I have installed a DC-DC charger in our Mazda BT50 that feeds an auxiliary battery in that vehicle. I did this because of the possible problems occurring with "smart" charging systems. Before you say the BT50 has not got the smart charging system, I read some forums and there seemed to be confusion in this area, so I went back to the dealer and asked. Two tech blokes answered in the affirmative, so I installed the charger. (P.S. it hasn't got a smart charger! according to another Mazda Rep).

The purpose of the aux battery was so I could wire all my "dirty" electrics to it, leaving the vehicle battery untouched and always charged. But the aux battery was also to augment the caravan 12volt power by 1. the battery itself, and 2 solar panels on the tow vehicle.

At this stage I will say that following advise and information on this forum, we had a Waeco 12v inverter fridge fitted, i.e. not three way, so we are totally dependant on power. OK with me so far?

Prior to the van pick up I was given the wiring diagram from Jayco and told to wire the plug up accordingly. Which I did. And have followed all instructions to the letter.

Connecting the van up, all worked as expected. Apart from the RCD tripping and having no 240v ac (happened weeks later) so at the moment all we have is 12v mainly from solar.

Now to use the power from the ute when parked, I made up a 5m extension lead, and wired it into pins 9 and 10 (which is labelled fridge supply), this was on the expectation I could utilize the vehicle aux battery, and even run the vehicle motor to have power when "Free camping" if things got bad.

How ever after connecting up the ext. lead, I found nothing happening, and traced the problem to under the van where the cables from Jayco's "tow loom" have nothing to connect to. Enquiring about this I am now informed that because I have a 12v Waeco fridge, these cables are not required. But what about using AUX power from the vehicle, sorry no can do. Your van only uses pin 2 to power auxiliaries. I cant charge the batteries using that. However pin 2 feeds the brake away battery charger, and the ESC system, it also feeds into the auxiliary terminal of the ST35 power supply, i.e. the aux in, and has fuse protection of 20 amps.

So apparently Jayco are doing what they say I can't do, but it is set p for the ST20-111, that can only except 20 amps. But the input is uncontrolled. Also I think the wiring is undersize.

Here is a brief on the Setec power supply:-

ST35 - III Series POWER SUPPLIES

Introduction

These instructions detail the installation and operation requirements for the ST35-III

power supply. These have been designed for operation in RV?s providing a DC power system, with optional battery back up.

The units operate from 240Vac and provide an isolated 13.65Vdc output at 35A for powering the load and charging of batteries.

All the necessary protection and operating features for the load and batteries are provided.

An optional DC input is also provided to enable charging of batteries and powering of the load from an external +13.8V DC power source.

Auxiliary Power Input: The power supply terminal ?Aux In +VE? provides an alternative option for powering of the load and float charging of the batteries when mains voltages are not present.

This input is to be powered by a suitable +12V system. (i.e. CAR). The voltage of the auxiliary power source should not exceed 14.8 volts.

When operating via the external input, current and voltage control for the battery must be provided from the external source. The ST35-II does not provide battery current limit or voltage control when operating in this configuration.

Trickle Charge is still functional when powered through ?Aux In +VE? terminal of power supply.

Suitable fuse protection must be provided for this input. A fuse rating not exceeding 30 Amps for ST35-II must be used.

Reading these instructions, I read that I can feed in from the vehicle AUX battery via pins 9 and 10, but I need to have some form of voltage and current limitation. The house batteries are 2x 100 Ahour, and it is a concern that if these batteries were low, connecting up the vehicle battery may result in a high current transfer.

Am I correct in assuming I have to use another DC/DC charger between the systems? or if the circuit is fitted with a 30 amp fuse, are the batteries supplying and feeding self regulating?

Hi
I would simply leave the Setec as it is & find a way of running two HEAVY cables, something like 3B&S [pos & neg] from the Aux battery in the tug to the house batteries in the van.
Just make certain they are well protected from cutting or rubbing through the insulation
Let the DC/ DC charger charge them when driving
There should be no current from the Crank battery when the engine is not running.
The Dc/ Dc should only start charging AFTER the alternator starts generating & suppling power
Also The DC/ DC charger will limit it's output current to what ever it is rated at.[ even with a dead flat battery]
Make sure you put good quality fuses in the positive feeds [ As near as practical to each battery]

Just remember that if the tug connection is still made when camped you WILL be drawing power from the Aux battery , unless you have an isolator in the circuit or unplug the connection between van & tug

Thankyou "oldtrack123" Just going over your points.
"I would simply leave the Setec as it is & find a way of running two HEAVY cables, something like 3B&S [pos & neg] from the Aux battery in the tug to the house batteries in the van.
Just make certain they are well protected from cutting or rubbing through the insulation". This is basically what I have done, made up a heavy cable ext. lead with m & F Narva 12 pin connectors at each end so I can connect up the vehicle when it is parked beside the van. I have wired them to pins 9 & 10, which are just terminated under the van, folded over and taped. The cabling for this lead is from the aux battery mounted on the ute, fuse protected and is heavy cable. So is it OK to couple the three (two house& the aux) even if there is a charge imbalance, would the batteries self regulate themselves.

Let the DC/ DC charger charge them when driving Yes that would occur.

There should be no current from the Crank battery when the engine is not running. The crank battery is isolated from this circuit by the DC/Dc charger.

The Dc/ Dc should only start charging AFTER the alternator starts generating & suppling power
Also The DC/ DC charger will limit it's output current to what ever it is rated at.[ even with a dead flat battery] . Correct & Correct.

