DC to DC chargers

  • Ewart and Vivian Halford
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09 Mar 2013 16:04 #1 by Ewart and Vivian Halford
Ewart and Vivian Halford created the topic: DC to DC chargers
Just a question to all you electrical wiz's out there.
I came across Sterling alternator to battery charger that can increase the alternators out put at low revs.
Does this work and does it damage the alternator.

Cheers all

Ewart oka 365
0428911147

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09 Mar 2013 16:24 #2 by Peter and Sandra OKA 374
Peter and Sandra OKA 374 replied the topic: DC to DC chargers
I've only heard of them being fitted to boats and yachts to maximise charge levels at relatively low constant revs, sort of like a smart regulator.
Maybe they make a "normal" DC To DC charger as well these days.
A DC to DC charger will do what you want, usually they need a minimum of 9v and will up the vltage to whatever the unit has been set at for the battery type.

OKA 374 LT Van, converted to camper/motorhome,
400ah Lithiums, 680w solar, diesel cooking heating and HWS,
Cummins 6BT, Allison 6 speed auto, Nissan transfer.

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09 Mar 2013 16:46 #3 by Outback Jack
Outback Jack replied the topic: DC to DC chargers
I have one of these, and to be honest the jury is still out.

Sterling ab12130

I have only done the one trip and did have battery problems, but I havent checked things out yet. They run really hot, so dont install behind the drivers seat like I did. I have very hot air blowing onto the back of the seat.

What the charger does do is allow maximum current into the batteries even at idle and increase the volts to 14.4 Volts

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09 Mar 2013 16:47 #4 by Paul Scherek
Paul Scherek replied the topic: DC to DC chargers
DC to DC converters are great. They are basically a multi-stage (smart) charger, just like a home charger, except that instead of running off 240 volts, they run of whatever your alternator puts out, as Peter says, even down to 9 volts. Because they are a proper multi-stage charger the house batteries get a much better charge, prolonging their life somewhat.
The DC to DC converter should be close to the house batteries, and then any voltage drop in the leads from the alternator to the DC-DC charger is less important, because the charger will still put out the correct voltage for the house battery.
Of course any voltage drop will cause the DC-DC converter to draw more current from the alternator to compensate, so it is still important to use the correct thickness of wire.
I use the Australian made Redarc BCDC1240. It puts out 40 amps to the house batteries (when they need it) and also has an inbuilt MPPT solar regulator. It makes the whole set-up for maintaining the house batteries very easy, by both solar and alternator.
I am not sure for me that the 40 amp unit was a good choice. My Oka had a 55 amp Lucas alternator, and after a night of running fridges and lights, the house batteries were taking their full 40 amps for about an hour before dropping back to a lower level. Add to this quite a few amps for the air con fans and compressor, and the poor old Lucas was trying to pump out over 60 amps. I should have bought the 25 amp version - it would take a bit longer to charge the house batteries, but the alternator would have been much happier.
Sorry, that is a long answer to a short question!
Cheers,
Paul

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09 Mar 2013 17:14 #5 by Ewart and Vivian Halford
Ewart and Vivian Halford replied the topic: DC to DC chargers
I have the 20 amp ctek that I took on the CSR, it was the largest on the market at that time, it didn't give me any trouble but it was to small for what I needed. Once the battery's got low it had trouble keeping up as we were only traveling for 5 or 6 hours most days and I drove the OKA at low revs. Some days it took ages for even the cranking battery to charge up so a 40 amp would have been better for me. I think I had an 80 amp alt at the time. I will admit I was asking a lot out of my house battery's and a three night stop over early in the trip did not help.

Ewart oka 365
0428911147

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09 Mar 2013 18:01 #6 by Outback Jack
Outback Jack replied the topic: DC to DC chargers
I have two brand new CTEK, but found they are a little low on current capacity.

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10 Mar 2013 07:23 - 10 Mar 2013 07:31 #7 by dandjcr
dandjcr replied the topic: DC to DC chargers
A dc-dc charger (such as the much talked about CTEK 250S or the Redarc BCDC1240 ) can't increase the output from an alternator, but it can bump up the starter battery voltage to better charge an auxiliary battery, especially if it's far away like in a trailer or caravan. They also commonly have a solar regulator built in which can charge both the auxiliary and starter batteries when the alternator is not running.

