter is engaged. (This is if you have the sense line runing of the alternator). I think they are great value at $39, just need to be aware of this small design issue.
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FollowupID: 16885   Submitted: Tuesday, Jul 15, 2003 at 17:30

baza posted:

Whoops, what I said above is not exactly true, if the voltage of the aux bat is higher than the main, current can follow back into the main bat. Just need to make sure you have a good main bat before heading off.
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FollowupID: 16934   Submitted: Wednesday, Jul 16, 2003 at 10:33

Luke posted:

I've built a Jaycar 12V battery manager kit - haven't installed it yet. The reverse current problem via the protection diodes is well documented in the kit instructions.


".....the MOSFET internal protection diodes will allow a 'reverse' current to flow if the auxilliary battery's terminal voltage is significantly higher than that of the main battery. While this is obviously not the case during normal charging operations, there can be circumstances where the auxilliary battery is fully charged yet the main battery's voltage has fallen due to a high current load - such as the vehicle's starter motor.

If the auxilliary battery can deliver a high current (most deep cycle batteries can' by the way), then the MOSFET diode current is only really limited by the conecting lead resistance Rl, and may rise to a high level - say 60amps. This in turn means that the MOSFETs will dissipate around 30watts each, thanks to the 1V drop accross the diodes. If this curent were present for an extended period, the MOSFETs would become rather hot and bothered due to the modest heatsinking capability of the diecast box.

Note thatthe MOSFETs dissipate less than 10W during normal charging operations.

While we were initially a little concerned about the above scenario, and considered a number of circuit configurations that would provide full two-way isolation between the batteries (by the use of a high current relay), we found that the additional expense and circuit complexity of the alternatives was difficult to justify.

And as the condition would only occur if

a) the auxilliary battery was of the high current type.
b) the load was applied for an extended period (unlikely with a vehicle's starter motor).
...and c) the main battery voltage dropped substantially under load (indicating a less than healthy battery), we reasoned that there was not too much cause for concern.

Of course, we'd have to addd that it's extreme folly to set off in your 4WD, camper or car/caravan combination with an unhealthy main battery - or for that matter, a vehicle that's difficult to start and needs to be cranked for an extended period.

Under normal circumstances, the potential for reverse current flow could be considered as somewhat of an advantage in a dual battery system, since any excess charge in the auxilliary battery will be automatically passed to the main battery via the MOSFET diodes. This in turn ensures that the main battery is always in the best possible state of charge for the conditions at the time, and can therefore crank the engine into life......."

End Quote.

Anyway, sorry for the long-winded post, but I thought the reverse current 'problem' should be documented correctly here.

I'm sure there are better (and more expensive) alternatives to the DIY kits. But I'm also sure that the DIY kits offer a good cheap and reliable alternative.



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FollowupID: 16974   Submitted: Wednesday, Jul 16, 2003 at 17:58

Ray M (Vic) posted:

One just has to be aware that if the main battery carks it for whatever reason it could possibly flatten the Aux. If this happens then you have no one to blame but yourself for installing this setup.Hooroo
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FollowupID: 16982   Submitted: Wednesday, Jul 16, 2003 at 18:25

Luke posted:

You're absolutely right Ray.

You've built two of those kits - were they for others or yourself? I'm interested to hear, aside from the reverse current problem, how you found the overall performance of the EA Battery manager kit?

Perhaps a slight modification of the kit to include a high current relay for absolute isolation would make the kit a worthwhile proposition, considering it's sensing capabilities and ability to apply low current charging as well as normal charge rates depending on the condition of both main and aux batteries?

The addition of said relay should be reasonably simple...


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AnswerID: 24951   Submitted: Monday, Jul 14, 2003 at 23:48

legerlos replied:

thanks matt $40 sounds great. legerlos
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FollowupID: 16844   Submitted: Tuesday, Jul 15, 2003 at 00:58

Rod posted:

Just to expand on this subject, buying a cheap setup with questionable reliability would not make you sleep to well out under the star's miles from anywhere. Does anyone have info on which Battery management system's to definately (stay away from) or recommend (the one to buy) all prices aside ?
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FollowupID: 16868   Submitted: Tuesday, Jul 15, 2003 at 11:45

Allyn (Pilbara) posted:

