Electrical: DC AC Conversions - Efficiency

[TWH99]

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

[theoldwizard1]

TWH99 wrote:
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 ...

The big win of higher voltage battery bank is less energy lost due to voltage drop (heating the wires) between the battery bank and the load. 24V is good, 48V is better, BUT other than inverters and a few other items, there are not many 24V/48V appliances.

TWH99 wrote:
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.

Twice the energy, so twice the cost.

TWH99 wrote:
Anything less expensive…like smaller gauge wire, less resistance with long wire runs due to higher voltage?

If you down size the wire much, you will have voltage drop issues again.

TWH99 wrote:
All things being equal, is their longevity or durability differences between a 12V and 24V system.

Not really, but I think that 24V inverters are built a bit more "robustly".

TWH99 wrote:
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.

They do sell 24VDC to 12VDC converters. If you are only going to be running lights (hopefully LEDs) and maybe a TV and a cell phone charger, this should be all that you need.

[TWH99]

Thanks TimetoRoll and Bigg Al,

My application is a Mobile 20 foot travel trailer that I will travel & l live in full time. I would like to run everything on battery, re-charged by solar Photo Voltaic cells. I will have a generator (Honda EU2000i) for back up. I will have a fixed, non-expandable physical space for the battery bank in the trailer. The wire run between battery bank and 3000 watt inverter will be less than a foot long and wires from solar panels to the battery will be relatively short also, but the longest less than 15 feet long.

1. Are these wiring lengths going to be helped significantly regarding resistance with the 24V system? I think I prefer the simplicity of a 12V battery bank for charging/operating the devices I have that can be plugged in to my 12 volt receptacles, and by-passing the inverter which uses power and produces heat.
2. I will run a 5K BTU window unit air conditioner that will have a start up surge of about 200 Amps (12 volt), will run with compressor on using about 40 Amps (12 volt). With 600 Amp hours of Lithium, 80% useable = 480 Amp Hours, I should be able to safely run the small Air conditioner for 8 hours. Will running approximately 500 watts continually for 8 hours through the Inverter be “hard” on it, shorten it’s life, increase energy consumption as it heats up.? Do I need to worry about any of this or is it only a concern when the constant load is much closer to the Inverters regular capacity. In my case if I had a high quality 3K Watt Inverter (3500 surge for 5 seconds) could I run it continuously at a 2000 or 2500 watt load for 8 hours without it skipping a beat or shortening it’s lifespan?
3. Recharging the 600 Amp hour battery pack. The Inverter/Charger combo unit says in it’s shortened spec sheet: Charger-125AC Efficiency – 88%. I’m guessing the efficiency relates to the Inverter and means for each 100 units of Battery power it draws it only converts 88 equivalent units to the device being powered…is that correct? What does the 125AC mean regarding the Charger? Does it mean it can pump in 125 Amp hours back into the battery each hour assuming the power supply can keep up with that? If not what does it mean 125AC.
4. As related to #3 above, if I am using a small Honda EU200i generator to recharge the battery bank (assume no PV solar panels working) and no draw on the battery bank, and the Amp hours on the battery has been drained down to 20% or 120 Amp Hrs, roughly how long will it take to charge the battery bank back up to 600 Amp Hours or what is the hourly rate of recharge in Amp Hours. I’m not sure what the relevant numbers are on the Honda spec sheet that relates to recharging a battery but one of these must be it. AC output – 120V 2000W max. (16.7A) 1600W rated (13.3A). and DC Output – 12V 96W (8A) and Receptacles 20A, 125V Duplex
5. Lastly, please confirm if this recharge calculation is correct for Solar Panels. A clean, properly angled, 100 watt panel, in 70 degree weather will be about 80% efficient during the peak hours equaling 80 watt charging rate per hour which equals 80W / 12V = 6.6 charging Amps per hour or 10 hours of full sun to put in 66 Amp hours into the battery bank.

Thanks to anyone who has stayed with me and is willing to answer some or all of my questions.
Tim

[time2roll]

TWH99 wrote:
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?

12v 200 amp/hours = 2400 watt hours (12 x 200)

24v 100 amp/hours = 2400 watt hours (24 x 100)

same energy, same weight, same foot print, same volume.

https://www.lithiumion-batteries.com/products/24-volt-lithium-batteries/24v-100ah-lithium-ion-batter...

http://lithionicsbattery.com/product/24v-110-lithium-amp-hours-gc2e-case-with-internal-neverdie-bms-...

[BFL13]

How far is the fridge from the battery bank? You need short, fat wires for 12v but thin, long wires will do the 120. If the inverter is close to the battery bank, as is proper, you can locate the fridge anywhere to be run on 120. You might be having a big inverter anyway like that, so then why not use it to run the fridge too?

There is more to this than electrical efficiency. Generally, when RVing you don't give a rat about efficiency, as long as you can get it all done. The main thing is not to run out of battery at 3AM and get to the next recharge time mid-morning.

Practical considerations are what count when RVing. Running long, fat wires is not practical. Efficiency is way down the list of things to worry about. 🙂

[Bigg_Al]

100 Amp Hours.

[road-runner]

I took a look at the Danfoss web site and couldn't find any information that would help make a conclusion about AC vs. DC efficiency. Maybe somebody knows where to find this info?

I got the impression that the Danfoss compressors are variable speed, which suggests to me that the motors are non-60 Hz AC motors, further suggesting they are driven using a variable frequency inverter. Is this inverter powered by AC or DC? Does the 120 volt AC input simply get converted to 12 or 24 volt DC? Does the DC input get converted to 120 volt AC? Will using your own inverter be more or less efficient? Or is it something altogether different? Without knowing info like this, actual measurements with using different power sources will be the only way to find the efficiency information. Is this data already available, or is it waiting for somebody to figure it out?

[TWH99]

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?

[MrWizard]

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

[TWH99]

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.

[time2roll]

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]

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

[MEXICOWANDERER]

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.

[time2roll]

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.

[ScottG]

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.

[vermilye]

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.