Independent battery management systems

Brian is on the money here. I strongly second his advice, but add my own - don't skimp on cable size when wiring up. If it's all under the bonnet, 8 AWG would be ok but 6 is preferred. If your aux battery is down the back, then definitely use 6 AWG. Mount the DC-DC charger as close as reasonably possible to the target battery.

Cheers

Hi Frank
Thank goodness I already have 6 AWG wiring in place so one less problem
Dave

Hi Brian

Thanks for your reply. I hadn't considered the DCDC charger as I must admit I dont know too much about them but it sounds like an easy solution. My understanding is you power the charger from the alternator or crank battery which is connected to the aux batteries. Is it suitable ( depending on size of DCDC charger used) to also charge a battery in a camper trailer via the Anderson plug whilst driving? Also is some form of isolator required to prevent crank battery drainage?

Cheers Dave

The DCDC charger can be connected directly to the crank battery, it has isolation included, so nothing else is required.
It should be positioned as near as practicable to the battery that is being charged in order to keep voltage drop to a minimum.

Brian, that only applies to some of them. A lot will work down to 9 V input, they will keep working until they destroy your cranking battery. Before you purchase any of these devices get on the web, download all their literature and study it carefully first. My device requires isolation, I do it with an ignition switched relay.

I thought that it was pretty obvious that I was referring to the Redarc unit as I stated that I know nothing of the other unit mentioned.
The Redarc unit will switch off if it detects an unloaded voltage of less than 12.7v.
This is the isolation to which I was referring.
There may well be lesser units out there that do not have this protective feature.

I have an DC/DC charger on the caravan, connected through an isolation relay.
But automatic isolation is not necessary – it’s a simple matter to switch the charger off when your alternator isn’t running – same applies to the van’s 3 way fridge.

Nomadic Navara,

"A lot will work down to 9 V input, they will keep working until they destroy your cranking battery."

In the great majority of cases that is simply not so. Because they will work down to 9V doesn't mean they'll pull your crank battery down to 9V.

In general they're designed to be powered by a charging system with a running alternator. The cranking battery will never go flat. This is beccause, most (but maybe not all - there may be exceptions) of them are designed to be isolators. They detect when the alternator is not running and switch themselves off, so they won't pull your crank battery down.

In regard to operating down to 9V etc, they are designed to accept the inevitable voltage drop in long cable runs under the high load they might impose. A long run of marginal cable at a 40 amp load could easily cause a 3 volt drop in voltage or more in a bad installation. So the source voltage could be 12, but at the charger it could be 9. That's what they are designed to cope with.

Cheers

Whole point of have dual batteries is so you don't flatten the starter battery, having both batteries joined together and running accessories of them whilst stopper if fort with danger.and is a stupid way of doing it....... Try push starting a 4wd!

Hi Dennis

Unfortunately there is no room under the bonnet for extra batts as its a 2.5 d22 Navara so aux is in the rear.

Cheers

Merry Xmas olcoolone
Our local auto electrician sees a lot Voltage Sensing Battery Isolator problems.
I have seen an ARB relay fail on someone else’s 4WD
I had a Redarc on mine which malfunctioned outside its warranty period.
But I must say Redarc impressed me with their aftersales service.
They express delivered a replacement, free of charge - without me having to supply proof of purchase. I never installed this, as I then went to a system of 2 Marine Batteries – directly connected – my fridges are connected in series with a voltage cut-off relays – I also monitor my voltages keenly.
With this arrangement I can run my batteries to a lower voltage and still get a start than I can on a single isolated battery. Both batteries are worked and charged equally - this results in a longer life.

Hi itsdave
If you can’t fit 2 under the bonnet the only way to go, in your situation, is a DCDC Battery charger

Hi olcoolone - I forgot to mention
Under your system if you flatten your battery through the car’s standard accessories, ie its lights or radio, the automatic battery isolator won’t save it – so good bye to your starter. In that case you had better be sure your auxillary battery is up to starting the car.
I prefer my stupid system.

I'm another advocate of the Keep It Simple Stupid system..apparently.
Has worked a treat for many years for me. I run 6, 8 and 10mm wire to the rear for different uses, lights, anderson, HF, inverter, and a winch at the front. I got sick of replacing stuffed up battery isolator and protection systems.
Merry Christmas olcoolone.

Thanks for the reply Mick

Had a read of your blog which made interesting reading. I'm starting to think that the Redarc BDC1240 could be the way to go as it would be capable of charging both my aux batts. Also liked the simplicity of it.

