12V Auxiliary Load Shifting with DC-DC charging?

Submitted: Friday, Feb 14, 2014 at 10:23
ThreadID: 106234 Views:5574 Replies:15 FollowUps:10
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Hello all, i have been doing weeks of homework on my Dual Battery System.

Looking at using a Redarc DBDC1225 or 40 or maybe a Ctek D250s Dual
I currently have a 6Amp 6 Stage DC-DC charger to look after my 105Ah Aux battery, which really doesn't replace enough charge during driving times.

I have read the 1000 posts of Ctek vs Redarc etc etc....
But one thing has me thinking, does anyone use a Solenoid to remove your load from the Aux Battery and place it on the Main Battery with Alternator while driving?

Thus allowing any DC-DC charger to correctly sense and charge the Aux Battery with the most efficiency?

I do this manually with my 6Amp DC-DC, otherwise the 6 Amps of charging is mostly going to run my fridge and perhaps 1 or 2 Amps goes into charging.

I know if i used a large enough DC-DC charger (25 or 40Amps) this in theory should be to much of an issue, but how does the DC-DC charger correctly sense the voltage and current needs of the Aux Battery when there is a load connected?

Many Thanks

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Reply By: ABR - SIDEWINDER - Friday, Feb 14, 2014 at 10:35

Friday, Feb 14, 2014 at 10:35
What is the load, if it is a DC compressor fridge don't worry about complicating things.

Also look at the 30A DC-DC unit.


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Reply By: Member - LeighW - Friday, Feb 14, 2014 at 10:37

Friday, Feb 14, 2014 at 10:37
It can't, when the charger is in the constant charge state it won't really matter, the load will just robe power from the battery and lengthen the recharge time.

If it is a constant load it will also cause the charger to take longer to switch into its trickle charge mode as the charger looks at the current and when it drops to a predetermined value it swaps to trickle mode, if your drawing more than the predefined value the charger will never go into trickle charge mode.

The trickle charger mode in this application is really only a gimmick and more suitable for 24/7 charging of batteries, if your operating the charger in AGM mode and can wear the longer recharge time then I wouldn't bother moving the load as its really not going to cause any issues.

If however you have a Redarc, will be doing long runs and using conventional lead acid setting I would consider moving the load as the charge voltage on the Redarc in that setting is quite high and could cause problems.


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Reply By: Salty_Dog - Friday, Feb 14, 2014 at 11:22

Friday, Feb 14, 2014 at 11:22
Thanks fellas, really appreciate the input.
Yes i know i am making something that should be simple, into something rather more complicated.
I'll detail my specs so you get a better picture of the situation.

Bosch 70 Amp Alternator (Non-Smart) output is 14.22/14.23v
Main Battery - Marine Cranking Battery (In car)
Aux Battery - 105Ah AGMhttp://www.sunxtender.com/solarbattery.php?id=8
(Bulk/Absorption 14.2 - 14.4v and Float 13.2 - 13.4v)

Current In-Vehicle Charging - 6 Amp DC-DC Mutli stager

Load - Waeco CF40
@ 12.5v - 4.9amps running
@ 14.1v - 3.7amps running
When not running, the CF40 still draws 0.43amps.

I might charge a laptop or a smart phone too.

With my small 6Amp DC-DC charger, i found "Load Shifting" was paramount to get any charge back into the Aux Battery.

I can imagine with a 25A,30A or 40A DC-DC, this is not as critical, or if it were chraged direct from the Alternator via a VSR.

My theory was to to remove any "Load" that may scavage the DC-DC Charging current and also in muck up the sensing of the Aux battery.

I see i might just be making more work for myself.
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Follow Up By: ABR - SIDEWINDER - Friday, Feb 14, 2014 at 11:43

Friday, Feb 14, 2014 at 11:43
I sell DC chargers and with what have there is no need for one.

Simply fit an isolator and decent cable.


Derek from ABR
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Reply By: Salty_Dog - Friday, Feb 14, 2014 at 11:54

Friday, Feb 14, 2014 at 11:54
Thank you Derek.

As of last Thursday, i have done just that, used a 140Amp VSR with good cable and 50Amp Anderson coupling.

