Solenoid v. other systems

Submitted: Thursday, Aug 01, 2002 at 00:00
ThreadID: 1623 Views:2023 Replies:7 FollowUps:6
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We are looking to put a second battery in our car to run a fridge, lights, etc... We have looked at the pirannah, ARB and other systems and they range in price from $650 - $1,050 (being a little more expensive because we have a hilux and have to put the battery in the tray). Several people we have spoken to, including auto electricians and 4wders have suggested that we just use a solenoid and save a lot of money. Others have suggested an isolating switch, however this carries a risk because of the need to remember to turn the switch. Can anyone advise as to any reasons why we should not simply use a solenoid and/or explain to me why we should invest hundreds of dollars more to get one of the more expensive systems. Are there problems with these more expensive systems or are they generally pretty good? Your personal experiences would be much appreciated.
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Reply By: Member - Sam - Thursday, Aug 01, 2002 at 00:00

Thursday, Aug 01, 2002 at 00:00
Michelle, I am having a Rotronics RFC12 unit fitted to my Hilux at this very moment. The unit (including an in-cab monitor and wiring harness for battery installation in the tray) cost me approx. $750 (not inc. installation). I would also recommending searching the forum using "dual battery" as your search string. There have been numerous posts on this subject. If you want any more info re the unit, check out or call 1800 644 620 and ask to speak to Rod Street. When I get my Hilux back at the end of this week, i'll let you know how the installation went. cheers, Sam.
AnswerID: 5319

Follow Up By: Michelle - Friday, Aug 02, 2002 at 00:00

Friday, Aug 02, 2002 at 00:00
hi Sam, thanks for the reply. i would be very interested to hear how you go. We have looked the rotronics system before also and have heard good reviews on it - just trying to get out of spending the extra cash if we dont have to
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Follow Up By: Member - Nigel - Saturday, Aug 03, 2002 at 00:00

Saturday, Aug 03, 2002 at 00:00
Good choice - I've had an RFC12 for a while and it allows my deep cycle to recover so much more quickly than all the other basic electronic isoloators
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Reply By: Allyn - Thursday, Aug 01, 2002 at 00:00

Thursday, Aug 01, 2002 at 00:00
I have just the isolation switch and am looking to upgrade to Rotronics or similar ASAP as I have been caught several times. It does not isolate as well as it should. Also find it very hard to get aux battery to proper charge level and as such have been through a couple of batteries.
AnswerID: 5320

Reply By: ExplorOz Team - David - Thursday, Aug 01, 2002 at 00:00

Thursday, Aug 01, 2002 at 00:00
Michelle,In all our cars we have used solenoid relay switching systems and we have never had any problems in our experience. The main points you need to be careful with:1. Cabling of the battery - you must must must use a heavy heavy guage DC cable. Go to the auto electrician/auto shop and get the biggest cable you can.2. Get a heavy duty starting solenoid with bolted connections.3. Make the cable runs are as short as possible - I know you are talking about the rear tray but do the best you can (again make sure it is a heavy guage cable).4. Replace/Add more earth leads from the - of the main battery to the chassis and engine mounts - I have never found a vehicle factory setup that installs enough wire on the - post to the chassis. I always replace these leads and run heavy cables to both chassis and engine block. Additionally on your AUX battery run good connections to the chassis and even back to the block if you can5. Make sure that the soleniod switching is run from the Engine ON position not ACC.Thats about the main points now when you install it all you want to put a multimeter across both batteries with the vehicle running about 1000RPM. To ensure that the second battery gets a good charge you want to make sure there is not to much loss batween the batteries. You will want on have a better then .25 volt difference between the voltage you read off each battery (ie. Main battery 13.45 and Aux 13.20 - measured at the same engine revs and basically the same time). If the loss is to high you will not get a good charge into the AUX battery. If the losses are high then you will need to look at your connections, cable and solenoid. I know you are running the cable a long way but the lower the loss the better the system. In fact it does not matter whether you are using solenoid or electronic system the quality of your cables and connections will determine how good your system is.Best points about the solenoid system are that if it fails you can easily replace it anywhere, cheaper, 100% isolation from MAIN battery and it never breaks. Off course there is a bad point in that you tend to run the first cell in the AUX battery a little hot just after startup - but make sure you always check the fluid levels and you will fine.
I get asked this question a far bit so I will write something in trip prep section on this issue. Hope the information helps you out - David
AnswerID: 5323

Follow Up By: Cam - Thursday, Aug 01, 2002 at 00:00

Thursday, Aug 01, 2002 at 00:00
Michelle, I to had a very similar settup to what you are planning to do & had a solenoid as the isolator. Very practical & cost effective. As David mentioned the key is to have good positive & negative battery contacts even to the point of welding the neg lead to the chassis. Also as the aux battery will be in the tray of the Hilux ensure that you pack you gear allowing plenty of ventialtion around the battery & also in a position where it is easy to get to to check fluid levels etc.
FollowupID: 2292

Follow Up By: Member - Jim - Thursday, Aug 01, 2002 at 00:00

Thursday, Aug 01, 2002 at 00:00
Just to add a bit more to David's post, when purchasing a solenoid get a "continuous duty" type.
Generally they are about $30-50, and just to confuse the issue they either last a reasonable period of time or they don't and the price doesn't matter.
You can always pick one up at Repco, or better still carry a spare.
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Follow Up By: Michelle - Friday, Aug 02, 2002 at 00:00

