AIMS 24v to 12v step down converter

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Desert_Rat

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I don't have the inverter/charger connected up yet because the manufacturer insisted on 2/0 cable (I ordered 2 AWG in error). Should be this week. But I thought it best to take the step down converter for a whirl to insure it'll work right. So far, so good. It's been running for over an hour now with every light, stereo and pump (except slider) on. As the cute little ammeter I bought on Amazon shows, over 30 amps DC is pumping through the system. it's slightly warm to the touch and its fan is pretty quiet.

I had to disconnect the converter, of course. Not that's it's working or anything, but I didn't want the charger back feeding the step down. The converter has been on the fritz for months, with lights flashing all the time and it burning up batteries. That's why I started on this project prior to my PV's being delivered. I wasn't about to spend a couple hundred replacing something I'd eventually trash anyway.

What's impressive about the step down is that it's actually doing what it should. It's not just taking 24v/30A and wasting off 12 of those volts, it's actually taking that 720W of energy and converting it to 12v. I know this because I have a 15A fuse in front of it, so it's not drawing 30A.

So far, so good. Happy New year to all!
 

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Gary RV_Wizard

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It's a voltage step down, not a rheostat, so it doesn't "burn off" voltage, but according to the specs the most output you can get from it is 30A x 12v or 360 watts. Up to 36A briefly, but 30A sustained. That's probably adequate since it is for house lights and power, not battery charging, but you might not be able to power a slideout motor through it, for example. The 40A model would have given you a bit more capacity for another $20.
 
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Desert_Rat

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It is an AIMS 40A model, and I agree that should be enough. Also agree that there's no way I'm powering the slider motor with it. As I've noted a couple of times, it demands up to 100A @ 12v so I'm going to power it with a separate, dedicated battery. I'm hoping a 200 CA motorcycle (or whatnot) battery will be enough but we'll see.
 

kdbgoat

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For RV use, you need to get CCA out of your head and concentrate on AH.
 

Lou Schneider

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Actually, for the application Desert_Rat is going to use this battery for (running the slide in and out) CA (Cranking Amps) is the correct specification.

The demand is similar to starting a car, the battery has to supply lots of current for a short period of time.  A deep cycle battery wouldn't be appropriate.
 

xrated

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Lou Schneider said:
Actually, for the application Desert_Rat is going to use this battery for (running the slide in and out) CA (Cranking Amps) is the correct specification.

The demand is similar to starting a car, the battery has to supply lots of current for a short period of time.  A deep cycle battery wouldn't be appropriate.

I don't know.....my slides take about 25 seconds or so (each) to deploy or retract.  I don't think I've ever cranked on an engine for that long at a time.  I might be more inclined to prefer a deep cycle vs. a starting type battery.  Plus, I doubt that my slide motor draws anywhere near the current a vehicle starter draws.
 

Gary RV_Wizard

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Also agree that there's no way I'm powering the slider motor with it. As I've noted a couple of times, it demands up to 100A @ 12v so I'm going to power it with a separate, dedicated battery. I'm hoping a 200 CA motorcycle (or whatnot) battery will be enough but we'll see.

So you will have a separate 12v battery, and presumably a charger of some sort for it?  All needed because of the small advantage of using 24v solar instead of 12v?    ???
 
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Desert_Rat

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xrated said:
I don't know.....my slides take about 25 seconds or so (each) to deploy or retract.  I don't think I've ever cranked on an engine for that long at a time.  I might be more inclined to prefer a deep cycle vs. a starting type battery.  Plus, I doubt that my slide motor draws anywhere near the current a vehicle starter draws.

That makes two of us. My intention is to buy the smallest AGM that will work, store it right next to the slider motor, then wire a cheap 2 amp charger through a wall timer switch and to the GFI port of my inverter. But you may be right that 200~ CA will not be enough. If it's not, I could use a full sized deep cell but I'd rather not.

http://www.autozone.com/miscellaneous-non-automotive/motorcycle-and-power-sport-battery/duralast-gold-powersport-agm-battery/178908_0_0?checkfit=true
https://www.amazon.com/Portable-Multi-mode-Battery-Charger-Motorcycle/dp/B014QXL438/ref=sr_1_10?ie=UTF8&qid=1483452119&sr=8-10&keywords=trickle+charger
 

Ernie n Tara

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Since I love to beat dead horses, I'll note that there are perhaps half a dozen responders to your various posts about using a 24V system to power 12V specific equipment who actually are knowledgeable about the field of electronics.  Nearly every, if not every, one of us has recommended against your 12-24-12 plan. There's a message there!

You're going to wind up with a convoluted/complex system and an expensive partial education in electronics with a concentration on interpreting and applying specifications.

Since I've made this kind of mistake myself, I appreciate how you've gotten here, but sometimes it does pay to listen ;D

That said, best of the new year to you and please be very very careful to be safe.

Ernie
 
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Desert_Rat

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I'll admit that the beaten horse is getting a bit aggravating, but I do respect that people will generally advise on what they're familiar and comfortable with.

That said, I do think it's counterproductive for this site to continue harping on 12v systems. More 1000+ watt solar arrays are being contemplated today because PV prices have plummeted, and, sorry, you're not going to design a good, efficient 1000+ watt system @ 12v. The math doesn't work, the amperage is more dangerous, and the cabling and termination more expensive. Electronics for 12v high amperage systems are less common, more expensive, and the stress on those electronics greater.

1000 (PV) / 12v = 83 amps
1000 / 24v = 41 amps
1000 / 36v = 27 amps

I'm installing 2,040w of PV  :)

Anyway, the company that makes another step down converter that I was researching finally got back to me this morning. Their unit supports 100 amps @ 12v, is rated at 94% efficiency, and is 12" x 7" x 3". It costs $500 but looks a lot more impressive then the AIMS unit I've installed. http://www.eaton.com/Eaton/ProductsServices/Electrical/ProductsandServices/Transportationsolutions/PowerConversion/Converters/24Vto12VPowerConversion/21100FamilyofDCConverters/index.htm#tabs-2. It would be recommended for anyone needing closer to 50a DC continual usage.
 

