24v or 12v if I want to run an electric fridge. - Fiberglass RV
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Old 10-29-2017, 12:23 PM   #1
Nik
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Red face 24v or 12v if I want to run an electric fridge.

I've seen some really nice Norcold electric-only fridges. On 24v they use 1/2 the amps compared to 12v. So I'm thinking of a small 24v system.

Stepping down 24v to 12v would be necessary for running the lights, pump and stereo. An extra expense, but not much.

Is it really the case that I'd get more life from the batteries running 24v, just for the fridge, because it uses 1/2 the amps. About 3 amps at 24v as opposed to 6 amps for 12v.

Thanks.

p.s. it just occurred to me that a 24v system on a travel trailer might have problems getting charged by the tow vehicle's alternator. Anyway around that?
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Old 10-29-2017, 01:02 PM   #2
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24 volts at 5 amps = 120 watts
12 volts at 10 amps = 120 watts
Watts = power
Switching to 24 volts will not make your batteries last longer and cutting the amps in half gain you nothing except that you could run smaller wire.
Plus a transformer works with AC not DC
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Old 10-29-2017, 01:12 PM   #3
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^^^^^^^

Steve said it perfectly.

And, if you really want to maximize your batteries with respect to refrigeration, use a propane fridge.
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Old 10-29-2017, 01:31 PM   #4
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24v

nothing gained with your idea our propane fridge so small I don't know if its worth it or not. be pretty hard to get a larger propane fridge in to as the camper is so small I have been contemplating though but then I would have to put my other tank back on!

in my bussing days people would think they could put in a huge battery bank such as 8 or 10 batteries and use an inverter to run an a/c. My bus had a 10kw generator 2 a/c and a 110v fridge!

The bus had no internal a/c so we ran the 2 roof tops for cooling on the highway. With all that running still got 8 to 10mpg sort of amazing when you think about it!

bob
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Old 10-29-2017, 02:07 PM   #5
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Seems I have a lot to learn. Thanks.

What I find strange is that Bigfoot's stock wires coming from the 12v battery on the tongue into the cabin aren't very heavy. Are heavy wires only necessary between batteries in a bank?
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Old 10-29-2017, 02:52 PM   #6
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The wires have to be of sufficient size to carry the intended load over the intended distance. That may sound vague, but it's not possible to determine if a wire is right by simply calling it "heavy" or "light".

Battery cables are quite standardized and have factory ends on them. Your batteries are probably connected to each other with off the shelf cables that would carry much more load than was intended inside the trailer, but were convenient to acquire and install. Those cables are not usually fused either.

The smaller internal house wires should match their size to the loads and be fused in case of overload.
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Old 10-29-2017, 05:44 PM   #7
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Nik, I am looking at the Nova Cool R3100 12/24 volt fridge for my Boler 13 foot. It uses 2.2 amps at 12 volts or 1.1 amps on 24 volt. With the 200 Ahr battery bank I am planning to use, I could run it continuously for over 45 hours without charging.
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Old 10-29-2017, 05:50 PM   #8
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Thanks for the lead on this fridge. I was all set to get a 3-way, but it seems that the electric 12v-only machines have really gotten better, and use a totally different 12v cooling system than 3-way, which don't really cool well with 12v.

I wish there were a 'modern' quality 12v fridge that also could use LP. An upgrade from the standard 3-way.
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Old 10-29-2017, 05:57 PM   #9
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Yeah, I looked at the standard gas absorption fridges and found that there were a lot of complaints about them not working well in a hot and humid environment. The compressor fridges are said to work much better. I like the Nova Cool since it was originally designed for marine applications so I figure it will survive what I throw at it. In addition, I don't need an external vent so no extra holes in the egg. You can configure it for A.C. and DC power.
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Old 10-29-2017, 08:15 PM   #10
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I run a Truckfridge TF130. It will run for three days off my two six volt golf cart batteries. I also run lights , water pump etc. I think it is 4.4 CF.
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Old 10-29-2017, 08:33 PM   #11
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Red face

Alex Adams,

The Nova Cool R3100 looks very nice. 2.2amps is amazing. And it's a big fridge by my standards.

You say 45 hours drawing 2.2 amps continuously on your 200Ahr system. Can you please explain that calculation? When you say 'continuously', I assume you mean keeping things cold continuously - or do you mean actually running the compressor continuously?

I'm starting with one Trojan SCS150 battery SCS150 | Trojan Battery Company which specs:

Capacity minutes (@25amps = 150 minutes).
Capacity Amp-Hours (20-hr, rate 100).

Capacity minutes is thus: 3.125 amps = 1200 minutes or 20 hours.

Can I assume you have the equivalent of two of my batteries? Giving you over 40 hours drawing 2.2 amps - with room to spare.

A lot of questions, I know. Please know I appreciate having my ignorance erased little by little as I hack away at this information.

Thanks.
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Old 10-29-2017, 08:45 PM   #12
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I wish there were a 'modern' quality 12v fridge that also could use LP. An upgrade from the standard 3-way.
The basic problem here is that the 12 volt truck fridges use a compressor system, like a household fridge. The absorption/propane fridges us a different principal. Absorption is a totally different process. In the absorption fridge, a small flame drives the system. To replace the propane flame, as the driving mechanism, with a 12volt resistance heater means a lot of energy is coming from the battery. A propane tank has many many times more energy in it than a battery. Batteries power other devices in the trailer too, so it's best to use the power source best suited to the task. And it's best to NOT use batteries to heat an absorption fridge.