To improve alternator performance you'd need an smart alternator regulator, such as the Sterling AR12V (we've got an earlier version). You need to make a couple of simple mods to the alternator but then these devices control the current though field windings to generate the alternator's maximum output at any speed, but they don't have a solar regulator built in.

In our case, maximum output is attained from tick-over speeds upwards for as long as necessary to fully charge both the starter and auxiliary batteries using its 4 stage charging process, plus we have a separate solar regulator.

David and Janet Ribbans - Oka 148
Oka148 profile here.
Visit our technical and travel blogs: here.
Last Edit: 10 Mar 2013 07:31 by dandjcr.

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10 Mar 2013 08:41 #8 by Peter and Sandra OKA 374
Peter and Sandra OKA 374 replied the topic: DC to DC chargers
Wouldn't it be easier and cheaper to just put more solar on the roof these days? Prices are down to under $1.35 a watt for 125w panels and a good MPPT regulator will fully charge the house batteries and then if required start to help the alternator charge the cranking batteries.
Charging with an alternator isn't free, there is always a power and/or fuel penalty and increased use does cause increased wear.
I've kept the house and truck batteries totally separate, the solar looks after the house batteries and has done so for over two years now and the alternator is just responsible for charging the cranking and aux battery.
I do have a VSR which can be switched on to charge the house batteries from the alternator but it has never been necessary yet.

OKA 374 LT Van, converted to camper/motorhome,
400ah Lithiums, 680w solar, diesel cooking heating and HWS,
Cummins 6BT, Allison 6 speed auto, Nissan transfer.

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10 Mar 2013 09:07 #9 by Paul and Sue Crompton
Paul and Sue Crompton replied the topic: DC to DC chargers
I also have a 3 stage smart regulator coming off the alternator. It has been on the vehicle for well over 10 years with lots of bush driving on corrugated roads and it seems still to be working fine (touch wood). The unit was bought originally from a marine shop. There has also been previous discussions on a similar subject in "Oka Maintenance - 120 amp alternators". Paul.

Paul Crompton - OKA 168

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12 Mar 2013 08:22 - 12 Mar 2013 08:24 #10 by Dean and Kaye Howells
Dean and Kaye Howells replied the topic: DC to DC chargers
I don't see the benefit of DC-DC chargers.

If the problem is undersized cabling giving excessive voltage drop I can see that using a DC-DC charger at the battery end will boost the charge voltage but in doing so increases the charge current in the undersized wiring which causes even greater voltage drop until the DC-DC charger reaches some sort of totally (power wise) inefficient equilibrium. To say nothing of excess heat generated in the undersized wiring. Wouldn't it be better, safer, more efficient and cheaper to dimension the cabling accordingly ? In this situation it seems to me that a DC-DC charger is adressing the symptoms of a problem and not the cause.

For a house battery why not run a 24 volt system ? Engels run off 24 volts, LED lighting can easily run off 24 volts with a small dropping resistor on each globe, if you need AC, 24 volt inverters are readily available. The beauty of this is cost and efficiency. A 24 volt systems current draw is half that of a 12 volt system with the same load and the cabling power loss is a quarter that of a 12 volt system, much more efficient. :) Or why not just use the correct cable in the first place ?

If the problem is the alternator running too slow why not decrease the alternator pulley size to make it spin faster ? (You'll need to re-calibrate the tachometer though). If the alternator has been changed already does it have the correct size pulley ? If it has come from a petrol engined vehicle the pulley may be too big (resulting in low output) for the OKA as diesel engines run slower than their petrol counterparts so need a smaller pulley to compensate.

Even with the best of charging systems flooded cell lead acid batterys (your average car/boat battery, deep cycle or otherwise) needs a finite time to charge. A half flat N70 will need a minimum of 6 hours to fully charge, two thirds flat double this time. This effect of this compounds if you're doing short hops and can result in a flat house battery after a couple of days.


One of the benefits of using AGM type batterys is that this is less of a problem. We use Optima batterys in #413 and though they have 2/3 the capacity (66 Amp Hours) as an typical N70 equivelant sized battery they can charge fully from half flat in 1 hour or from 2/3 flat in 2 houre. This more than compensates for their smaller capacity.

Deano
Last Edit: 12 Mar 2013 08:24 by Dean and Kaye Howells.