I just thought a proper DC-DC charger might care for my 105Ah AGM battery better?
Granted, its not in the vehicle much, only Camping times.
Rest of the time it sits in the workshop connected to 240v with the 6Amp multi stage charger on Float 13.2volts (which is 95% of the time)

I do understand that the battery may never reach 90 or even 100% with a load connected to it anyway.
Given i am mostly trying to get as much charge back into the Battery on my short 1 or 2 hour drives, the Alternator might be the best option?

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Follow Up By: ABR - SIDEWINDER - Friday, Feb 14, 2014 at 12:11

Friday, Feb 14, 2014 at 12:11
2 hour drive, the alt is the only option I would use.


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Reply By: Salty_Dog - Friday, Feb 14, 2014 at 12:36

Friday, Feb 14, 2014 at 12:36
Thanks again Derek.

If i could ask in a little more detail.
When i pack to go away, the Aux Battery is at 100% as it has been tendered on the 240V charger.

When i drive to my destination, i have started plugging the Waeco into the Vehicle power, hence leaving the Aux Battery untouched until i need it that night.
Am i kidding myself doing this?
Should i just plug the fridge straight in to the Aux Battery and plug the Anderson coupling (via VSR) in to the Aux Battery the moment i set off?
AnswerID: 526430

Follow Up By: ABR - SIDEWINDER - Friday, Feb 14, 2014 at 12:45

Friday, Feb 14, 2014 at 12:45
If the fridge is cold and cycling then connect it to the aux battery straight away.


Derek from ABR
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Reply By: Member - PJR (NSW) - Friday, Feb 14, 2014 at 13:00

Friday, Feb 14, 2014 at 13:00
I was talking to Redarc at a recent 4WD show to see what we should do to improve our system. We have three identical Allrounder 100 AH batteries under the bonnet. One is set aside as the crank, with all the usual standard loads (crank, factory car stuff etc - NO 4WD aftermarket accessories etc) with the other two in parallel for all 4WD accessories, which includes fridges, radios (UHF etc), driving lights, camping power and flood lighting etc etc. All the aftermarket stuff we load the car up with.

The ONLY exception to the rule is the winch as it is wired to the crank battery. I was advised that it is best to wire it to the battery that is directly connected to the alternator. But NOTE. We always winch with three batteries in circuit and the motor running.

There is also a separate fuse box and relay panel under the bonnet for all the 4WD stuff. No mix of any power anywhere except when the Redarc is activated by a switch in the car. The Redarc is the only link between the two battery systems.

The car is a 100 series 4.2TD, which had two batteries in direct parallel as standard.

There is a 200 amp Redarc solenoid between the crank battery and the other "pair" which allows all three to assist with cranking and winching.

He said that what we had was excellent. No need to go to any special staged charging etc.

So may I suggest that you leave what is on the main as is and reserve the second battery as 4WD stuff only. Move anything 4WD to the second battery EXCEPT the winch.


PS I did not read all the previous posts - sorry if I am taking someone's thunder.
AnswerID: 526432

Follow Up By: Allan B (Member, SunCoast) - Tuesday, Feb 18, 2014 at 11:19

Tuesday, Feb 18, 2014 at 11:19
I like your system Phil. It is in accord with my long-held conviction of "Keeping Things Simple". I built a successful engineering career on that principle.

In truth, my Troopy system with 3 batteries has become quite complex partly as an experiment and partly because.... well I have to have something to do when I'm not outback! It works well but I won't publish the detail here as it will probably attract all sorts of criticism from some.

I did consider that the Redarc dc-dc chargers would look after the AGM auxiliary batteries better than simply the alternator, but I have no evidence of that unless I get more than 5 years from them. (I wasn't getting more than a couple of years from AGM's before the dc-dc)

I may not even wait until the AGM auxiliaries cark it before changing to your system. Might be a better way to fill the days than sitting on this forum! LOL

Phil, how have you found the "Allrounders"? How long have they been in service in your vehicle? Are they shielded from engine heat? Any other comments you would like to pass on?