Compressor fridges use a low amperage motor and compressor unit to make cold.

This battery draw, in my case on my boat, ran about 1/3-1/2 of the time under the best of conditions. So, you can count on a continuous 12 volt draw, over time, for as long as the fridge is running. It never stops and you have to keep up with it in battery charging. While charging your batteries, there is a loss. So now you might add 20% more energy to the battery than it actually holds in available power. Then, the faster you draw the battery down, the less you get overall. So if you combine loads, you may wonder where the lost power went.

Meanwhile, the propane absorption fridge is operating silently and will for weeks off of your propane tank. Yes, it may need a fan in hot weather, or some insulating to make it the best it can be, but it's a marvelous system that is very well proven. And you can use your batteries for other loads.

Just remember that a 12 volt absorption fridge will never be efficient because it was never intended to be. The energy is simply going to make heat and heat is what the propane does the best. Heat is what batteries do the worst. It's not a good comparison to judge 12 Volt absorption with 12 volt compressor. Not the same thing at all. It would be like complaining that you can't use propane to charge your computer.
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Old 10-29-2017, 08:49 PM   #13
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So I have a total of 200 Ah of battery capacity in Sealed Lead Acid batteries. Since you are not to draw down the batteries below 50% capacity that means I can draw 100 Ah total. If the compressor runs for 1 hour, it will have used 2.2 amps or 2.2 Ah. 100 Ah divided by 2.2 amps is 45.45 hours assuming the compressor runs all the time, which it won`t.
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Old 10-29-2017, 09:04 PM   #14
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Nik,
Forgot to mention that you can only draw a battery down to 50% charge or you will begin to damage it. Take the full amp/hour rating and cut it in half for your actual available power. Then remember that it takes more energy than that to recharge it. About 20% more or so. Then remember the amount you get out is determined by how fast you draw it out- faster draw means less overall available.

Then, while charging, you have to take into account the bulk and absorption phases of the charge regimen. So, an hour on the generator may only give you half of what you expected.

You have to do the calculations, but there is a lot more to it than that.
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Old 10-29-2017, 09:16 PM   #15
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Thanks Alex, that's very helpful.
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Old 10-29-2017, 09:19 PM   #16
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Thanks Raspy. I read about preserving batteries and not going down to 50%.

I'd have to carry a lot of battery weight to keep food cold. I can see why LP is still very popular with Campers. You make a very good case for gas. And I totally get it.

One, I just find the Dometic three way units seem built so poorly.
Two, I met a young guy traveling in his van with a small cooler style fridge, an $850 unit that ran at 38 degrees. His system had 4 batteries and 200 watts on the roof. I got inspired by that.

But I really don't want that four battery weight and I'm not sure I'll be in full sun all the time at all. In fact I seek shade always.
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Old 10-29-2017, 09:20 PM   #17
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So I have a total of 200 Ah of battery capacity in Sealed Lead Acid batteries. Since you are not to draw down the batteries below 50% capacity that means I can draw 100 Ah total. If the compressor runs for 1 hour, it will have used 2.2 amps or 2.2 Ah. 100 Ah divided by 2.2 amps is 45.45 hours assuming the compressor runs all the time, which it won`t.
Your calculations also assume there is no other 12 volt draw in the trailer. Often, usage rises to meet capacity. Some loads that must be considered are lights, computer and phone charging, stereo or DVD/television, parasitic detector loads, fans, water pump, inverter, etc. Now you are down to an unknown, but lower number. Meanwhile, the propane system would be cruising along and good for weeks more with no maintenance.

Your batteries, in your example, will have to be re-charged every other day. Or less if you runl the other loads I mentioned. What is the charging system for this entire everyday job? A job that must go on relentlessly on schedule to prevent damage to the batteries and keep your food cold. Every day.
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Old 10-29-2017, 09:33 PM   #18
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Yup, I get it.
On my fourteen days camping this fall I kept daily electrical usage to a minimum. But that was before I installed the stereo which now draws extra 7.5 amps.
Naturally there's a lot of other demands on the battery. The argument for LP grows. I only wish the Dometic LP fridges didn't feel so crummy and look like a high school science experiment.
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Old 10-29-2017, 10:09 PM   #19
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But I really don't want that four battery weight and I'm not sure I'll be in full sun all the time at all. In fact I seek shade always.
This is one of the reasons I don't want solar on my trailer roof. I have no intention of parking in the sun all the time.

My favorite charging system is a portable solar setup that I can set out to make up for the majority of my use. We end up charging computers and phones and we watch movies sometimes at night along with LED lights and water pump use. It all adds up and I'm surprised at how fast my (4) T105 6 volt bats go down.

I charge with the seven pin plug while towing and with jumper cables from the truck to the trailer batteries while parked. Sometimes with a Yamaha 2000 watt genny, but we're taking it less and less. I'm working on a new portable solar array.
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Old 10-29-2017, 10:19 PM   #20
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I have 200 watts of solar that I can move about 25 feet away from the trailer. Here is a pic of my camp from a trip in September. The panels are on a clothes drying rack from Amazon and are held on with spring clamps.
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