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

[BFL13]

I carry the Honda 3000 in the back of the truck towing the 5er. (Between the storage box behind the cab and the hitch. --long box truck.) I don't need a gas can--I just fill up the Honda gen ( if it needs it), when I am getting gas for the truck. Long 120v cords are no problem for voltage drop to get from the Honda to the charger(s) in the 5er.

At home I slide the 3000 off the truck onto a dolly that height, and roll that away till next time. I also take out the 5er hitch and storage box so I have a usable truck bed again.

If starting over I would have two Honda 2000s in parallel instead of the one 3000 for easier handling and more total power.

[TWH99]

Side Note to BFL13 - once again posted before I saw your most recent post.
I would prefer to use the EU3000is but I will need my generator to be mobile. I won't have compartments in my small trailer to put in any size generator. It will be in the bed of the tow vehicle. The EU3000is is 130 lbs dry so that could be dangerous for me to repeatedly load and unload into the truck bed. The EU2000is is 45 lbs dry, easy to move and if I need more power, I can get the companion EU2000 to run parallel and double my watts or AV's from 1600 to 3200 bringing it up to the performance level of the EU3000is at about the same price. The big reason to me for getting the EU3000is (if weight was not a factor) is the remote control start accessory available for $360 from the link below...that would be sweet to have. The install looks a bit time consuming but relatively easy per the very well made install video. More comments/questions coming on your 8:00pm entry but I don't want to stack up too many questions before I get answers. I did that before and decided I need to ask one or two questions and then wait until I have solid answers that I understand before moving on to the next thing. I really like the way member "Westend" replied to one of my entries above. If you guys can do that too (copy my original and then comment after each question) that would be great. The examples using real number scenarios to back up the theory just stated helps even more. I can't always understand the answers completely but between the text description of the answer and the numbers/equations of the answers, I can usually get it. I then re-state what I think I understand for confirmation. thanks Tim
http://www.pinellaspowerproducts.com/view/HEUREMOTEKIT3/

[BFL13]

Your output watts is charger voltage x amps to the battery. However the battery voltage starts at whatever, say 12.3 and rises as the charge goes on. I am not sure which voltage to use for the output, charger (as set with a variable voltage charger) or actual battery voltage on a spot check.

See my Kill-A-Watt results posted earlier.

You need to dial down the Magnum to be at or under the Honda's 13.3A and see what DC amps you get as output on low batteries (so they will accept those amps for sure---if the batts are not low, they won't accept that many amps and you will get the wrong idea)

How low is low enough depends on the size of the bank in AH. Bigger bank means you can be higher in SOC and they will still accept the amps. You will find out by trial and error or just way overshoot and not have that as an issue.

Once you know how many amps you can charge with initially, it is easy to say how long it will take to recharge the bank to any SOC, given that you know the charger's high voltage set point (for doing the Absorption stage), the size of the bank in AH, and the SOC at the start of the recharge.

(A veiled threat to post my ugly graph again! 🙂 )

[TWH99]

Thanks for taking the time to edumacate me.
BFL13, I sent this before seeing your last post so all references to you below relate to your previous post.
See specs for Charger at bottom.

OK roadrunner and BFL13, that helps. I did not realize the "charge" going into the battery was at 12 volts. I understood the charge coming out of the battery was at 12Vin order to (A.) feed 12V outlets/receptacles and (B). feed the inverter to go through the conversion to AC to feed the devices/equipment needing AC. So you are saying the Honda Inverter generator is delivering 120V/13.3A/1596W to the charger which then changes that current to 12V, same wattage 1596 so the Amps convert to 112.4 per W=V*A and that is then delivered to the battery bank. Thats is great news, much better to add amp hours at a rate of 112/hr than 13.3/hr. I understand the power loss due to charger efficiency (87%) per spec= 97.7 Amp. Does the Power Factor correction of .95 then bring it down to 92.8 Amps being sent to the battery resulting in 92.8 Amp hours being added per hour of charging at this rate?

Roadrunner -I thought Lithium batteries took a big charge quickly. What is the specification I should look for on a battery that gives me some indication as to how much current a battery CAN draw in a hour. (I am aware that charge rate and duration is programed or tailored per battery mfg and type. I knew that was part of the function of the controller between the solar panels and the battery, does the charger that receives the AC from the Generator also get programed for a charge rate and duration cycle?) I had heard Lithium batteries charge up fast (relative to other chemistries) up to 80% of full and then the last 20% of charge is at a slower rate but that it does not hurt the Lithium battery at all to quick charge up to 80% and then quit and use the battery back down to 20% before charging again. Apparently Lead Acid NEED to be fully charged each cycle to keep them healthy.

BFL13 - in your last sentence above, are you saying with a PF corrected Charger, the Honda Generator could possibly run on Eco mode and still deliver enough energy to the Charger for it to deliver 12V- 92.8 Amps to the battery bank? ...thus not needed to run at normal full power...1596 watts?

The Spec sheet for the Charger says the following:
125 ADC Continuous Output at 25 degrees Centigrade
87% Charger Efficiency
>.95 Power Factor
18 Input current at rated output (AC Amps)

[BFL13]

Dragged out my Kill-A-Watts results for my PF- corrected 100 amp DC charger, and for my non-PF corrected 75 amp DC charger. Being run with a Honda EU3000is doing 126.8V AC unloaded. (Loaded voltages shown in results)

A. 122.7V, 15.57A, 1910VA, PF 0.97--DC output 102A, 14.8V (Batt 14.44 and rising)

B. 123.8V, 13.64A, 1693VA, PF 0.73--DC output 75.0A, 14.8v (Batt 14.08 and rising)

You can see the lower output (75 vs 102) means less of a voltage sag at the input.

