Nissan Navara d40 2006 dual battery

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I agree with what Frank says, especially that the 8 B&S cable is a bit light on.
But even that size cable should not be producing the reported 2 volt drop.
Are you absolutely certain that you have 8 B&S?

As to the question of connecting an IGN signal to the charger blue wire.....
I am unsure if your vehicle has a "smart alternator" however it may overcome your problem. Your charger currently has a turn-on voltage of 13.4v and a turn-off of 12.8v. With the IGN connected to the charger blue wire the turn-on is 12.2v and the turn-off is 11.9v.
However that 2.0 volt drop on your cable is confusing the charger and really needs to be addressed.

Same as Frank and Allan.
You can use a multimeter set on volts and length of wire to connect the multimeter between the main +terminal and the Chargers input + terminal. That will verify any voltage drop in the positive supply line, should be less than 0.5 v if possible and the negative line can be checked in the same way. As Frank mentioned, the voltage drop, VD, no one wants it, is happening in both lines so the additive effect if present may be causing the issue.

Although many don't like it I use the chassis, like Frank said is an option, and an additional heavy cable from the chassis to the main battery negative and so the charger negative is also a heavy cable to the chassis. From the main to charger there is a very heavy orange welding size cable and using this method sees not much voltage drop at all.

As others have said check connections, 6B&S would have been better.

Some chargers will stop charging if the voltage drops, then check the input voltage again, if it recovers they assume the voltage drop is due to wiring loses and eventually will either ignore the drop or throttle back. Have you tried letting it run for some time to see if it adapts?

My method on my BT50 with a service canopy and aux battery in the canopy:

0 gauge +ve from starter battery through to a distribution board in the canopy close to where the battery is. 0 gauge -ve from distribution board to OEM chassis earth point close to starter battery. 8 gauge pos and neg from distribution board to the 25A DC-DC charger and from charger to battery (cable run about 1.5m total from charger to battery).

thanks guys, i found the problem, first i tested all connections and they were all good, then i followed the test suggestion by RMD, big drop on red cable but no drop on Black, so as you all suggested the 8B&S is probably undersized, even thou the charger manual recommends 8B&S upto 6 meters, even when i spoke to Projecta a while back, they send me a diagram with 8B&S black and red from main battery to the back and after many tests they asked me to do, they finally said there is a big resistance somewhere that i need to find and fix other than that everything looks fine.
since i already have a 4awg black cable, i am going to replace the red with a 4awg cable and set it up the way Frank has got his with the distribution board. i will let you know how the system works when i have the work done.

Follow-up deleted after reading RMD's Reply below

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Why would you have.... "key on and motor stopped"?
Seems not a good idea at any time.

G'day Allan
Just mentioned it because it might happen sometimes with ign left on OR the owner may have a switch on that wire to a +ve source which acts as though the ign is On. Therefore the unit would still be active to the lower voltage level of main battery and depleting main battery charge reserve. On a freezing morning in camp up in high country with main down to 11.9 v and trying to start a glow plug assisted Diesel will be interesting with little punch in the battery.

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Just one question Kam, are the voltages at the different points all being read by the one portable multimeter...... or do you have some installed meters?

Hi Allan, i use the same portable multimeter

Does the voltage at the main battery really stay constant or is it the same each time you check when the dc unit stops charging. If the alternators has a blown diode the battery voltage will reach it's normal regulated voltage, BUT under load the loss of 2/3 of current ability will see the loaded voltage of main drop to below the dc unit threshold and so it will then cycle on and off as the dc unit applies and disconnects it's inverter section as the main battery voltage level is discharged and then subsequently recharged. Without the dc unit in circuit, test alternator by turning on everything and see what voltage the alternator can hold the main battery up to. Sometimes the alt charge wire is too small in size and that also limits the output of alt to battery.

i was sure the main battery voltage stays constant at 14.4, but i run a test as you suggested, and it stays at 14.2 under load. so i assume the alternator is OK, right?
another question, when i upgrade the red cable from main battery to the distribution board at the back with a 2AWG (i am intending to upgrade to this size cable), what kind of fuse is available to fit at the main battery to connect this cable to? at the moment i have an 8awg inline fuse.

With the cable sixes you are talking, a bolt-down type type of fuse and holder such as this one by Redarc would be the best option, IMO. That one is expensive, but there are cheaper options - search on eBay or Jaycar for "bolt down fuse holder" and take your choice.

A quality circuit breaker is another option but they can be affected by engine bay heat. I had a 70 amp one in an engine bay. It continually popped with a 40 amp load. I now have a bolt down fuse.

Cheers

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I absolutely agree Frank. Bolt-in fuses are the only way to go for high capacity applications.
....I like Redarc but 70 bucks for a fuse plus holder is outrageous.

Jaycar, Narva and others do the same for about $24.

And there is an even cheaper way for the fuse located at the battery. Use a fuse without the holder, attach one end to the battery post terminal and connect the other end to the cable with a lug and short 8mm bolt & nut. Cover the fuse with heatshrink. Voila.... less than $10 all-up. Totally reliable and fused right where it should be. And that is how mine are.

I originally had a quality(?) 60A thermal circuit breaker in the engine bay but it failed, not from heat, but from fine dust ingress. Now it's a fuse! But the same CB's on the cabin distribution board are OK. Here they are good as they also perform as isolator switches.

Blade fuses?????..... I have had two fail due to oxidised contacts in the last month. No current demand, one supplies a voltmeter the other a relay.
Both of these came from stock I purchased some time ago. Methinks I'll chuck the lot out and get new quality stock. Bolt-in fuses would be a bit of overkill here.

Good result, glad we could help.

Cheers