duel batter system question: DC/DC or Isolator

[FNQGU]

So in the case where the 2nd battery is a couple of meters from the starting battery (ie. in the rear of the wagon/ute) you would mount the dc-dc charger in the back, not under the bonnet near the starter?

[Cuppa]

So in the case where the 2nd battery is a couple of meters from the starting battery (ie. in the rear of the wagon/ute) you would mount the dc-dc charger in the back, not under the bonnet near the starter?

That is correct.

[FNQGU]

And if the mounting point is starting to get out to say around 1.5m (cable length), I am guessing thicker cable even things out? My 2nd battery is actually mounted beneath the vehicle up under the passengers side, about mid-way back. I was thinking of mounting a dc-dc charger down low on the cargo barrier behind the rear seats in the wagon to avoid water related problems.

Also, if you run 6 B&S cable through to a camper trailer with two further batteries, is there then a need for a second one of these units on the camper?

[Cuppa]

Thicker cable is the way to go. 1.5 metres is not much of an issue, but remember you have to double the distance to account for negative & positive. (Total length). Check out the table at the bottom of this page it gives recommendations for wire size to ensure minimal voltage drop for given current carrying capacity & cable length. You will see that for 3 metres with a 40amp charger the recommended cable size is 10mm2 (not including insulation as is the case with autocable). This equates to somewhere between B&S 6 & 8. Go to the next size up ie. 6 B&S & you'll be good.

In theory if you connected the battery under the the vehicle to the two in the camper trailer in parallel so that the charger saw them all as a single battery you would only need a single charger, but I think you would want to be using much heavier cable which could become both expensive & impractical. If you consider that many under bonnet dual battery systems have the batteries joined using 35mm2 cable for relatively short runs I think you would want to go much heavier to the camper trailer if setting up in this way.

Best to have a dc to dc charger in the camper trailer taken from the crank battery. Depending upon what your intended use of the under vehicle aux is, you might be ok with just an isolator solenoid eg. If using it when winching with the motor running. But if planning to have say, a fridge in the vehicle running off it I would say that you would need two dc to dc chargers.
You could have solenoid between crank & under vehicle, & then dc to dc between under vehicle & camper trailer batts, with the dc to dc mounted in the trailer, but depending upon the cut in & cut off thresholds of the solenoid, you could find that you were not getting as much to the trailer as you could with a direct connection between crank & trailer batts (via dc to dc in trailer) if you follow me.

For crank to trailer .... If you are buying a Redarc charger ask them for their recommendation on cable sizing, their technical service is excellent (a good reason for buying their gear). .... According to here they recommend 16mm2 between crank & BCDC1240 when it is is mounted in the rear of the vehicle. heavier again in trailer. 4B&S is about 21mm2. Welding cable is more economic

When working out how best to do my set up Redarc were great. I emailed them a copy of my proposed wiring diagram & they promptly got back to me with their comments.

Cuppa

[FNQGU]

Beauty, that was great. Thanks for the links too. I have been thinking along those lines but you have saved me further research and firmed up the plan. I currently run a 40 litre fridge in the wagon from the aux battery, and will probably run a 60 litre fridge in the camper from a single 120Ahr Full River battery. I have a 1kva Honda but would like to stop taking it in the future, or at least stop relying on it, and will be working out the solar input from other info on the site (mostly also from you) and some cracker info on ExploreOz.

That, or I am going to lash out on one of these... http://patriotcampers.com.au

Nice and simple with enough essential kit, but bullet proof by the looks of it and heaps of clearance.

[megatexture]

I have never really understood why Ctek have chosen to limit the D250s Dual to 20 amps, requiring the additional Smartpass unit to increase charging current, when it could easily all be done in one box. There is no doubt that Ctek is quality gear, & I have never heard anyone complain about their choice to buy the 'Dual'.

It was my lack of understanding for the need for two separate units if more than 20 amps charging rate was required that saw me choose the Redarc, that and the fact that 20amp capacity was less than I needed for my solar panels output. If The BCDC1225 had been released when I was buying, I may well have chosen it, but I have since come to understand there can be good reasons for fitting a larger amp unit, so now I would still choose the 40 amp unit.

Regarding the need for a higher charging rate, this is another horses for courses issue involving pattern of usage as well as battery capacity. If ctek's Smartpass is required to boost the 'Dual's' charging amps the Redarc's cost becomes competitive. Either will do the job just fine unless there is a specific need for the full 80amp capacity of the Smartpass (In which case an alternator upgrade may also be wise. Folks I know who have used their full alternator capacity for more than short periods - charging LiFePo4 batteries - have found alternator life to be short).