HueyPilotVN

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I for one will not bother you by being a 12 volt Missionary.  It is your money to invest in whatever system you think you want.

I am however reminded of a debate that took place over 100 years ago. 

The question was about which system "Direct Current" or "Alternating Current" was best for transmitting electric power over long distances.  There were some famous people lobbying on either side.  The DC proponents even went so far as to hold up the Electrocution of a convicted criminal as an example of the dangers of AC.

Another strange and little know fact about early electrical systems is demonstrated in a remote and strange location.  If you ever visit the California Ghost town of Bodie you will see an electric transmission line going into town that is straight as an arrow.  The theory back then was that if the transmission line deviated from a straight line that the electricity would jump off and run straight.

I am more than content to watch and learn whatever lesson I can from this different approach of using 24 volts as the standard in Desert Rat's system.  It doesn't cost me a cent and it may well be illuminating and probably not any more dangerous than me riding around on my 36 volt golf cart with a solar panel on top.

Good Luck Desert Rat.
 

Gary RV_Wizard

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More 1000+ watt solar arrays are being contemplated today because PV prices have plummeted, and, sorry, you're not going to design a good, efficient 1000+ watt system @ 12v.

And nobody argued with that improved efficiency. The debate was over the cost and design effects of having to adapt that nice efficient 24v solar system to power another system that is completely 12v. So far you are adding a step-down transformer plus a secondary battery and 12v charging system as a result of the choice.  I don't think that is going to be the end of it, either, but it's your RV and your money...
 

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boatbuilder

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Just about every inboard powerboat over 40' has 24V DC systems with 50A DC-DC converters.  There are a lot of them out there.  I have used the Eaton, Newmar Power, and Sure Power brands with good results.  I have not replaced any of them due to an actual product failure.  There are some boats that tap off one battery of a 24V bank to get 12V power and use a Vanner Equalizer to keep both batteries charged but I am not a big fan of that set up. 
Liberty Coach is using a 54V(if I recall) Li-on battery bank with DC-DC converters to step it down to both 24V and 12V. 
 

Gary RV_Wizard

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Any load measured in 100's of amps rather than single digits, and seconds rather than hours, is closer to "Cranking Amps" than 20-hour rate deep cycling (amp-hours). 

Of the various standard measures, power usage in modern RVs is probably better reflected by the RC (Reserve Capacity) measure than either AH or CCA. RC is measured as minutes @ 25A.
 

Lou Schneider

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Gary [RVer Emeritus] said:
So far you are adding a step-down transformer plus a secondary battery and 12v charging system as a result of the choice.  I don't think that is going to be the end of it, either, but it's your RV and your money...

It's not a step down transformer, it's a step-down converter.  The difference is significant.

A transformer is a simple, passive device that changes one AC voltage to another.  They're used throughout the AC power grid and they are the key that makes long distance transmission of AC power possible.

A DC to DC converter is more complicated.  Basically it's a combination of an inverter that creates AC from the incoming DC, a transformer or other passive circuitry that steps up the voltage, and a rectifier that converts the AC back to DC. 

All DC to DC converters contain numerous active devices.  All active devices have finite lifespans and can be damaged or destroyed by heat or by exceeding their voltage or current ratings.  Their lives may be measured in days, months, years or decades but sooner or later they will reach their end of life and fail.
 

Gary RV_Wizard

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It's not a step down transformer, it's a step-down converter.  The difference is significant.

LOL, Lou. I figured somebody would point that out, but most RV owners won't know or care about the internals. "24v gazinta and 12v gozota" is all that most readers will care to know.  ;)
 
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Desert_Rat

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boatbuilder said:
Just about every inboard powerboat over 40' has 24V DC systems with 50A DC-DC converters.  There are a lot of them out there.  I have used the Eaton, Newmar Power, and Sure Power brands with good results.  I have not replaced any of them due to an actual product failure.  There are some boats that tap off one battery of a 24V bank to get 12V power and use a Vanner Equalizer to keep both batteries charged but I am not a big fan of that set up. 
Liberty Coach is using a 54V(if I recall) Li-on battery bank with DC-DC converters to step it down to both 24V and 12V.

I think most 18 wheelers today are 24v as well.
 
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Desert_Rat

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Gary [RVer Emeritus] said:
Any load measured in 100's of amps rather than single digits, and seconds rather than hours, is closer to "Cranking Amps" than 20-hour rate deep cycling (amp-hours). 

Of the various standard measures, power usage in modern RVs is probably better reflected by the RC (Reserve Capacity) measure than either AH or CCA. RC is measured as minutes @ 25A.

Well, come on Gary, do the math then (I know I can't). I need up to 88 amps, 40 amps continual, for 30 seconds. What's the smallest battery that I can get away with?

Sad story: So I emailed "Battery University" (.com) today seeking that answer. Battery University supposedly prides itself on being the foremost authority on batteries, but I suspect now that it's just a front. Got a response telling me to ask this so-and-so retailer instead. The guy who answered in sales fumbled around answering the simplest of questions. So apparently, neither Battery University or this retailer know what the heck they're talking about. Such is life today. You want something done? better do it yourself.
 

xrated

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Gary [RVer Emeritus] said:
LOL, Lou. I figured somebody would point that out, but most RV owners won't know or care about the internals. "24v gazinta and 12v gozota" is all that most readers will care to know.  ;)

I caught it too...but I was at work and didn't have time to respond.  8)
 
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