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Follow Up By: Member - PJR (NSW) - Tuesday, Feb 18, 2014 at 15:04

Tuesday, Feb 18, 2014 at 15:04
The batteries are three years old and no hassles at all. I can't say how much work was involved in splitting the system as I did not actually do the work. I was told that it was easy though. The biggest hassle was finding a good battery carrier to hold the third battery. ARB had one that fitted where the fuel filter was down close to the left hand firewall. The filter was moved to in fron t of the carrier and the Redarc 200 amp solenoid was mounted on a bracket on the side of the ARB carrier. It does not look like a difficult but that is a bit like hindsighrt as it is finished. The compressor was moved to a voide at the side of the RH drawer. When using the compressor I lift the void cover just to give the compressor that bit of cooling.

Lots of 50 amp cables run from a new fuse box under the bonnet behind the air cleaner and away from the cars stuff, all the way to outlets in the back seat and right down the back near the compressor and fridges and some flood lights. All with their own 15 to 30 amp fuses with LED indication of a blown fuse.

What else do you need?

But there is one problem that is not evident until later. The VMS box running on the crank rebooted when the car was started. There is a simple fix if you want it.

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Follow Up By: Member - PJR (NSW) - Tuesday, Feb 18, 2014 at 16:34

Tuesday, Feb 18, 2014 at 16:34
Stick to emails, not messages, for a tick please Allan. Then we won't get off topic.

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Reply By: Salty_Dog - Friday, Feb 14, 2014 at 13:18

Friday, Feb 14, 2014 at 13:18
Thank you all.
Next outing it will using the Alternator via VSR.

Yes, i always pull my fridge temp and contents down to 2 degrees before i pack it in the Vehicle (when i have time anyway)

I will monitor the results and charging rates using this method.
I undertstand ambient temp plays a HUGE role in my Battery draw from the fridge.
Last weekend it was in Echuca, 42 degrees, Vehicle locked all day, little ventilation, cabin temps were maybe 45 degrees....
When i got back to my Vehicle, the Fridge was still icy cold, but the Fridge compressor duty cycle would have been near to 85% of not more.

That 105Ah battery normally gets me 3 full days running the CF40 (Battery at 35% SOC) with out any form of external charging (not ideal)
In those 42 degree temps and conditions it lasted 1.5 days and the Battery was dangerous low at 15% SOC.

Hence my new concern to get charge back into the Battery fast.

I realise to solve this issue, i either need to add an additional 105Ah battery or use Solar during the day.
Sadly i was away from my Vehicle, so the idea of leaving a 200Watt panel out isn't the best.
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Follow Up By: Member - LeighW - Friday, Feb 14, 2014 at 13:26

Friday, Feb 14, 2014 at 13:26
Running the fridge with a simple VSR will have virtual not impact on the batteries charging I wouldn't bother precooling just pack with you frozen food etc and and plug it in.

The simple VSR will charge your battery much faster than a 2oA charger would.


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Reply By: Salty_Dog - Friday, Feb 14, 2014 at 13:56

Friday, Feb 14, 2014 at 13:56
Thanks again fellas for the input.

One reason i was considering the DC-DC route, was the option of having a Solar Input.

But after much reading, i think i would only ever connect a Solar Panel with MPPT reg direct to my Standalone Aux Battery anyway.

I have read that the Ctek D250s Dual is very "lossy" when through putting Solar.
Ie: 11Amps of Solar in, 7 Amps of Solar Charge out.
I do appreciate there has to be some loses, but that seemed counter productive to me.
The same chap then bought a Redarc DBDC 1225 and got a far better through put using his Solar Panel, more like 11Amps in, 10Amps out.

All things considered, and for the times i'd use one, i think the "KISS" approach is easier... Just plug it directly into the Battery (with Reg) while the Vehicle is parked at camp.

I was also concerned about running a 40Amp DC-DC charger (like the DBDC1240), as it has a 48Amp line side current draw (when delivering the 40Amp output), and i was worried that might strain the 70Amp alternator?

I'm no expert, so i'm not sure if this is a concern i should be worried about, or if charging direct from the Alt is doing the same thing anyway?
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Follow Up By: Member - LeighW - Friday, Feb 14, 2014 at 14:33

Friday, Feb 14, 2014 at 14:33
Your correct a good quality stand alone MPPT charger is the way to go for solar, it will be more efficient and you have redundacy if you loose either the in car charging for some reason.

