Jump Start dual battery via dash switch

Submitted: Tuesday, Dec 05, 2017 at 11:09
ThreadID: 135950 Views:5732 Replies:9 FollowUps:13
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I have done some reading on here and elsewhere on info looking at using the Redarc isolator/VSR in my Dmax to jump start off my second battery in the tub via a momentary push button, but there seems to be some conflicting info.

I know people may say why bother for the odd time you need it; I'd like to have the functionality of it as I've been caught twice on interstate road trips in the middle of nowhere, and having to unpack the whole back of the ute to disconnect the rear battery and all my other accessories and jumpstart (I use a Century marine pro, not an AGM), I’d rather just use a switch. My jumper leads aren’t long enough to reach from the rear to the starter.

I'm not that techy, but I see there is the blue wire coming off the isolator with a spade connector on it which is supposed to be used for the override function. There seems to be arguments on how to wire it up, and whether to use a fuse and what cable sizing. This is where I need some assistance by those in the know. Any recommended wiring diagrams would also be appreciated.

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Reply By: new boy - Tuesday, Dec 05, 2017 at 11:34

Tuesday, Dec 05, 2017 at 11:34
12 years ago on my 1st Patrol I had a simalar set up there was a 3 way switch on the dash ,the alternator would only charged the auxillary battery until for a few hrs every month I switched over and charged the starting battery . Should I forget and flatten the starter I then had to hold the switch down and this linked the 2 battery,s and I could start . Worked a treat The auto who put it in had a client who went round Aust and never charged the starting battery. I,m less techy than you Top End so no idea what he used to perform this magic.
AnswerID: 615367

Reply By: Glenn C5 - Tuesday, Dec 05, 2017 at 11:51

Tuesday, Dec 05, 2017 at 11:51
Hi TopEnd This may help.https://www.redarc.com.au/wiring-diagrams
AnswerID: 615370

Reply By: RMD - Tuesday, Dec 05, 2017 at 12:08

Tuesday, Dec 05, 2017 at 12:08
Top End
You do not need to unpack anything in the rear if you have a VSR or a normal continuous duty solenoid.

A jumper lead across the VSR main terminals will connect the aux battery to the front IF you need to.
Alternatively, for only when you need such a function, you can fit a Battery isolator switch across the VSR with two leads, one going to either side of the VSR. That will provide a definite supply as it locks both systems together. Open bonnet, turn KEY on bat heavy switch and all is ready.

Even if you use either method, the return negative line from the rear battery has to be large enough to supply the current or else nothing will operate properly.

I use a normal constant duty solenoid and have the system lock itself or I select and it drops out when ign turned off. That allows an in cab jump start anyway.
I also use the Batt Isolator so the solar, can if required, be connected to main battery to maintain it too.
AnswerID: 615371

Follow Up By: Top End Az - Tuesday, Dec 05, 2017 at 12:25

Tuesday, Dec 05, 2017 at 12:25
Thanks I will check it out. Yeah I didn't know the Redacr isolator had this functionality already until recently. Would have been handy to know on the side of the road.
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Reply By: Allan B (Sunshine Coast) - Tuesday, Dec 05, 2017 at 14:38

Tuesday, Dec 05, 2017 at 14:38
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Hi Az,

As you already have a Redarc SBI-12 isolator in the engine bay, all you need is to connect a momentary pushbutton to its blue wire with the other side to earth. When you press the button the SBI will close and join your two batteries together.

BUT.... you need to consider the current that has to flow from the aux battery to the starter. Starters typically draw well over 100 Amps.... just look at the size of the cable from your cranking battery to the starter..... it would be at least 20mm2 even for that short distance. You would need at least that size (and preferably 32mm2) for the distance between your aux battery in the tub and the SBI in the engine bay. Together with a fuses or circuit breakers of at least 100A capacity (maybe more) at each end. If the cable is too small it will be unable to supply sufficient current to crank the engine or even worse will overheat and possibly catch fire.
This assumes that your cranking battery is at a very low state-of-charge and the aux battery will be contributing all of the starter current. In reality, the cranker may just be too low to effect a start but will seek some contribution from the aux battery which then will not need to supply such a large current, in which case you may get away with a smaller cable.......... but can you be sure of that before the smoke rises?

