DC Charging options

Hi forum members

I’ve been lurking for quite sometime now and learned much from reading the threads. I find myself in a bit of a dilemma concerning the most appropriate DC charging system to use in our new vehicle.

It bit of background and intended use.

My first auxiliary is in the back of the ute and runs an inverter to power and low powered coffee machine which draws a total of 36 DC amps to make 50 cups. (This data is based on hard empirical data i.e. I’ve tested and measured and retested and re-measured (and drank lots of coffee while testing). When the machine is idle, i.e. not making coffee it draws 26 AC watts.

So, I need a charging system to power it, the AGM battery, back up fast. I’m not too concerned about treating it with kit gloves and prolonging the life of it. It just has to get max amps back into it as fast as possible. The 2nd auxiliary is in the van that is being towed and most likely connected to the 1st auxiliary via 4 B& S cable and a 175amp Anderson plug. The van battery will be about 7 – 8 metres from the 1st auxiliary in the ute. The van battery powers LEDs and devices both 12v & 240v via an inverter respectively.

What is the better option a 100 – 200 amp Redarc like isolator or a 30 amp DC to DC charger?

It gets confusing when auto sparkies I know claim that once the cranking battery is full the alternator supposedly senses this and then only sends out a trickle charge to the auxiliary batteries. Is this true in all or any cases? (I have a new MN Triton). I guess the only way to find out is measure the current after the engine has been running a while


I dont want to get the wrong one to begin with and invest in the wrong system and would appreciate any help from you especially if you have any empirical data on the matter.

Thanks in advance

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Reply By: Ozhumvee - Thursday, Jun 10, 2010 at 08:06

Thursday, Jun 10, 2010 at 08:06
A DC to DC charger of the biggest capacity possible would be the go.
I take it you intend to run the ute battery while it is charging? I'd reckon it would need to be sitting on at least 1500rpm to provide enough alternator output though.
The other solution would be a gennie running a large capacity 240v charger.
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AnswerID: 420245

Reply By: drivesafe - Thursday, Jun 10, 2010 at 08:50

Thursday, Jun 10, 2010 at 08:50
Hi filosoph, for a start, as long as you use a genuine automotive grade AGM, like an Optima, Odyssey or Exide Orbital Spiral type AGMs, then you can charge and discharge them with as much current as you have available or need.

If your using any other AGMs and then you can expect a very short life span from these types of AGMs because the can not tolerate high current charging or discharging.

Next, unless you can afford a Stirling DC to DC 50 amp charger, you would be wasting money with these other toys.

By the way, the are quite a few Mobile Coffee Vendors who are using the Stirling units with great success.

As to the story about an alternator stepping once the cranking battery is full charged, this is just total crap.

Some alternators on different makes of vehicles, step back based on a time period from when the motor was started and have absolutely no idea if the cranking battery is fully charged or on it’s last legs.

Last but not least, with the charging current you need, give your alternator a chance to do what it’s designed to do and that’s to provide high currents when needed, like when your batteries are low. Your alternator will recharge the battery set up you have, much, MUCH quicker than any of these toys could ever hope to do, so just get a relay/solenoid type isolator.
AnswerID: 420250

Reply By: Member - John and Val - Thursday, Jun 10, 2010 at 09:16

Thursday, Jun 10, 2010 at 09:16

Not too sure about your measurements and actual requirements. I take it that when heating water the inverter is drawing 36 amps from the battery. That's a fair load on any deep cycle battery, though if it starts out fully charged it should stand it for a while. The 50 cups is interesting - that's at least 10 litres of water to be boiled. I assume you don't aim to serve 50 cups one after the other? How many cups per day and how long does the machine run to produce a cup? This info will help us understand your actual electrical demand. Where does the idle current (26 ac watts) go?

