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Old 10-01-2010, 04:44 PM
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Default Dual battery Setup Wiring and Cable Size Questions

I brought an 80L compressor fridge that draws 5w, hence the reason for the dual battery setup, (cheapie off ebay, will have to suck it and see how she goes).
I brought a Redarc SmartStart microprocessor-controlled battery isolator, also a flea bay special less than $100 delivered to the door.
I have decided to by an AGM battery, Fullriver brand off ebay either 105AH or 120AH model.
It is my understanding putting AGM batteries under the car bonnet is fine as far as safety goes, but AGMs batteries should be kept away from the extreme temperatures of turbochargers, and if they must be placed close to the turbocharger then heat shields should be used. Battery brackets for the 2.8 TD GQ I have seen are on the passenger side next the firewall and the turbo. I am therefore considering mounting the battery in the rear of the cab.
It is easier to be able to remove the fridge and battery if I decide to leave it at the campsite whilst camping this way. The run between the fridge and the Auxiliary battery will be nice and short.
I do not plan to setup my wiring so I can crank off my auxiliary battery. Worst case scenario is I can lump the auxiliary battery around to the engine bay and use jumper leads.
Here is some info sourced from the internet and my thoughts/questions

WIRING SETUP- Ground
Some wiring diagrams suggest that you should use the chassis to earth the auxiliary battery. Cant think of a bigger ground cable than the chassis.
http://www.fridge-and-solar.net/dual_bat.htm
However the above link suggests running a ground cable back to the spare battery, just in case there is a point of resistance in the chassis somewhere along the line.
http://redarc.com.au/knowledge-base/tech-tips/better-parallel-charging
Redarc suggests that it may be advisable ground the auxiliary battery in the back of cab to the chassis, then to run the ground back to the cranking battery to ensure that auxiliary gets a full charge, and the path of less resistance is not to the cranking battery. Note this is not there typical wiring diagram.
WIRING SETUP- Positive
The positive cable joining the cranking battery and auxiliary battery should also be protected against short circuit, at both batteries. This is a no brainer. fridge-and-solar.net suggests the most economical way to protect the cable is with Mega fuses of 100A at both batteries. Other sources indicate auto reset breaker is the way to go, 50amp auto breakers can be picked on ebay for as little $8.00. They have some 100amp circuit breakers too but not auto breakers.

CABLE SIZE
Dont be fooled by amp ratings on cables, max rated cable is a safety spec, you need to work out the
In theory one should be working on the calculated voltage drop. The formula is sourced from here:
http://redarc.com.au/knowledge-base/how-to.../calculate-voltage-drop
Volts drop= (Length x Current x 0.017) / Area
Length= Total Length of wire in metres (including any earth return wire).
Current= Current (amps) through wire.
Area= Cross sectional area of copper in square millimetres.

Notes:
This formula only applies to copper at 25C, voltage drop increases with wire temperature, at approx 0.4% per C.
0.017- This figure only applies to copper.
Area is in square millimetres of copper, there can be confusion on how cable size is rated, with some manufacturers stating wire diameter rather than area, some even including the insulation.

Dont forget that the current through the wire will heat it up, so even if it is only a 25C day, the wire will be hotter, which will increase the voltage drop. This will keep increasing until the cooling effect of the surrounding air on the wire balances the heating effect of the current. This demonstrates why it is important not to skimp on wire size when wiring a trailer.
http://www.motorhomesaustralia.net/cable.html

I am however confused, I have read that the alternator will compensate for this as the cable run is to charge the battery, not to run the fridge.
How many amps is the alternator on my patrol pumping out?
Rule of thumb stuff, 6B&S / 13.5mm2 is as a minimum from what I have read.
I was going to use an Anderson plug in the back so the battery would not be wired up permanently and make it easier to disconnect and not to have 2 loose battery connectors floating around the back waiting spark up. But 13.5mm2 wiring means I am looking at some heavy duty Anderson plugs and maybe solidering up my connectors because I dont have a cramping tool to take such heavy duty pins.
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Old 10-01-2010, 05:56 PM
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I went redark,2B&S,Ran from neg to neg on both batts.Also earthed aux. to the motor & chassis,100 amp inline fuses at each batt.Never had drama's. Go overkill. You can get the cables crimped at any autosparky,or as i did at battery world for nothing where i bought the lugs.Earths ya biggest enemy and voltage drop!So go the biggest,or atleast close to batt. cable.
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Old 11-01-2010, 04:44 AM
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I assume you have the aux. in the engine bay. Did you run the earth link the redarc link above where the cranking battery was only earthed via the aux?

Anybody with ideas about the best place to feed thick cable though the firewall and into the back cab. Currently looking at a hole in my firewall a cable I think is for electronic breaks for towing run, but from there my only idea is to run the cable under the plastic door sill trims into the back of the cab.