Cheers
Allan

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Follow Up By: Top End Az - Tuesday, Dec 05, 2017 at 15:08

Tuesday, Dec 05, 2017 at 15:08
Hi Allan. Appreciate putting a response together for us. Yeah my nephew did the dual battery install so not sure on the size of the wire to the auxl I'll have a look at it tonight, but it's the points you raise that had me wondering if I have sufficient capacity in the wiring to consider this as an option.
And thanks for confirming the wiring at the SBI end of things; I was pretty sure thats all you had to do for that end of things.
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Follow Up By: Dean K3 - Tuesday, Dec 05, 2017 at 15:59

Tuesday, Dec 05, 2017 at 15:59
Simple check if your wires are type that would be used to power up a fridge or lights ie <6 mm then its far too light duty to allow for current required.

Ideally something as a rough guide 16mm2 upwards about same size as my little finger ( a very subjective sizing guide) is starting to get there -winches and starter motors will usually use 35mm2 cable (figure I see quoted fair bit)

I have a linking button on my national luna smart solenoid I have learned hard way not to push this button when trying to 2nd battery jump start -blew the internal circuit breaker non repairable item.

I would hope the redarc doesn't have this feature for reliability reasons, reading of the technical info suggest it doesn't

I use a pair of jumper leads across from one battery to another is I need to do a emergency start
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Reply By: Peter_n_Margaret - Tuesday, Dec 05, 2017 at 17:44

Tuesday, Dec 05, 2017 at 17:44
I have a RedArc 2 way 200A VSR that is quite a few years old.
It is close to the crank battery and connected to the house batteries by a 70mm2 (huge) cable (it is 6 or 7m long) with 200A fuses at each end. I use a chassis earth return.

I have wired the earth wire to a switch in the cab. I normally leave it OPEN, so the VSR is not operating.
I also wired the "momentary push button" to an on/off switch, also in the cab.

So now I can use the VSR as intended, leave it non operational or lock it closed.
This lets me use the alternator to charge the house batteries or the solar to charge the crank, use the house to help power the winch (at the front) or the alternator to help power the winch (when it is at the rear) or use the house to crank the engine (or just help when the Perky is hard to start in sub zero temperatures).

This has been 100% successful. We once had a crank battery die on a trip and waited a couple of months until we got home before buying another.

Cheers,
Peter
OKA196 motorhome
AnswerID: 615377

Reply By: Hoyks - Tuesday, Dec 05, 2017 at 18:46

Tuesday, Dec 05, 2017 at 18:46
If you have a good heavy earth from the second battery to the chassis, you can get away with just the1 jumper lead and join positive to positive.

As said above, trying to run power back through the solenoid may not do it a lot of good and if you have fuses at each end of the system (which is strongly advisable), they might not survive the experience either.
AnswerID: 615379

Follow Up By: Idler Chris - Tuesday, Dec 05, 2017 at 22:00

Tuesday, Dec 05, 2017 at 22:00
There is a fire risk associated with a battery in the back of the vehicle sending all those amps to a starter motor, and its a complication I do not want in my vehicle. I check my battery before any trip and replace it if at all unsure. I also carry a Suaoki G7 battery pack and have a solar regulator attached to my starting battery. So if the battery is flat and if the Suaoki fails, I just hook up the solar panel, go for a walk and just leave a bit later.
Chris
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Follow Up By: Frank P (NSW) - Tuesday, Dec 05, 2017 at 22:26

Tuesday, Dec 05, 2017 at 22:26
G'day Chris,

I can't identify the fire risk you mention.

My 2nd battery is in the back of my BT50 because there is no room elsewhere. I have 0 gauge cabling between batteries with a 300A fuse at each end, both near the batteries. Positive cabling is sheathed in split conduit, cable tied above the chassis except where there is a risk of pinch from the body or some other component and routed clear of sharp edges. Without going into excessive detail, I can manually switch the system to crank and/or winch off both batteries.

I believe in normal use there is no fire risk.

In the event of an accident and a short circuit in my cabling, I expect one or both of the fuses to blow, which I hope would circumvent an electrical fire caused by my remote second battery.

Am I living with a false sense of security?

Cheers

FrankP

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Follow Up By: Idler Chris - Tuesday, Dec 05, 2017 at 22:56

Tuesday, Dec 05, 2017 at 22:56
Hi Frank,

No you are not living with a false sense of security IMHO. You have identified the risks very well. I can't live with that word "hope", Mr Murphy is just too active for me to give him any assistance.