"What is the better option a 100 – 200 amp Redarc like isolator or a 30 amp DC to DC charger?" The 100-200 amp rating is the isolator refers to the maximum current it can handle, not what it will supply. (That's determined by the capacity of the alternator and the nature of the load.) The isolator is there to separate the auxilliary battery from the main battery when the alternator isn't charging. Some are claimed to only connect the batteries after the main battery is fully charged - in my experience they connect almost immediately the alternator is working. You need some kind of isolator to do this. For your purposes I think probably your best solution is to use an isolator in the engine bay and a 30 amp DC to DC charger close to the battery.

What the sparkies are telling you about the alternators cutting back to a trickle charge after a while is sort of correct. (Actually, it is mainly the battery characteristics, not the alternator that determines the charge rate.) The charging voltage required by the main battery is temperature dependent, so the alternator output is made temperature dependent to match this. The alternator output voltage is reduced as the alternator ( and battery) temperature rises. One problem with simple charging systems for the auxilliary battery us that with the battery being stuck out the back somewhere it doesn't experience the temperature change. Due to losses in the wiring and the alternator voltage being reduced, the aux battery never gets a full charge. That is why a DC to DC charger close to the aux battery is a very good way to go. (These chargers take the available voltage and increase it up to what the deep cycle battery actually requires.)

With 2 auxilliary batteries in seperate locations as you have, it becomes a bit complicated, as you really need the dc-dc charger close to both. Would it be possible to put both in the van? If not, suggest give the charger to the battery running the coffee machine, and run heavy cabling from there to the second aux battery - not ideal, and there should be another isolator between them unless you want both to share the load - getting complicated.



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AnswerID: 420256

Follow Up By: drivesafe - Thursday, Jun 10, 2010 at 09:36

Thursday, Jun 10, 2010 at 09:36
John, mate your not doing the math.

How can a 30 amp DC to DC toy compere with the 70+ amps that an alternator can provide to a single Spiral type AGM battery and if the alternator is bigger, the amps going into these types of batteries can be even higher.

I’ve seen where a Discovery 3 with a 140 amp alternator has supplied a charge current of 97 amp going into and Optima 55 Ah battery. There is no way these toys can compete with this sort of charging current.

And as you have already pointed out, as Pat has two auxiliary batteries, one of these DC to DC toys would actually take a much MUCH longer time to charge his batteries and further to this, if I remember correctly, the Triton does not reduce voltage and charges at 14.? all the time so this makes it a total waste of money to even consider one of these toys.

BTW, of the vehicles that do reduce voltage, in most cases, it has nothing to do with temperature and is, as posted above, a time based step down, controlled by the vehicles engine management system.
FollowupID: 690435

Reply By: Member - Boobook - Thursday, Jun 10, 2010 at 10:53

Thursday, Jun 10, 2010 at 10:53
Pat, before you can get a decent answer from anyone you need to correct the comment below and add details.

"My first auxiliary is in the back of the ute and runs an inverter to power and low powered coffee machine which draws a total of 36 DC amps to make 50 cups. (This data is based on hard empirical data i.e. I’ve tested and measured and retested and re-measured (and drank lots of coffee while testing). When the machine is idle, i.e. not making coffee it draws 26 AC watts."

This statement can not be correct and / or complete.


a)the coffee machine uses 36 AH to make 50 cups ( and how did you measure that?) or
b)the coffee machine draws 36 DC amps while making the 50 cups ( over an unspecified time) or
c) Something else.

Sorry to be pedantic but without this information it is impossible to answer your question meaningfully.

AnswerID: 420279

Follow Up By: filosoph - Thursday, Jun 10, 2010 at 18:39

Thursday, Jun 10, 2010 at 18:39
It appears my post did not make it let me re-write it again
FollowupID: 690505

Follow Up By: filosoph - Thursday, Jun 10, 2010 at 18:53

Thursday, Jun 10, 2010 at 18:53
The 36 DC amps is the total DC amps used to make the 50 cups of coffee, which took just under 50 mins. The coffee machine was plugged into a digital measuring device and then into a 10amp wall socket. Each cuppa goes through a process of surging to about 1500AC Watts for a few seconds, then to 1200 -1400 AC Watts for about 10 -20 seconds then down to about 800AC watts then once the coffee is delivered the machine goes into idle mode which measures at 26 AC Watts.

