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TWH99

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Posted: 09/12/17 01:04pm Link  |  Quote  |  Print  |  Notify Moderator

I am an Electricity Novice:

Assumptions: Boon-docker - does not use shore power, uses Solar Panels and Batteries, or generator. Refrigerator does not have propane option...electric only, AC or DC.

Question: what is the most efficient method to power a RV Danfoss Compressor Refrigerator? Do I use the 12V battery direct (DC) to Refrigerator or 12V battery to Inverter making 120V (AC) to Refrigerator?

If a device can run on both AC and DC, does the rated input power (in Watts) of the device always remain constant when doing the Watts = Amps x Volts equation.

Below is the specification given for the Refrigerator. Is this enough information to figure out the Amperage it will consume from my batteries? Assuming Rated Input Power for the device is the same for AC and DC, then Amps used if I ran the energy from my battery through my inverter changing it from DC to AC, would be .54 Amps (Amps = Watts/Volts) 65W divided 120V = .54 Amps. And if the refrigerator compressor ran for one straight hour I would consume .54 Amp hours from my battery (not counting the roughly 10% energy loss from the Inverter process). If I ran the energy directly from my battery to the refrigerator and the watts or rated input power stays the same, I will be using 5.42 Amps (65W divided by 12 Volts). So I use about 10X the energy from my battery going DC instead of running it through an inverter to produce AC.

What am I doing wrong here? I’ve always heard, if possible, it is much more efficient to run a device directly from the battery (DC) instead of running it to an inverter that changes it to AC to run the device. My calculations above prove the opposite.

Assuming my calculations are incorrect, how does one figure out how many Amps a device will or can use under maximum draw for an AC setup versus a DC setup when the device is capable of doing it either way? Is the data provided below enough to make that calculation?

Input voltage (AC) 120 V
Input frequency 60 Hz
Rated input power (AC) 65 W
Input voltage (DC) 12/24 V

Thanks for helping this beginner.
Tim

vermilye

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Posted: 09/12/17 01:21pm Link  |  Quote  |  Print  |  Notify Moderator

What you are leaving out is that when you use an inverter to run the refrigerator on 120V, the power is still coming from the battery. The input to the inverter will be around 10X the current coming out of the inverter (wattage is the same, so one tenth the voltage will have to push ten times the current).

With the losses produced in the inverter, I suspect you will be better off running directly from the battery.


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ScottG

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Posted: 09/12/17 01:21pm Link  |  Quote  |  Print  |  Notify Moderator

Watts remain constant, minus any conversion losses like converting AC to DC or DC to AC - but those losses are relatively small.
Volts X Amps = Watts.


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time2roll

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Posted: 09/12/17 01:31pm Link  |  Quote  |  Print  |  Notify Moderator

I would be making an inquiry to Danfoss directly. You will lose 10 to 15 percent converting 12vdc to 120vac.
Probably best to go 24v option and lithium batteries.


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MEXICOWANDERER

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Posted: 09/12/17 02:03pm Link  |  Quote  |  Print  |  Notify Moderator

My Danfoss motors all use 24vdc.

When I power them off the 12 cell bank the aggregate total amp hour usage for refrigerator and freezer = 116 amperes hours. They are 18 CF units.

When I use a 24 volt power supply AC to DC 2,784 watt hours.

Through the power supply is 3,313 Volt Amp Hours.

But storage vs domestic power is one thing, and

CEF via AC powered battery charging, another.

If only interested in power drawn by the compressor use the topmost figures as a rough estimate, battery charging (CEF) may not be an important factor.

TWH99

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Posted: 09/12/17 03:13pm Link  |  Quote  |  Print  |  Notify Moderator

Thanks Vermilye, scottG, timetoroll, mexicowanderer,

I was surprised to get so many responses so quickly. So I think I’ll cast a line in again.