Friday, Aug 02, 2002 at 00:00
Hi David Thanks very much for this information. unfortunately i don't understand half of the technical stuff but we are intending to get a professional to install it, so i will give him all of the info above. I presume you have installed all of these yourself, but can you advise whether it would be best to go to an autoelectrican or go to somewhere like ARB to get it done?
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Reply By: Graham - Thursday, Aug 01, 2002 at 00:00

Thursday, Aug 01, 2002 at 00:00
How much do you really need/use the 2nd battery......
We only occasionally need the 2nd battery (a spiral cell deep cycle Optima) and its normally in the caravan, using a solenoid in the car to charge it thru an Anderson Plug.... if we want to go bush without the van we just take the battery out of the van, put it in a small battery box in the rear of the Pathfinder and run leads to/from the Anderson Plug. The battery is the same physical size as the std. battery in the car so if it packs up we can just swap it over.... Battery cost $400 but seems well worth it.......

my .02c worth
AnswerID: 5333

Reply By: John - Friday, Aug 02, 2002 at 00:00

Friday, Aug 02, 2002 at 00:00
Michelle, remember your ennemy is the combination VIBRATIONS + BULL DUST. If you keep it simple, you probably can repair it. if you don't repair it, see you on the track waiting for spare parts etc...
AnswerID: 5351

Reply By: ExplorOz Team - David - Friday, Aug 02, 2002 at 00:00

Friday, Aug 02, 2002 at 00:00
Michelle,What city/location are you in?I think either choice will be ok - Yes I always install everyting in my vehicle (fuel tanks, batteries, radios, etc) as this means that I know everyting about it where it is how it works etc etc and thus I can fix it all. In fact I also fully service my vechile and I would strongly recommend that a certian level of understanding be gained by everyone about their vehicle. This will mean you can fix it yourself should you break down in the bush. Anyway, either you local auto electrican or ARB will be able to do a good job for you - Get them to show you the cable that the intend to use and ensure that the center copper section is about 5mm in diameter. The cable is the key to reducing the loss.BTW: I am an electronics engineer and I have fully studied electrical loss in vehicle wiring, (this is a pet subject of mine) so get the cable right and then the connectors etc that follow will have to be correct as the cable gauge will determine how it can be terminated.Good luck with it... DavidBTW: If you like to help and information gained via the use of this site could I ask you and everyone to consider joining our Membership Program (only $30/yr initially and $20 to renew) as it helps to keep this site running.
AnswerID: 5353

Follow Up By: Ray - Friday, Aug 02, 2002 at 00:00

Friday, Aug 02, 2002 at 00:00
Hi David,

I look forward to the article, I to have an engineering qualification though in mechanical, and am suprised how much misinformation there is about this issue. This has also been an area I've been very interested in and I've tried to source info from various professions. It really is seen as a bit of a black art as while it obviously complies with theory many assumptions have been made over the years by all including auto elects. This happens in all areas not just auto elect so a good indepth accurate article should be well received by all. Even articles by popular mags are very light on in content. I understand you will need to write to the audience but that doesn't mean content should suffer or again assumptions will be made. For example (not picking on you) most people talk of wire size in mm when it's really square mm which can confuse or at work cause fires. To easily isolate voltage drops in a circuit such as dual batteries many wouldn't know to use a multimeter to test between both positive or both negative connections on the same circuit to find in which side the drop is in, either +ve or -ve. Or in your example of connecting the solinoid to the active on circuit it should be on but not starting or draw will occur from the aux battery. Really the cheaper after market units are to make it easy and reduce mistakes but the dearer ones allow charging to be done in parallel with priority to the main battery which is a far better system.

I look forward to your article, we are all learning, and more so from your well prepared site.
FollowupID: 2309

Reply By: Wayne - Sunday, Aug 04, 2002 at 00:00

Sunday, Aug 04, 2002 at 00:00

I have a dual battery system in my GQ Patrol using a solenoid system.

It works like this

The two batteries (main and auxillary) are connected through the solenoid using battery cable. Both are earthed to the engine so that either of them can be used to start the vehicle.

When I turn the ignition off or to accessories the two batteries are isolated from each other.

ALL of the accessories in the car, (lights, fridge etc) are connected to the main battery. This means that whilst camping out only ONE battery is supplying the power. Imagine this - your auxillary is supplying power for your fridge and camp lights etc and someone has accidentally left a door ajar in the car, the interior light is left on all night and the main battery ALSO gets discharged. Now you have two flat batteries. Whilst many would say this is not likely to happen, I bet they listen to the radio, tape or CD player without thinking which battery is being discharged.

The solenoid needs power to activate and this wont happen if the main battery is dead flat. So I have a momentary push button switch located under the dash that will power the solenoid from the auxillary battery so that I can start. (ie push in button and hold, turn ignition key, let go of switch and start vehicle)

Both batteries are starting type batteries not deep cycle (you might be interested in reading the article in Australian 4WD Monthly June 2002 edition page 79-82 (back orders through or 1800 801 647) about batteries for dual battery installations. I have had both batteries for over 5 years starting my diesel.

The only thing I need to do more regularly is check that they are both working OK as with my system one battery can be faulty and the other will start the vehicle and you wouldnt be aware.

My brother is an auto electrician working for a major trucking company (Volvo) and has setup this same system in a hilux he owned with the battery in the tray. Just make sure all the connections are of a high quality - you should be able to get an autoelectrician to make the cables up for you if you wanted to install the system yourself.

Good luck!
AnswerID: 5387

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