Thanks for your help
Dave

Dave,

If you're thinking of connecting your vehicle aux (front of the tray?) and your CT battery in parallel through an Anderson and then charging them from one DC-DC charger, then I don't think that's the way to go.

It might work in a fashion, but IMO it will be a severe compromise. I don't think you'll get the best daily use from your batteries, and probably get reduced calendar life as well. There are many reasons why this is so but this post will get pretty long if we go into that now. But feel free to ask if you want more info.

If you can afford it go for two chargers, one for the ute tray, one for the CT. A quick look at eBay suggests you'll get a Redarc 1220 for $320-ish and a 1240 for a hundred bucks more. If you bought two of whatever you might get a deal. And don't get stuck on Redarc - there are other brands to consider - Ctek and another local product, GSL spring to mind.

It would be wise to consider now if you're going to add solar, and if you did, would it be the CT or the ute or both? That may influence your choice of charger. Or you could have a separate solar reg and hook it up to whichever battery you want.

Be wary of going for the highest output amps, like 40 or more. Many (but not all) deep cycle batteries like to be charged at only 10 to 15% of their amp-hour capacity. So a 100Ah battery might only want 10 or 15 amps at the bulk stage, not the 40 that a BCDC1240 charger (or a conventional alternator system) might deliver. But then, there are other batteries that will take all you can give them. What your batteries want will be one consideration in deciding the charger(s) you buy.

Hope this helps.

Cheers

Oops, the Ctek link doesn't work. Try this.

Where the auxillary is not located near to the starter battery - the DC/DC charger is the best option to bring the auxillary up to full charge. Don’t worry about arranging to use it as a top up only.

Ross,

"What systems disconnect the main battery from charge when it is up to fully charged?"

Rotronics does that. Charges the crank battery then attends to the aux, but continues to monitor the crank and switches back to that when needed. Clever, but doesn't take account of modern ECU or temperature compensated systems - unless you count the "cheater" diode they use.

Cheers

Frank P
I realize you can have it happening that way by why would you want the main to be intermittently connected and disconnected just because a small difference in volts is seen by a pre programmed little brain.
I would do anything to avoid that.

Me too, Ross. It was the first crossed off my list about 6 years ago.

Cheers

Ross, Rotronics claim that not having the cranking (main) battery on line means that the auxiliary battery will charge quicker. They even have a graph that shows it see this link.

They also have a bit on modern alternators not being very effective for charging batteries. see this link - they claim the Rotronics Fast Recovery Independent System will improve the charging.

Ross, your statement that the alternator (regulator ) sees the DCDC charger in the same way that it sees a discharged battery is totally incorrect.
The alternator will see the discharged battery in a similar vein for the first few minutes, but the rapidly rising surface charge on the plates of both the crank and the second battery will soon cause the regulator to ramp down its output to just a trickle (assuming no other loads).
If there are other loads, then the alternator will supply only sufficient current to satisfy their requirements with little remaining to charge the second battery.
The DCDC charger by contrast, will continue to constitute a load to the regulator and will be treated as such with full load current being supplied as long as the charger demands it.
A standard alternator charge setup will never charge a second battery either as fully or as quickly as will a properly sized and fitted DCDC charger.
Any decent auto elec could easily demonstrate this fact on an alternator test bench for your complete satisfaction as to the veracity of my statement.

Whilst disconnecting the crank battery will indeed allow the second battery to charge both more fully and more rapidly, the very real danger that you face with any system that disconnects the crank battery from the alternator in order to charge the second battery is that should something go awry and both batteries inadvertently become disconnected from said alternator for even just a few seconds, then you risk destroying every computer, blowing every light globe that is on at the time, plus numerous other potential disasters.
This accidental disconnection could be as simple as a loose Anderson plug, battery terminal, corroded wire, etc.
Not a risk that I would be taking.

Hi Leigh

I tried ringing Rotronics but the no is not available. Also have been told by two businesses that supplied and fitted the Rotronics system that they couldn't get hold of him and had also heard he had closed. Unfortunatly no one can get hold of him to confirm this.

Dave

Does seem odd though as it costs to have a website hosted so would have thought they would have pulled it down?

G'day Peter and Merry Xmas.

Have you ever tried to get a Ctek fixed? It's for that reason that locally made and supported products are a major consideration for me.

Cheers and all the best for the festive season.

Mick

Merry Xmas to you Mick as well.

I know what you are saying most stuff is throw away these days.

The vendor of the Ctek is also the repair agent and includes an additional 2 year warranty above the original 2 years so I think it is a pretty low risk option.

Have a good one

Peter