After the 50 cups were made I took a reading of the cumulative amount of AC Watts used during the just under 50 minute period and then converted them into DC Amps which came out at 36 DC amps to make 50 cups of coffees, i.e. lattes, cappuccinos, long blacks, short blacks and hot chocolates as well as some straight hot waters.

Hope this clarifies


FollowupID: 690506

Follow Up By: drivesafe - Thursday, Jun 10, 2010 at 20:38

Thursday, Jun 10, 2010 at 20:38
Hi Pat, going on your post above, you have only measured the AC consumption and you have probably used at least another 10% more power.

By the way, the amount of power consumed is measured in amperes and amps is used to ( put simply ) measure the pressure.
FollowupID: 690525

Follow Up By: Member - Boobook - Friday, Jun 11, 2010 at 06:46

Friday, Jun 11, 2010 at 06:46

I worked backwards from your latest posted figures ( and assuming that the 800W is for about 20 seconds. You will use about 51 amp hours of energy from the battery.

Be aware that watts and amps are instantaneous figures. You are interested in Watt hours and amp hours ( as well as the maximum Amps for cabling etc)

That means you will use about the capacity of a 100Ah battery if you treat it right or you could possibly use an 80AH. Anything less probably won't make it through the 50 cups.

The other piece of info is how long before you need to make another 50 cups.

IMHO the only way to charge this is directly to your isolator as some others have suggested. Depending on your alternator capacity the 100 A Redarc will do the trick. I guess it will take about 4 hours to get the system back to fully charged.

Have you considered a Honda etc generator instead.

FollowupID: 690557

Reply By: Battery Value Pty Ltd - Thursday, Jun 10, 2010 at 12:19

Thursday, Jun 10, 2010 at 12:19

I second drivesafe.
Spiral wound lead/tin are the fastest lead acid batteries, in terms of current discharge and recharge.
They can accept charge from 0 to 95% within a lightning fast 45 minutes with the right charger/s.

And yes, they can be charged quickly by alternator up to 85 or 90% SOC.
If you come home, connect a good quality charger to it and let it top off the charge overnight.

Just by coincidence, I did a test a few days ago: put the spiral wound baby into the
Paj, and turned on the high beam. Left it sitting there for 2 hours 15
minutes, after which time the voltage had dropped to 11.8V under load.
Then I took the battery inside and used two 25A fast chargers* to
boost charge the battery at 14.8V at a combined max charging current of 50A.

I could watch the terminal voltage creeping up as the 50A pushed up the SOC
faster than the police would allow it. But surprisingly there was hardly any
warming of the battery, maybe 5 to 10 degrees above ambient if at all.
After 30 minutes or so, the voltage was 14.4V and the current started to
taper off. After another 15 minutes, the chargers went into float charging
mode, which signals a SOC of around 90 to 95%.
*(see photo of the setup attached)

Sorry, if I'm boring you with this, but I do this battery
charging/discharging/load testing thing on an almost daily basis. And from
what I've seen so far, these pure lead/tin spiral wound batteries react
extremely quick to applied charge, they just suck it in. They have me
excited as you might have guessed....
So if you're after really quick recharging times, this is the technology of
These batteries certainly will help you save in the long run, due to reduced
generator run times, and due to their longer cycle life.

Best regards, Peter
AnswerID: 420290

Follow Up By: filosoph - Thursday, Jun 10, 2010 at 18:37

Thursday, Jun 10, 2010 at 18:37
Hi Peter

Once we are on the road there is little chance of 240 volt charging. Refer to my posts to you via walkaboutcoffee email address.

I like the idea of the of the batteries you are talking about now in the above post. Are they the same ones you emailed me about a few months back?

FollowupID: 690504

Reply By: filosoph - Thursday, Jun 10, 2010 at 19:08

Thursday, Jun 10, 2010 at 19:08
Hi Peter

Would it be possible to hook up multiple DC to DC smart chargers in the same fashion? Or would the isolator system still be best value , i.e. bang for buck?

