battery cable size

Either 2 or 3 Gauge (2 B & S) would be good Toyotas use that between the twin batteries in the 100 ser..
You will need the little push button in the cab to make the isolator do that.

Where is the battery going, to be 4m away

If you only want it to help start if teh main battery goes flat, I suggest you would be far better off with one of those LifePO4 battery packs that fit in your hand.

For a couple of hundred dollars you will save carrying 25-30Kg around and the cost of wiring and an isolator.

Regards Philip A

How often do you get a flat battery?

For me it is a extremely rare event, so I didn't wire up the car to have redundant systems like that. To make it work you not only have to have heavy cable that can cope with the power, but a relay that can handle a large number of Amps running in both directions, as well as fuses that can take it too. After the cable has snaked its way around the engine, cab and chassis you probably have a meter or more than is ideal.

What I did do is have the cable enough to charge the 2nd battery and a heavy earth cable from the 2nd battery to the chassis.

If I need to self jump start, then I run a single jumper lead between the positive terminals and start it that way.
9 times out of 10 you are just giving the starter battery a helping hand, rather than full amps to start from the 2nd battery, for that 1 in to where the main battery is fried, then you may as well just replace it with the 2nd battery and head for town.

... and get the dealer to switch off the smart charge 'feature'. My BT50 doesn't have it and we have nowhere near the battery issues that Rangers seem to have.

Nick,

A second battery to run a fridge, etc. is usually of a deep cycle type, providing lower amps over a long period and a deep cycle battery is not generally suitable for use as a starter battery which requires high amps for a short period.

You really need to determine what your primary use is for each battery and obtain a suitable battery type to use for each specific purpose.

There are people who will insist on a particular size and so forth ...... but auto electrics is always a compromise.

lets start with the starter .... most small diesel starter motors draw in the range of 100 to 150 amps at stall will a battery at full voltage.
The actual cranking time is very short in terms of electrical loads.
The actual cranking current may be considerably less than the stall current perhaps as low as 60 to 80 amps.

if you are considering winches, most have a 6ish Hp motor that will draw in the range of 400 amps stall and a couple of hundred amps on typical winching

Factory starter wiring in most vehicles is very light compared to the ideal.

I wont talk in wire guage because we live in Australia and all calculations are made in mm2

Many commercial jumper leads are lighter than 10mm2.

10mm2 would be in my opinion an absolute minimum ..... it would be perfectly adequate for charging the battery and delivering AUX loads from the alternator.
However it's probably pushing things a bit far to crank off ...... but it would probably get you started in most cases.

Any bigger will make bugger all difference to charging efficiency, at those lengths.

I typically use 10mm2 solar twin to feed switchboards and 50 amp andersons, because it is redily available, easy to handle and the bang for buck cable in that size range

If you are expecting to crank, I would be looking at 20mm2, because it is commonly available and still not too big to handle.

It would be the minimum I would look at If winching ..... 25mm2 seems only a little bit heavier but if you are winching you need every extra but of help you can, and it still is a handleable size ...... some would go to 30, 35 50 or 70 mm2 but it starts to get expensive and hard to handle

for comparison and because people insist in talking in American wire guage.
#8 is a little smaller than 8mm2
#4 is a shade larger than 20mm2
#3 is about 25mm2
#2 is about 32mm2
#0 is just under 50mm2

As for the "ranger foolisheness" good luck with that.

cheers

Nick, I have a set of jumper leads that I made up with top quality battery clamps and 70mm welding cable. Those leads are long enough to go from my 2nd battery in the tray to the main battery.

Reason I have this setup is as Bantam and Alan have said, you really can't protect high current wiring of that length in a vehicle unless you have either unlimited space or unlimited money for the protection. A HRC fuse of say 600a will be around 200mm long

My fridge battery is an agm and I am not worried about it starting the engine if needed, so far I haven't needed it.

In reply to some comments :
The battery i'm using is one that i have moved around from vehicle to van etc, its a dual purpose 100ah .
There are a lot of good suggestion here , but keep in mind this is a simple quick fix jump start in the event that i might get a flat battery just at the wrong time .
so if i use 26mm2 size cable over ~ 4.5 meters be ok
In the L/C i have the charging wire that runs to the back of the wagon inside the chassis from engine bay & is in an air hose for insulation from any damage .
cheers

While your protection methods are commendable, there are situations where other members concerns are valid.

EG if you have cable through the chassis and you get rear ended and the chassis ends compress and short the cable, disaster looms.
One can only protect against the expected but must allow for the unexpected also.

Personally if I couldnt get the second battery under the bonnet I would use my heavy duty Jumper cables for a flat battery