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12 Mar 2013 09:23 #11 by PeteFox
PeteFox replied the topic: DC to DC chargers
Dean
As I see it, the advantage of battery to battery charging is than a standard alternator can't charge an AGM battery with the correct charge curve.
An AGM must be periodically brought to around 14.4v (absorption) in order to prevent sulphation. Failure to do this will result in premature failure of batteries or at best reduced capacity.At the completion of charging, the battery should be held at float voltage, something an alternator may not be capable of doing, continuously exceeding float voltage in a fully charged battery is also terminal, particularly with an AGM where the electrolyte cannot be replaced.
I agree it is poor practice to use undersized cabling and then use a B to B charger to compensate, but it is the lesser of 2 evils.

Pete Fox OKA266 MultiCab
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12 Mar 2013 19:09 #12 by Paul Scherek
Paul Scherek replied the topic: DC to DC chargers
Absolutely right, Pete, alternators of the age of Oka's typically put out 13.8 to 14 volts, not enough to fully charge an AGM battery. The real benefit of DC - DC chargers is their ability to correctly charge whatever battery style we choose. Most can be set to suit the battery chemistry in use.

Typically, a standard alternator at 13.8 volts can only charge an AGM to about 80% of its potential capacity. As we are told never to discharge a battery below 50% of its rating, that leaves only 30% available. With a good DC - DC charger we can get a full 50% of the batteries capacity, or in effect 40% more usable power.

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12 Mar 2013 21:45 - 12 Mar 2013 22:12 #13 by Tony Lee
Tony Lee replied the topic: DC to DC chargers
Quite easy to find DC-DC chargers with a rated charging current equal to either long-term rating of the alternator, or the maximum rating of the battery so you can't do better than that - and as described, will have the capability of fully charging any battery. Given that most of us drive for perhaps three hours a day when in travelling mode, that time, plus input from a modest solar system, does have the capacity to keep batteries fully charged on most days.
Still, I would aim to provide most of my requirements from solar and the vehicle charging would be just a nice extra for those periods of low solar input when you need to put a few amp-hours in to provide basic services if you needed to stay put for a few days. Either direct charging through a split charge relay, or via a DC-Dc unit will do.

As for fitting a 24V house system to a 12V vehicle - don't even think of it. Bad enough fitting a 24V house system to a 24V vehicle as I installed in the MC8.
Yes, slight reduction in cabling costs with 24v but it is a vastly overrated factor since size of cabling to most appliances is based on mechanical considerations rather than irrelevant voltage drop calculations and in return you lose the ability to easily jumper the two systems together as I did for several thousand km when my engine alternator failed in Darwin and the parts weren't obtainable until I got back to Sydney. Why give up unlimited two way redundancy for no benefit. The Airstream is a big rig all 12V system and it has a 2500W inverter running a large convection microwave with no voltage drop problems simply because the inverter is mounted inches away from the batteries in the next compartment.

Some appliances just can't be obtained in a 24V version. Stove and HWS ignitor systems for example. Entertainment systems, radios, TVs and the like aren't readily available in 24V. Many 24V items are more expensive and far less available in 24V than in 12 - eg pumps. I get around that in the MC8 by fitting a 20 amp 24V to 12V converter, but it does add complexity, even though I can run 12V pumps off it if I can't get 24V units. 24V Webastos are available but then you have a unit that is forever married to that vehicle. In the OKA you would be adding complexity going to 24V house and then adding more complexity to undo the complexity and the nett practical gain would be zero.

The only possible exception to the all 12V would be to install the often-cheaper 24V solar panels teamed with an MPPT controller.

Those who haunt other forums and read blobs might remember Cuppa's first RV - a 24V bus where Cuppa (I think) also installed a 12V alternator as well as the existing 24V unit and ended up with a dual system that caused problems rather than solving them
Last Edit: 12 Mar 2013 22:12 by Tony Lee.

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13 Mar 2013 04:13 #14 by Peter and Sandra OKA 374
Peter and Sandra OKA 374 replied the topic: DC to DC chargers
Agree with that Tony, I found the Humvee was a joy to work on electrically as it was a 24v military spec vehicle and the elcetrics were pretty much bulletproof.
Problems started up though when the time came to civilianize it for 4wd touring here in Oz. Things like fitting stereo's CB's HF radio and then charging all the myriad of things we use these days like GPS and phones etc. Initially I fitted a Charge Equaliser which allowed you to pull 12v from either battery (two 12v batteries in series to provide 24v), it then floated the batteries at the same voltage but after the second one failed I ended up fitting a 24v DC to 12v DC charger to third 12v battery which works fine.
BUT the DC to DC charger over a period of a month or so will flatten all three batteries despite being turned "off" as it still senses the voltage on both batteries (12 and 24) and this small drain flattens the batteries eventually.