The advantage of the VSR is it will replenish the bulk of the charge much faster, the initial inrush current will be quite high, depending on the battery and the SOC, for a 50% SOC on a 100Ah battery I would expect to see somewhere around 60A inrush dropping to around 40A after a few minutes, and around 30A after 20 minutes or so. Keep in mind though that this is no worse than starting with a flat cranking battery.

If you had both a flat cranking battery and a flat aux you might want to disconnect the aux for awhile, but I myself wouldn't worry about it.


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Reply By: Salty_Dog - Friday, Feb 14, 2014 at 15:07

Friday, Feb 14, 2014 at 15:07
Thanks Leigh,

My particular AGM battery has No "In-rush" current limit at all, so long as the Voltage is correct.

Is there any deterioration in Battery life from high in rush currents?
I guess i cant have it both ways, i want a fast recovery charge, so that is what it is going to be.
I understand for the longest life, i should cycle my AGM between 100% and 90%, but that is just not practical in the real world.

The irony in all of this, is this is the same Dual Battery set up i had 20 years (with a 200Amp Solenoid) and it worked fine for 6 years of Aux Batt life ;)

Re: Solar, I have Plasmatronics PL20 Reg in the workshop, whether i use that with a new 200Watt panel, or an MPPT that comes with a panel kit, i'm not sure?
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Reply By: ABR - SIDEWINDER - Friday, Feb 14, 2014 at 15:49

Friday, Feb 14, 2014 at 15:49
Don't get confused now, stick to the isolator method.

Keep the solar separated from the cars circuitry.


AnswerID: 526448

Reply By: Salty_Dog - Friday, Feb 14, 2014 at 16:04

Friday, Feb 14, 2014 at 16:04
The confusion i hope.

Aux Battery - via 25mm cable and 140Amp VSR (2 meters distance from Main Batt)
Solar - When it happens will be connected standalone to the Aux Battery.

IE: Physically removing one 50Amp Anderson (VSR) and connecting another (Solar)

Yes, i understand the Voltage from the Solar Reg would have "closed" the VSR, hence joining everything together.
But in my case, it will be old school, connect one option or connect the other ;)
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Reply By: Salty_Dog - Friday, Feb 14, 2014 at 16:05

Friday, Feb 14, 2014 at 16:05
No Confusion i meant to say.
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Follow Up By: ABR - SIDEWINDER - Friday, Feb 14, 2014 at 16:12

Friday, Feb 14, 2014 at 16:12
No problem, I think you know what you are doing now.

Have a great weekend.

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Follow Up By: Member - LeighW - Friday, Feb 14, 2014 at 16:13

Friday, Feb 14, 2014 at 16:13
Stick with the VSR, unless the battery specs specifically state to limit the charging current it won't be a problem.

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Reply By: Salty_Dog - Friday, Feb 14, 2014 at 16:23

Friday, Feb 14, 2014 at 16:23
Thank heaps for your help today fellas, its been very much appreciated.

The Battery Manufacturer quotes the below, so i think i'll be ok, so long as the Voltage is on target.

"Utilizing pure lead calcium grids, the plates are thicker than the industry standard for longer cycle life, increased reliability and power. The low impedance AGM design allows for excellent charge acceptance and there is no current limit required with controlled voltage charging. "

AnswerID: 526455

Reply By: The Bantam - Sunday, Feb 16, 2014 at 10:06

Sunday, Feb 16, 2014 at 10:06
As others have said, stick with ya simple VSR....that 6 amp Dc to Dc charger will be doing you no favours at all...and will provide you no advantage.


AnswerID: 526550

Reply By: Salty_Dog - Monday, Mar 03, 2014 at 21:27

Monday, Mar 03, 2014 at 21:27
As a follow up, after 2 trips away with the Fridge and 100Ah Battery fitted.
The VSR has been the Go! Far superior to the 6Amp Dc-Dc charging option i had.
The Aux Battery tops up in no time! And i mean, NO TIME!

In fact, the VSR stays closed for at least 30minutes after i turn the engine off, as the voltage in the Aux Battery stays up around 13V, 12.9V for a while, even with the fridge running!

In short, the VSR has been the perfect "short driving" solution for charging my Aux Battery.
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