Lifted from another thread some useful info, which has made me rethink the 100amp breaker idea more carefully, and thinking of a smaller breaker, say 50amp matched to 6B&S cable ( 13.5mm2)

Auto circuit Breaker – Better than a fuse
The main problem with using a fuse is that, to avoid having to replace it every so often when there is a high current surge, you have to fit a much higher current rated fuse than you would need when you are using an auto resetting CB. This in turn then means if you want to keep the whole circuit safe, you then need to fit thicker cable. On top of this, when you take into account that a circuit breaker goes open circuit quicker than a fuse will, the CB adds so much additional safety to the circuit.
Auto circuit Breaker – Size of breaker
By drivesafe (edited/shortened by me)
“I supply 30 amp Circuit Breaker ( CB ) with our small controllers and the only time we have had the CBs tripping in and out is when the vehicle is first started and the auxiliary battery is very low and this only happens with AGM batteries. This tripping in and out is not only not a problem but can actually protect the battery. Contrary to much of the misleading hype surrounding AGM batteries and how quick they can be recharged, most AGM batteries can not tolerate full inrush currents and having the charge current limited by the CB tripping in and out is a means of protecting the battery.
The original basis for our use of 30 amp CBs is that 30 amp CBs are designed to handle up to 45 amps for up to an hour before tripping and the fact that conventional batteries ( all that were available when we started producing DBS 20 years ago ) will never take anywhere near that sort of current unless there is something wrong with the battery, so we designed our dual battery set to be not only a voltage sensing type but also to have over current protection and even with thousands of my dual battery systems in service, I have never heard of the reason for a battery not fully charging being caused by an auto resetting CB tripping in and out.
If connecting two batteries while the motor is running then in a worst case scenario using high end battery like an Optima or Odyssey, the current flow can easily be around 100 amps, depending on how low the battery is and the output voltage and current capacity of the alternator. A 30 amp Circuit Breaker ( CB ) will tolerate this sort of current for as much as a few minutes before tripping. Once tripped and cooled, it will cycle like this allowing the battery it charge.
As the battery charges, the current requirement falls off rapidly to the point where the CB no longer trips.”
VERY FEW dual battery systems ever require to handle much over 40 to 50 amps on a continuos basis and are never likely to have to handle surges of over 100 amps.
If you use a 50 amp CB, you need to fit nothing smaller than 6B&S ( 13.5mm2 ) cable and if you use a 100 amp CB or fuse ( fuses are not recommended in any DBS ) you need to fit at least 3B&S cable (25.7mm2 ) to make the cabling safe. This size cable is great size cable to reduce voltage drop to almost zero but not practical from both a cost point of view and because of the difficulty of installing such thick cable.

Argument for 100amp circuit breaker:
Yeah one for example. The owner could hear the isolator constantly tripping in and out (one inline 30A cb) and wouldn't properly recharge his aux. This was in his previous vehicle with an N70 main and a 90a/h Full River AGM aux. I never got to see the set up as he traded the vehicle and kept the dbs which I put in his newer campervan with 100A auto reset cb's (as recommended by redarc) inline near each battery. Is all working well now.

Auto Circuit Breaker – plastic back
If using CB's to link an alternator to a VSR/Battery manager/solenoid, or linking battery to battery, DONT use metal bodied CB's! Use good quality plastic bodied units, or mount the metal ones onto an insulated base material.
Failing to do so is a good way to increase your fire risk. If either of the two methods is impractical for you, use two breakers, "back-to-back", with each breaker as close to each battery and/or alternator as possible.

Auto Circuit Breaker – quality brand metal back
drivesafe : “I ONLY use Cole Hersee ( Made in the USA ) CBs and they are metal bodied and out of the many thousands I have supplied with my gear, not one has failed.”

Cable Sizes and applications
6B&S cable ( 13.5mm2 ) has a 100% cycle rate of around 100 amps and a 20% cycle rate of 230 amps.
Some advocating: 6B&S cable is fine for jump starting, just as well as 50 amp Anderson plugs can be used with the same size cable to make jumper leads.
Australian regulations for ELV wiring state that no more than 5% voltage drop is allowed in installations but for 12V installations only 3% drop is recommended which is about 0.36 volts.

Last edited by brissybrew; 11-01-2010 at 06:10 AM. Reason: more info
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Old 11-01-2010, 09:47 AM
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Bit too much info for me! My crank is earthed to motor and chassis (factory)And as i said i ran 2B&s for all my wiring also between the two negs on both batts.The way i look at it is go atleast batt.cable size wire to have the least probs.The redark's got anti spike etc. The inline fuses are only there if the wire wears through somewhere and shorts!Which iv'e also covered mine in the split condute and hav'nt had it tight where it could rub through on something!.
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Old 12-01-2010, 06:05 PM
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I just noticed I posted this thread in the wrong forum, should have posted in auto electrical. I will repost there please post responses in that thread.
http://www.patrol4x4.com/forum/showt...d=1#post343555
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