With bignev's future camper plans I would think that he may be better off considering a higher than 20amp charge rate.
The higher the rate the faster it will bring the battery (ies) up to 3/4 full, (less driving time) but after that charging amps are reduced & an 80amp or 40amp charger will only be able to put charge into the battery at the same rate as a 20amp charger. The important part is that it (a smart charger) will still put in, which an alternator alone won't, but because the higher amp charger had put the bulk charge in sooner, it has more opportunity during your driving time to bring the batter(ies) up to full, & /or allow the solar to do so before it gets dark. The 20amp unit is better suited to longer driving periods and/or smaller battery capacities. IMHO of course.

Oh, and another advantage of using a dc to dc charger (aka battery to battery charger - B2B) of whatever brand in a dual battery system is that unlike an isolator solenoid it allows for a different charging voltage for the aux battery, useful if you have two different battery types ( Eg. a 'wet' crank battery and an AGM aux battery) where each have differing charging requirements. Basically another factor in treating the batteries more kindly to extend their lifespan.
Their ability to boost available voltage is also useful in accommodating voltage drop from long cable runs (eg. To camper trailer) & thus should be mounted close to the battery which is being charged.

Cuppa

The dual is recommended for battery banks between 40-300ah and they recommend the smartpass for banks up to 800ah

Bcdc 1220 50-100ah
Bcdc 1225 75-200ah
Bcdc 1240 200> ah

[Cuppa]

The dual is recommended for battery banks between 40-300ah and they recommend the smartpass for banks up to 800ah

Bcdc 1220 50-100ah
Bcdc 1225 75-200ah
Bcdc 1240 200> ah

Yes, but why not have a single 'box' suitable for different charging requirements, instead of requiring 2 boxes if you want to have an output greater than 20 amps. I still don't get why the need to do it this way.

I also think that whilst the suggested matching of chargers to particular battery capacities may suit some folk, they certainly won't suit all. It's too simplistic & cannot take everybody's desired pattern of usage into account. To do so would have to include how often, & how long the user will drive for, & whether they have solar & if so how much. This would obviously be hugely variable, hence the simplified recommendations, as to do anything else is likely to harm sales, because many folk don't know what their style is, or will be. So the recommendations IMHO are as much about marketing as they are about reality.

The quoted recommendations support my suggestion that manufacturer's recommendations can only be approximations

Ctek's 20amp output - up to 300Ah
Redarc's 20amp output = up to 100Ah

Big difference!

[megatexture]

It may be a case of " would you like fries with that" or there maybe something behind the scene that makes the diffrence for example the redarc 20amp is a 3 stage charger whilst the ctek 20amp is a 5 stage, the two extra stages are desulphation and pulse stage so not sure how that would make a huge diffrence ..

[BigRAWesty]

Once again some top reading. And if you stick to those future plans I'd definitely follow cuppas advice.
If however you are currently doing just short trips a straight isolator is still an option and when time comes with the camper trailer add the dc-dc charger to that.
A little more expensive but it gets you a dual setup cheaper to start, then optimum charging in the camper when it arrives.

I personally went an isolator setup with jump start override.
Use 25mm2 welding earth wire to join the batteries. Got 6mm twin core running to the back for the fridge and other bits..
Now for the fun fact..
It cost sub $200... Lol.
Redarc isolator is $120 ish.
Cable $20..
Split conduit to hide the bright orange cable.. $10

Battery.. Free.
Check out some heavy industrial joints like truck companies, or earth moving joints.
Most machines run 24v and as soon as the batteries look like dying they change em out.
Now if you find a place grab a couple as you may get a knackered one. But the one I'm using now can carry my fridge and camping lights for 2 days by itself.
Can't complain for free.
And like I said, save the cash for when you do get the camper and then setup for your longer stints and lash out for more top end gear..
That's my opinion anyway..

[gaddy]

Just thought I would add the results of my usage over 7 days at straddie with no solar or generator charging

The car has a projector 150 isolator
7 days running a engel fridge driving no more then a hr or 2 each day and and sterio on evey night , on the last day the car sat for 4hrs in the sun locked up 10 min drive to the barge 1 hr barge ride and 10 min drive home , fridge ran all last night , checked second battery voltage at 9.30 this morning 13.5 volts

The camper has n70zz marine . No charging system at all and no led lights
The lighting consists of 2 x 12 volt down lights above the stove and sink and 2 by standard roof lights , and also runs a 500 gph bilge pump for the shower , when I checked the battery at home today showing 12.5 v , we are quite ecnomical in what lights are on after dinner and washing up usually only a 12 volt fluro

So all in all very happy and will not be changing anything anytime soon

Gaddy