I have Rotronics RDC12M and can thoroughly recommend it. Top dollar though but I need reliability and to date have lasted 2 years without either battery replacement whereas before both were going under every 12 months. I have friends who have had same batteries for 4 - 5 years with Rotronics and another who is into 8th year and only now looking like a replacement is necessary. Therefore I can see it paying for itself easily and the extra peace of mind made it all worth it.so many places, so little time !!!
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AnswerID: 24963   Submitted: Tuesday, Jul 15, 2003 at 08:34

Matt (W.A.) replied:

Having said all this about my Neighbour, I have an ARB Dual Battery System that runs from a Solenoid and has an In Cab Override Switch so that if I run the Aux Battery Down too much and it doesn’t have enough Juice to switch the Solenoid then i can do it manually, It has also being set up to not have to Re-Wire any of my Accessories either Can listen to the radio all day when I’m working on/In the Pajero
it also has a Surge protector Fitted, all these little things add up and i preferred to have the pro's do it! Cost me $690 I have also herd that Rotronics Controllers are Reliable and Piranha Too.

Keep It On The Rough Stuff

Matt (W.A.)
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AnswerID: 24968   Submitted: Tuesday, Jul 15, 2003 at 09:18

al replied:

4X4 monthly did a bit about dual set ups in the bush mechanics section last June's mag I think. The guy shows you how to fit a manual switching system from go to whoa , he uses marine switches which are very robust and being manual there's not much to go wrong. Don't know anyone who has one bit it may be worth trying and its cheep +++
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FollowupID: 16865   Submitted: Tuesday, Jul 15, 2003 at 11:18

Member - Melissa posted:

We've had marine switch systems on two old 4WD's over the years and yes they are simple and reliable BUT the onus is on the operator to manually swith between batteries as appropriate. We had occasions when we'd forget and it was a pain! For the sake of ease and convenience we would not go back to a manual system.

:o) MelissaPetrol 4.5L GU Patrol &
Camprite TL8 offroad camper
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AnswerID: 24995   Submitted: Tuesday, Jul 15, 2003 at 14:36

micko replied:

G' day i'm running a smart isolator dual battery setup in my hilux. It cost me about $150 for the isolator from an auto electrician and about 70-90 for the rest of the heavy cable and connecters to do the job. i installed it myself as it was very easy to do .i've had it for about 6 month now and it has worked great. i do beleive that my local arb shop fits these setup's but at a lot larger price.
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AnswerID: 25004   Submitted: Tuesday, Jul 15, 2003 at 15:14

Martyn (WA) replied:

I have the ARB solenoid set up in my GQ, faultless so far, normal battery and a deep cycle as the auxillary, I've fitted a battery monitor as well with a little LED display that tells you the charge rate of the auxillary, both battery conditions the battery condition whilst stationary, plus the charge rate of the main battery when the engine is running, handy little gadget. Go Go gadget.....Keep the shiny side up
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AnswerID: 25015   Submitted: Tuesday, Jul 15, 2003 at 17:21

Member - Grinner replied:

I was using the DickSmith unit for 2 1/2 years and found that it just didnt charge the second battery quickly enough. Also it hasn't been mentioned yet that you need to build the DickSmith / Jaycar unit yourself. $40 gets you a Circuit board and a bunch of little components. I've bought (but not got around to fitting yet) a REDARC battery isolater www.redarc.com.au, cost $130 and only has 3 connections, -ve, +ve main battery, +ve second battery. It automatically senses the voltage of the batteries and kicks in and out as needed.

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AnswerID: 25028   Submitted: Tuesday, Jul 15, 2003 at 20:07

Member - Willem replied:

I have a Rotronics MH10 in my dual setup where one battery is housed under the passenger seat. The whole setup excluding batteries cost $260 including heavy duty cable and I fitted it myself. Works a treat and have had no troubles with it.

An old saying goes........you pay peanuts...you get monkeys !! :-)Cheers, Willem
Never a dull moment
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Reply 8 of 12
AnswerID: 25044   Submitted: Tuesday, Jul 15, 2003 at 21:59

Member - Oskar(Bris) replied:

I took the advice/circuitry of a battery company in brisbane and installed the whole thing myself and the system seems to work really well. They sell the Rotronics unit as well but they have installed this circuit/system in hundreds of 4x4's and have had only positive feedback.
Basically they simply use a solenoid (100 amp cont. duty. 400+ amp peak $50-$70) connected to the -ve side of the main battery (not the +ve), so that when the ignition key is in the off position ie. when you are not in the vehicle or at night etc the aux battery supplies the power to run everything like lights and fridges etc. and the main battery stays fully charged for starting. One upside of breaking the -ve side is that you don't get the pitting of the solenoid contacts that you do when you break the +ve side (so they say).
By adding a switch in the cabin you can isolate the main battery while driving and send all the charge to the aux bat. (obvious benefit to fast charge the aux which may be run down a bit). This is a basic explanation only.
Or by using a "normally ON" 2 way rocker switch as your switch in the cabin and then add a tempory "memory" cable to the -ve on the main battery you can take your Deep cycle battery out for use as a trolling motor power source etc or just isolate the Aux.
They supplied an 85AH (under-rated at 75AH) deep cycle, 500CCA (USA) with 1 year warranty and all the cables and components for about $300. And the battery will also double as a cranking battery.
If you are interested I can send you the circuit diagram.
I know this response is a bit lengthy but I did go into it fairly extensively.
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Reply 9 of 12
FollowupID: 16908   Submitted: Tuesday, Jul 15, 2003 at 22:09

Luke posted:

Hi Oskar,

I'd be very intersted to read your circuit diagram please.


Thanks very much,

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FollowupID: 16914   Submitted: Tuesday, Jul 15, 2003 at 23:10

Waynepd (NSW) posted:

Hey Oskar,
Me Too Please
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FollowupID: 16989   Submitted: Wednesday, Jul 16, 2003 at 18:49

Brian posted:

I'm pretty interested in this set up as well Oskar, could you send me the circuit please?? I am thinking of battery management as we are a couple of weeks away from buying our fridge.... I was going to follow Australian 4X4 monthly's Roothie and install a 3 way marine switch.. apparently cost under $40...your suggestion may yet find its way under my bonnet...
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FollowupID: 17027   Submitted: Thursday, Jul 17, 2003 at 01:20

Member - Toonfish posted:

yeah i better have the cicuit diagram as well thanks .



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FollowupID: 17406   Submitted: Wednesday, Jul 23, 2003 at 01:45

Colin posted:

Hi Oskar,

I'd be very intersted to read your circuit diagram please.


Thanks very much,

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AnswerID: 25053   Submitted: Tuesday, Jul 15, 2003 at 22:43

legerlos replied:

g'day oscar, yeh mate i would definetly like a look at diagram. chidgybolan@hotmail.com thanks legerlos
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FollowupID: 16915   Submitted: Wednesday, Jul 16, 2003 at 00:31

Member - Oskar(Bris) posted:

No worries it's on its way
Please remember, however, that the circuit as it stands is presented as an idea that may or may not work. I cannot take any responsibity for what anyone else may do with it in its current form or in any modified form. If you choose to use it it is your own responsibility.
All I can say is that I have installed it in its current form in my own vehicle and I am very happy with it. I take responsibiltiy for my own mistakes only.
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FollowupID: 16916   Submitted: Wednesday, Jul 16, 2003 at 00:31

Phil R posted:


As mentioned above with the Dick Smith kit, you are getting a bag
of electronic components, bare circuit board, a bunch of wires,
a die cast box that you have to drill and fit components into. You have
to solder the components into the circuit board, once assembled it
has to be tested and if it dosn't work then you have to do some
fault finding, Worse case you fry one of the components and or
damage the circuit board. You will need some basic knowledge
in assembling electronic circuits and be able to identify and read components, also you will need to know how to use a multimeter etc.
This sounds like a cheap way to go at $40, but if you have any doubts about your circuit building capabilities then steer clear unless you know
someone that can help you. If you are keen to learn go for it.


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FollowupID: 17045   Submitted: Thursday, Jul 17, 2003 at 15:46

Member - Oskar(Bris) posted:

Did you get my E-mail?
I lost the original one and I had to scrounge it back from one of the others. I'm not sure I used the attachment system/thingo correctly so if you didn't get it - sorry.
I can send it again if you want.

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FollowupID: 17407   Submitted: Wednesday, Jul 23, 2003 at 01:51

Colin posted:

Hi Legerlos,

I'd be very intersted to get a copy of Oskar's circuit diagram please.


Thanks very much,

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AnswerID: 25216   Submitted: Thursday, Jul 17, 2003 at 19:42

legerlos replied:

osker, yeh mate,got the e mail but it had no circuit diagram and yes i still would appreciate a look,thanks legerlos.
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AnswerID: 26500   Submitted: Friday, Aug 01, 2003 at 10:16

Matt replied:

I have installed the DSE 2nd battery controller in my 1997 Land Rover Discovery

Have a look....
2nd Battery for Disco

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