As I was composing that last question, near the end, I realized what Vermilye stated.
With either option I was considering, the ultimate source of the energy to run the device was a 12V battery. The inverter option just added another loop that consumes some additional energy. And no matter how optimistic I am, I can’t create energy per the conservation of energy law. So the inverter sucks out the DC equivalent amount of amps from the battery to run the device, then uses an additional 10% of that amount to turn it into AC and then sends it off to the device. It’s the inverter that needs all that energy to do the conversion but once it completes it’s job of ratcheting up the voltage by a factor of 10 it sends that final product along with reduced amperage by roughly a factor of 10.

You guys brought up 24V systems and Lithium batteries. I have heard other RV people say, if going with a large setup and starting from scratch (not up grading 12V) a 24V system is best. Is it as simple as increasing the volts (doubling it in this case) cuts the Amps consumed in half when running the same device at the same wattage…resulting in battery Amp hours also being consumed at half the rate? If that’s not it please explain.

I will be doing a Lithium Battery Bank. Is a 24V, 100 Amp lithium battery roughly the same price and physical dimensions and weight of a 12V 100 Amp lithium battery. Same question for equivalent performance but higher voltage Inverter and Charger and Controller? What else will be more expensive to set up a 24V system over a 12V system? Anything less expensive…like smaller gauge wire, less resistance with long wire runs due to higher voltage? All things being equal, is their longevity or durability differences between a 12V and 24V system.

I’m guessing the reason for not upgrading from 12V to 24V is all the major components must be also be upgraded, thus there is not much from the old 12V system that is useable in the new 24V system.

thanks again for sharing your time and knowledge
Tim

time2roll

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Posted: 09/12/17 03:27pm Link  |  Quote  |  Print  |  Notify Moderator

24v 100 amp will consist of 2x 12v 100 amp batteries.

Two separate batteries or integrated into one box... either way the 24v would be twice the size and weight as it also contains twice the power. Best with lithium to get it in a single battery so all series cells are balanced automatically.

Complications are a new charging system and a method to create 12v for existing accessories. Most things are more efficient at 24v because the amps are cut in half.

TWH99

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Posted: 09/12/17 04:20pm Link  |  Quote  |  Print  |  Notify Moderator

Weight and Volume will be limiting factors for me. You say at 24V, Amps needed to run same device are cut in half but are the Amp hours of each battery case also cut in half and if yes, isn’t that a “wash”.

For Example: lets say my weight & volume maximum is 4 Lithium batteries at
150 Ahr each for a total of 600 Ahr at 12 Volts. Is their a significant advantage to connect the same 4 batteries so they become 24V and only 300 Ahr ? Or does the Amp hour remain at 600 and just the voltage changes? Sounds like free energy again or creating new energy.

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Posted: 09/12/17 04:57pm Link  |  Quote  |  Print  |  Notify Moderator

effiecncy, doesn't change, 24v does not significantly increase efficency
what it does do, is 1/2 the Dc amps for the same watts load which means you can use smaller wire

that is one of the reasons 1200 watts is 10 amps at 120v 100amps at 12 means much bigger wire going to the inverter from the batteries

and why power transmission lines are HIGH voltage,

if your installing a Large inverter in the RV then 24v means smaller wire and less amps for the same amount of 'WATTS'

the fridge is NOT a big concern

base your design on how big an inverter you want, then decide on 12v or 24, lead acid, or Lithium

if you move around a lot the lessor weight of lithium is some advantage
if your stationary OFF grid, that advantage is lost, and the expense of lithium is many times the cost of lead acid, and AGM becomes a more cost EFFICIENT decision,

when buying lithium even a small 200AHr 24v system, aka 400aHr 12v is about $4000
, space/size is very close to AGM, the difference is the weight, NOT dimensions aka storage space used


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TWH99

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Posted: 09/12/17 06:16pm Link  |  Quote  |  Print  |  Notify Moderator

I still need some clarity regarding Amp Hours when wiring batteries in series.

If I have two, 12 volt, 100 Amp hour batteries and I wire them in parallel I will have a battery bank of 12 Volts with 200 Amp Hours.

If I wire those same two 12V, 100 Amp hour batteries in series, I will have a 24 Volt battery bank. How many Amp hours will my 24 Volt battery bank have?

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