Cheers,

Chris
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Reply By: bellony - Wednesday, Dec 06, 2017 at 00:16

Wednesday, Dec 06, 2017 at 00:16
I run a Redarc SBI12D that splits the standard batteries on a 100 series cruiser. I did not want to run another cable to the dash with a momentary switch for the start assist feature so I just use male/female insulated push together terminals on the blue wire and tuck them away in a 150mm section of split tubing. In the case of a flat starter battery I just have to pop the bonnet, push the 2 together, start the car and then disconnect again.
Simple, as long as you have the wiring to handle the current as already discussed.
AnswerID: 615383

Reply By: Injected - Sunday, Dec 10, 2017 at 00:59

Sunday, Dec 10, 2017 at 00:59
What i do for customers that want a VSR SBI 12 fitted ( with decent cables to allow jump starting) is get a small aligator clip, attach a small piece of cable with a flat spade male terminal on end. Plug onto the blue wire and attach aligator clip to the Aux post of VSR and that will close the circuit allowing to jump start.
AnswerID: 615451

Follow Up By: Allan B (Sunshine Coast) - Sunday, Dec 10, 2017 at 08:33

Sunday, Dec 10, 2017 at 08:33
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The "Aux post of VSR" that you refer to is I guess one of the smaller posts at the base of the SBI-12. These posts are in fact the terminations for the coil of the solenoid. One of them (the RHS) is in fact earthed so your technique would work, but the user needs to remember which post or there is a problem.
Surely it would be simpler to permanently attach to that post a short cable with a male spade that could simply be plugged onto the blue wire when required?
Cheers
Allan

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Follow Up By: Injected - Sunday, Dec 10, 2017 at 19:47

Sunday, Dec 10, 2017 at 19:47
I was refering to the Aux post at the top of the VSR, one is Batt ( from crank ), the other is Aux ( to aux/ second battery).
if you connect the blue wire to a power source ( from second batt ) the solenoid closes and allows jump back.
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Follow Up By: Allan B (Sunshine Coast) - Sunday, Dec 10, 2017 at 23:16

Sunday, Dec 10, 2017 at 23:16
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I am sorry 'Injected', I misunderstood your definition of "Aux post".
I was also wrong about earthing the blue "over-ride" wire to cause the solenoid to close. From memory I thought it was necessary to earth that wire but on checking I find that it needs to be connected to a +12v source (the aux battery stud) to activate as you said. My sincere apologies.
Cheers
Allan

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Follow Up By: Injected - Monday, Dec 11, 2017 at 01:02

Monday, Dec 11, 2017 at 01:02
No worries Alan, thats whats good about forums like Exploroz, its all about sharing and learning.
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Follow Up By: Top End Az - Monday, Dec 11, 2017 at 16:26

Monday, Dec 11, 2017 at 16:26
Thanks guys; I haven't been on here for a few days but Injecteds idea could be the go looking at it. I will let you know how we go with it
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Follow Up By: Allan B (Sunshine Coast) - Monday, Dec 11, 2017 at 19:36

Monday, Dec 11, 2017 at 19:36
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Incidentally, if anyone is considering using the Redarc SBI-12 (or similar) with a remote button to engage the solenoid for jump-start or winching, the Redarc wiring diagram falls short. They show a light-duty wire from the Aux Battery post of the SBI to a pushbutton and returning to the blue wire of the SBI.
What they don't show is a fuse in that positive from the SBI to the pushbutton. Without it, if a short was to occur on that light-duty wire, there is no limitation of the current and you could well have a fire in your vehicle. A 5A fuse close to the SBI is required.
Cheers
Allan

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Follow Up By: Dean K3 - Tuesday, Dec 12, 2017 at 11:28

Tuesday, Dec 12, 2017 at 11:28
Allan,

that comment you made about fuse sound very much like same issue I have with the national luna smart solenoid system. Blown two of those non replaceable fuses.

Thus why if needed I drag out jumper leads hook up directly then disconnect immediately after engine started.

saved my goose 2 or 3 times now doing it this way -not that I like making a habit of it
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Reply By: Member - Phil G (SA) - Thursday, Dec 14, 2017 at 23:18

Thursday, Dec 14, 2017 at 23:18
It would be way simpler to just buy a Lithium Ion jump starter and carry that. I have one that is the size of a book weighs very little and will jump start a Landcruiser V8 diesel. Cost $250 at a caravan show late last year. Can also be used as a power pack. Awesome bit of lightweight kit. Also saw them for sale at ARB.
I recently got rid of my Redarc Isolator as it has been relegated to my antique collection!
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