AnswerID: 420349

Follow Up By: drivesafe - Thursday, Jun 10, 2010 at 20:48

Thursday, Jun 10, 2010 at 20:48
If you use two spiral AGMs, you will actually limit the amount of charge going into them if you use a DC to DC charger on each battery, which means you will take much longer to fully charge them.

With these batteries you can charge them MUCH faster from your alternator, furthermore, like your calculations above, you need far more than 30 amps going into these step ups to get 30 amps coming out of them.

This is not only a waste of power but you will probably exceed the total output of your alternator which means you will actually have far less charging ability using two step ups than if you just use the alternators output.

In your case, you would be wasting a hell of a lot of money to get a much slower charging set up.
FollowupID: 690529

Follow Up By: Battery Value Pty Ltd - Thursday, Jun 10, 2010 at 21:31

Thursday, Jun 10, 2010 at 21:31
Hi Pat,

yes, theoretically you could have multiple DC/DC chargers hooked up in parallel.
But this has to be tested first, to confirm they won't affect each other in any way.
But as has been said before, this is an expensive option because you'd need 2 to 3 such 30A units to equal the output of an alternator, in terms of Watts.
You also have to account for inefficiencies in these units, which means you probably are limited to a maximum of two of these in parallel which will suck in at least 75A, while outputting 60A.
Thus, the DC/DC have a charging rate disadvantage of 20% over pure alternator charging in the rapid bulk charging region (where it matters most) between 10.5 and 13.8V.

To take full advantage of the extraordinary quick charge acceptance of the spiral wound AGM batteries, you want to look into upgrading your alternator to a 120 or higher amp rating.
As to the power consumption figures we arrived at earlier, I recommend 4 of these 50Ah batteries in parallel for a whopping 250 amps max charging current.
This setup will allow you to get enough hot coffee flowing to fill 200 cups before recharging.
Plan on 1 hour charging time per 100 cups with a high rated alternator.
Plus top off charging overnight, and during 'work free' days with a good mains powered three stage charger.

Best regards, Peter
FollowupID: 690539

Reply By: filosoph - Thursday, Jun 10, 2010 at 21:03

Thursday, Jun 10, 2010 at 21:03
Looks like the Triton has a 95amp ( or is it ampere) alternator. If the above rationale is continued would it be most frugal to get a 100amp isolator? If so what is the best one out there? Best one meaning bang for buck and might as well be this fiscal period.
AnswerID: 420378

Reply By: filosoph - Wednesday, Jun 30, 2010 at 09:51

Wednesday, Jun 30, 2010 at 09:51
On another point.

If the auxiliary battery/s is receiving 14.2 volts and the cranking battery and the red arc 100 amp isolator are receiving 14.45 volts is this an acceptable voltage drop?

I keep hearing different auto sparkies claiming that after the cranking battery is "full" then the alternator will only trickle charge. I dont see how this would be possible but I guess the only way to be sure is to some how measure the amps going into the battery not just the volts. I'm using one of those $20 multimeters to measure the volts.

Also how would I test the amount of amps going into the battery?

It's on a mates car that I've rewired for him. Cranking battery to his RedArc then to his 1st aux batt. then continued on to an Anderson plug to the 2nd aux batt in the trailer. the difference between the two batteries was .04 of a volt i.e. 14.24 in the 1st and 14.20 in the 2nd The 1st battery is in the back of the ute about 5 metres from the Redarc and the 2nd battery is about 8 metres
AnswerID: 422562

Reply By: Maîneÿ . . .- Wednesday, Jun 30, 2010 at 10:23

Wednesday, Jun 30, 2010 at 10:23
when the Aux battery is connected direct to the Cranking battery by the battery isolator, the Alternator REGULATOR will only 'see' one battery voltage, this is the voltage of the two batteries averaged out together, because they are now wired in parallel and as such are basically now only one large battery.

The Alternator’s regulator should maintain the charge of this ‘large battery’ at the average 'float' voltage of ~13.8v.

This of course assumes the battery cabling is correctly sized and is secured solidly.

Maîneÿ . . .
AnswerID: 422569

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