Make sure you put good quality fuses in the positive feeds [ As near as practical to each battery]. Done

Just remember that if the tug connection is still made when camped you WILL be drawing power from the Aux battery , unless you have an isolator in the circuit or unplug the connection between van & tug. That was the reason for installing the aux battery, to augment the van 12v system particualy in the winter time when solar would be at its minimum.

So would I be correct, that you are suggesting that I disconnect the lighter gauge wire (13 AWG) being feed from the aux battery to pin 2 that feeds into the aux input of the Setec unit, which would then only feed the brake away battery and power the esc. system (leaving them still powered). And then utilise the heavy cable (8 AWG) coupling via pins 9 & 10 (heavy duty pins) to feed the two house batteries direct from the aux battery in the vehicle, that there would be no need to have voltage or current regulation between the vehicle and the van?

I might add that this will also mean for me a re-wire at the aux battery, it is not the neatest of jobs (factory install), I cannot close the lid on the battery box because of the birds nest.

-- Edited by iana on Friday 1st of April 2016 12:06:46 AM

My gawd my head is hurting. I have put an Anderson plug on top of my car battery and run a lead direct to the auxiliary batteries. When I turn the engine off I unplug the lead. Works for me but not everyone of course. I suppose I'm saying it seems to be making it hard work. Maybe a member will have an easier way. Luck with it anyway. Woody46

Forget about trying to reliably charge house batteries or running a fridge via your trailer plug. The wiring is just too small. The cross-section of copper is what is important, not the outside diameter of the insulation.
Anything smaller than 6B&S /6AWG is a waste of time for wire runs over a couple of meters.

Unlike 230V AC, the voltage drop with low voltage DC is so severe that even with 6AWS the 14volts at the tug battery will be down to 12.8 under load after 5 to 6 meters and you'll never get a full charge into either your aux or house batteries.
There is a relationship between resistance and current with voltage (Ohms law). The lower the voltage the more severe is the problems of long run resistance and DC over AC is worse still.

If your extra aux bat is not in the van next to the house bat getting power from the van's DC-DC charger (ie, it's in the back of the fourby/ute) then you need another DC-DC charger in the back of the fourby/ute next to the aux bat.
Most modern DC-DC chargers have an isolator function inbuilt when the charging voltage drops below a certain level. If yours doesn't then you need a battery isolator next to the tug battery.

For your van house batteries it is better to mount a 50A Anderson to the rear of the tow vehicle and the van drawbar with a short flylead to join the two.
You only need to run single heavy duty wires for the positive from the batteries to the Anderson plugs. The entire vehicle chassis (tug & van) are heavy duty negative rails, so use a short run heavy wire from the Anderson plugs negative to a substantial connection on the chassis for both the tug and van to lessen the overall circuit length. Do not rely on the towball coupling as a suitable negative connection.

Always the DC-DC charger goes right next to the battery so if you have one in the BT50, good, but if you also have to run that aux battery power from the back of the fourby to the van then you run into that long run wire problem there too.

Just consider voltage drop and wire sizes to be interrelated.

Next consider the size of your battery storage in Amp Hours at 50% because your battery voltage drops as the stored charge drops, and taking it below 50% can drastically shorten the battery lifespan (cycles).

Next consider the charge you can put back into your battery Vs the charge you are going to draw from the battery.
7.5 amps from the fridge overnight (12 hours of no sun for solar) is 90 amps. This means you need approx 200Ah of storage just to run your fridge overnight, but, a compressor fridge is not usually running its compressor full time. Compressor fridges are cycling on and off depending on the internal temperature requirements of the fridge, the ambient air temperature and how you use it, so no-one can say for sure that 'X' model fridge uses exactly this 'Y' amount of power.

Inverter fridges are considerably more efficient than the older 3way absorbtion style fridges but you have to have enought stored juice to run them, whereas a 3way can be switched to gas when free camping.

Thank you Hylife. I knew someone could put it in a way even I could understand. One little extra thing I'd like to say when the night temperature is around 25 minimum( FNQ ) fridges work damned hard. Once again thank you. Woody46

Woody46 wrote:

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Thank you Hylife. I knew someone could put it in a way even I could understand. One little extra thing I'd like to say when the night temperature is around 25 minimum( FNQ ) fridges work damned hard. Once again thank you. Woody46

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That they do. Even 3way fridges (absorbtion) running on gas struggle to maintain cold when the ambient temp is high without some additional venting/fan assistance.

After much studying of the Setec ST-III manual, and from reading different comments on the web, I have concluded that the DC external input, only supplies power to the 8 x load outputs, i.e. lights, pumps, TV etc. If 240v ac is not detected, the power supply will only allow trickle charge to the battery (0.08A). In other words the only time the Setec ST-III charges the house battery is when it detects 240v, if it doesn't, it only allows trickle charging to occur.

This explains why a lighter gauge wire 13 AWS is used as the 12v supply from the tug. (I have two 12v supplies from the tug)

The 12volt fridge is connected direct to the house batteries in the van (as is the solar). Since the fridge will run on quite low voltage (cut out is at about 9.7v) I see no reason why I can,t continue with my present set up (plan), but I have to forget about charging the house batteries in the van. This part of the exercise will be just switching the fridge over from house to the vehicle aux battery. The house batteries will continue to be charged by solar only or by 240v via the ST-III power supply when at a powered site.

I also see no reason why I cant take the supply from the vehicle aux battery (via the twin core extension lead) to the DC external input, this will enable us to have lights, etc. Because I am not trying to charge any batteries, the voltage can be lower and not cause problems.

I did measure voltage drops on the system today, but the load I used was 24v (my mistake) I'll get a 12v load and repeat the test to get some idea what the voltage drop actually is.

I am also planning on getting some portable solar panels, but I want to get this part of the journey sorted first.