OKA 374 LT Van, converted to camper/motorhome,
400ah Lithiums, 680w solar, diesel cooking heating and HWS,
Cummins 6BT, Allison 6 speed auto, Nissan transfer.

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09 May 2013 12:19 - 09 May 2013 13:20 #15 by Kav1050
Kav1050 replied the topic: DC to DC chargers
I am working on installing a second battery and solar to the Travelander, i have had a fair bit of experience with running 12v systems on various setups, camper trailers etc but i am in now way an expert and would appreciate a review from someone who has some knowledge and any advice.

I am Trying to get to a stage where i can camp for a week and have the power last without the use of a Gen set.

This is the equipment i will be using:
2 x 120W Solar panels in parallel = 240W Max / 12v = 20A Max
2 x Ritar RA12-120A/H Battery's = 240A/h spec sheet here http://www.recell.co.nz/new/Ritar/images/RA12-120.pdf
CTEK D250S Duel DC/DC charger also is an MPPT controller for the solar panels also

I will be running 1 x 40ltr Engel and 1 x 32L Engel a 12v water pump occasionally and LED strip lighting at night for a few hours and occasionally a 700W inverter (maybe 10min use a day for toaster or something) 12v charging of ipad and phone

I have done some calculations based on worst case scenarios and the most amps i will draw in a 24hr period is 86. This is much more than i antisipate.
If i can get 5 hrs sun per day i will be able to get at least 18 A/Hr assuming i keep the panels pointed in the right direction

As a backup i will have a 120 Battery charged from an independent 80W solar panel which is on a tracking stand to follow the sun. Incase there are a few days without enough sunlight.

I have attached a drawing i did for the wiring lay out

Other things to mention
Batteries will only take 36Amps per hour max each
Cable sizing i will workout based on estimated amp draw and length to keep the Voltage Drop under 4%
Solar will sit on top of camper when driving then go on ground on stand at camp and get moved to follow the sun
32Ltr will be on freeze i estimated 60% duty cycle for this at 2.5A current draw and 40ltr 50% Duty cycle at 2.5A Current draw
Allowed for isolation switch so i can kleep the OKA auxiliary battery charged very quickly which could then in-turn DC charge to the camper system when engine off.?
32Ltr is only to keep bait frozen (and cool beer quickly) if there was no sun for a few days this could be turned off and bait thrown out. I have a throw net if needed

Regards
Kav

OKA 423 Shane and Lisa :)
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Last Edit: 09 May 2013 13:20 by Kav1050.

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09 May 2013 13:44 #16 by PeteFox
PeteFox replied the topic: DC to DC chargers
Kav
I have one of those CTeks in my Oka and it is a good performer with the alternator input but IMO it is a terrible solar charger. All it wants to do is trickle charge unless the batteries are right down. The other downside with these is that there is absolutely no feedback which means some sort of monitoring equipment is needed. Most solar regulators have at least a voltmeter and an ammeter.
I have left it installed for the alternator charging but I have disconnected the solar side and installed a dedicated MPPT solar regulator. I have heard of other users of these having the same issue.
Seeing that you are really wanting good performance on the solar side and unless you are absolutely committed to the CTek I suggest you reconsider.

If you do reconsider then you should look for an MPPT regulator that can take a 24v (nominal with series connection) input from the panels to charge the 12v batteries. This would halve your voltage drop to better 2% drop using the same size cables. Your 4% voltage drop is effectively a 4% drop in power
I have a couple of Powertech 30A regulators ( in Oka and at our farm) that do just this. They are Chinese, but not your usual rubbish and are available on EBay for about $180. There are many others.

Pete Fox OKA266 MultiCab
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www.byles.net/www.oka4wd.com/forum/membe...oka-266?limitstart=0
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09 May 2013 14:12 #17 by dandjcr
dandjcr replied the topic: DC to DC chargers
Kav, it's not a bad set up so a quick reply, not in any particular order or importance.

Solar panels are rated in perfect conditions so you would be wise to assume that only about 70% of the rated output can be obtained, due to sun angle and power, temperature, regulator and wiring losses. So your 240w solar panels would probably only produce 170 W maximum, or 12 Amp when charging at 13.8 v. The MPPT regulator in the CTEK charger may boost this.

Is your relay intended to supply additional power from the Oka battery (note CTEK is only a 20 A charger) or to allow solar only charging? Not sure it's needed for the second one, but for the former it would need to be a heavy current type (ie 100 Amp) to carry the loads you indicate and connected from the Oka battery directly to your 2 batteries. You could replace this and the Cab Switch with a manual battery switch but it would need to be a heavy current type and fitted near the source of the current (alternator output and batteries).

You show batteries 1 and 2 connected in parallel. That's OK while charging but unless they are both the same type, age and condition, one will gradually discharge into the other. Not a major risk if they are charged regularly but worth noting if left for any length of time.

Re your R-PTC Fuses. Are you referring to manual or automatic resettable fuses? I would recommend the manual type since if one blows/opens, it indicates a fault condition which needs investigation, you don't want it just to reset with the fault still present. But manual fuses do need to be in an accessible location and close to the source of the current for best protection.

Make all wiring as thick as possible (eg starter cable for fridges and inverters) since any voltage drop means a significant power loss which is proportional the the square of the voltage drop. A voltage drop from 12 to 11.5v is only 0.5 v (4%) but the power loss is 11.5x11.5/12x12 (9%). Note ammeter current shunts drop voltage too so you're better off just monitoring voltage rather than current and at full power, your inverter would take nearly 60 Amp.

Hope that helps

David and Janet Ribbans - Oka 148
Oka148 profile here.
Visit our technical and travel blogs: here.
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09 May 2013 16:19 #18 by Paul Scherek
Paul Scherek replied the topic: DC to DC chargers
Kav, I have the Australian made Redarc equivalent to the C-tek. www.redarc.com.au/products/product/40ain...low-voltage-version/

It has a proper multi-stage charger that charges at up to 40 amps and likewise has an MPPT solar charger. It can be set to suit several types of batteries and so far seems to work very well. With 40 amps available, when the sun doesn't shine for a few days the Oka engine can be run up for a relatively short time to get some charge into the house batteries.

I haven't fully tried the solar part yet - a quick check when I fitted it showed a charge of 10 amps, but that was late in the day, and I can't even remember how many watts my panels are. However, the DC-DC converter from the alternator seems to work very well.

The regulator lists at about $700, but I bought mine from ebay new for about $400.

You should also note that your batteries A/h rating is probably specified at a 10 hour or 20 hour discharge rate. Using heavy current items like toasters reduces the A/h capacity quite a bit.

It is also worth considering putting more insulation around your fridges, especially the freezer. By this I mean proper polyurethane, preferably at least 25mm thick. This really can help reduce your power usage.

Cheers,
Paul
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09 May 2013 16:53 #19 by Dean and Kaye Howells
Dean and Kaye Howells replied the topic: DC to DC chargers
Kav, some good info here. You haven't shown a isolate switch for the inverter. These can have an idle current that results in significant battery capacity loss if left on.

A question here, do these multi input (alternator and solar) units simply disconnect the solar panel output from the battery when the alternator is running or do they manage both inputs in tandem ?

Me, I'd avoid the electric toaster like the plague as this consumes power like there's no tomorrow. Of the devices you list this is the most hungry by far. Avoiding this will decrease your power requirements substantially.

I'm in the process of refitting #413 and the input here is good for me too. My intent is to leave the the vehicle batterys alone with the exception of LED lighting on the main vehicle (non start) battery which is insigificant in power requirement and run the main 40 litre Engel fridge/CoolZone crisper and 17 litre Engel freezer off a new 300 AH (approx) flooded cell battery (non deep cycle) that I was given. Not ideal but a very good price :). We don't have any 240 volt appliances/chargers etc but have a 300 watt pure sine wave inverter just in case. I'm toying with the idea of fitting a 250 watt (24 volt) solar panel over the cabin and running a 12 volt output MPPT solar regulator from this to keep the house battery charged with a C/O switch to charge from the vehicle alternator just in case. Any comments ?

Deano :)
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09 May 2013 16:54 - 09 May 2013 17:43 #20 by Outback Jack
Outback Jack replied the topic: DC to DC chargers
If you have the dollars I wouldnt bother with AGM batteries. Go with Lithuim.

The Redarc will not be able to charge them, but a Sterling DC DC charger is switchable to suit battery.

This is on Ebay and although not sure of seller, the specs give you some idea on Lithuim Batteries.

Ebay Seller. Look at Specifications
Last Edit: 09 May 2013 17:43 by Outback Jack.
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