You can see the non-PF corrected 75 amper wanted 13.64A input which is more than the 13.3A Honda 2000. BUT if the 75 amper were PF corrected, it would not want so much input, so IMO the Honda 2000 could run a 75 amper or so if it were PF corrected--such as that Magnum, dialled down so it would be under the Honda's 13.3

BTW I got away with that 15.57 input on the Kill-A-Watt, which can fry at over 15A. Don't do that! You might not be so lucky. 🙂

Note 125/102 x 1910VA = 2340VA /18 amps = 130V, so that 18A business for 125 amps output doesn't quite work. Needs a bit more input or matches with less output, more like 118a. Whatever, don't worry about it. VA gets weirder the more you poke at it. 😞

[road-runner]

When working with different voltages you need to think in terms of power. For example, 13.3 amps at 120 volts is 1596 watts (volts * amps). The charger converts that power to, say, 14.2 volts, and you can find the current at that voltage and power, which is 112.4 amps (watts / voltage). Run that for an hour and you've added 112.4 amp-hours to the battery.

In reality it's not quite that simple. The charger is less than 100% efficient in its voltage conversion. Not knowing the exact efficiency, 85% is a reasonable guess, so that 112.4 amp output becomes 95.6 amps. It's unlikely that your battery will draw that much current for an hour, so you won't even be able to add 95.6 amp-hours in an hour. If your charger wasn't power factor corrected there would be another output reduction, but you don't have to worry about that one.

[BFL13]

"If the Magnum Inverter spec says: Input current at rated Output is 18 Amps"

You appear to be confusing AC with DC values. Rated output is 125 amps DC, ISTR, while input at 125 volts AC might be 18a for 2250VA

The Honda spec is in VA not watts. That is why you must know if the charger is PF corrected or not. A PF corrected charger can be run with a lower VA amount to get the same DC amps output, or looking at it the other way, you can run a higher amp charger with the Honda if the charger is PF corrected.

[TWH99]

Battery Charging Rate Question:

If the Magnum Inverter spec says: Input current at rated Output is 18 Amps.
(Does this mean it can only push out or charge the battery at a maximum of 18 Amps (assuming the power source is also at least 18 Amps)? Is an 18 Amp Input, operating for one hour roughly equivalent to adding 18 Amp hours to the battery charge?

Generator Honda EU2000i Generator spec is: AC output rated 1600 Watts 13.3A (max 2000 Watts 16.6A)

Are those the specs, listed above that I need to figure out the charging rate?

Does the 13.3 AC output of the generator mean it will roughly add 13.3 Amp Hours to the battery if the generator runs for one hour.

In this setup, will the generator be the limiting factor, only able to put out 13.3 Amps continuously.

If the Inverter spec: Input Current at rated output = 18 Amps is the incorrect spec for the calculations I am looking for, the spec sheet can be found at the link below. If any one knows the correct spec and can find it on this sheet, please pass on that info.

https://pdf.wholesalesolar.com/inverter%20pdf%20folder/MSH4024M-Specifications.pdf?_ga=2.188029736.1232624645.1505241136-2016977354.1505241136

[wa8yxm]

I use 12 volt direct to power mine, ALL the time. it only draws 3 amps running.

No matter which method you use (DC or AC) there is a converter I am told and conversion loss but if you invert to 120 volts at best that is 90% efficient (matters not if PSW or MSW) and then you re-convert to compressor power.

So running 12 volt direct you only have the single converson, 10% savings.

[MrWizard]

your talking about spending thousands of dollars for lithium

lithium is not heavy, it can be bulky, 100amphrs of 24v can take more space than 100 ampHrs of lead, but they are very light weight
but they do not lend themselves to extra batteries being carried around and jumpered, they have very strict battery management protocols and built in cell balancing, special charge requirements

although effective use, do to deeper discharge capability, means 200ampHrs of Lith
can supply the same use as approx 300ampHrs lead acid, where it is better to NOT discharge below 50%
you can take Lith down to 20%
in a have to make do all the way dead situation, i would take the 300amphrs of lead acid
also you can't charge cold lith batteries, no cold weather camping unless you have heat blankets for the lith batteries

a lot of stuff to think about

[westend]

TWH99 wrote:
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.Lengths don't typically change. It is the increased voltage that allows smaller wire.
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? Almost impossible to answer as there are many grades of inverters on the market, some better, some worse.
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.125AC is probably the rating at which the device operates within it's specifications. 125AC input = 88% efficiency to produce (?) charging amps
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 If using the 12V power output of the Honda generator, you'll probably never recharge the battery bank. It outputs very small current. If you power a charger with the generator it will output more charge current to the batteries. We don't know what that current is.
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. Theoretically correct but you may see more charge than calculated.

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

[TWH99]

thanks for the comments theOldwizard1
"Check into getting a "soft start" for the A/C."
Yes, soft start is a good idea!
BFL13 - another good idea, stow extra batteries in tow vehicle. My batteries will be inside trailer so might be a hassle to "jump" them. They are so dang heavy I don't see myself moving them around. Would have to create a door on exterior of trailer to access the batteries. I think my biggest problem will be not having enough roof real estate for the solar panels. But this "plan" is still about 2 years out in the future so maybe there will be more compact options for solar panels then.
Piano tuna -yes, by passing the inverter for all 12v needs is the most efficient.

Another question: are the 3-way refrigerators, when used in the 12v mode, way less efficient (needs more watts to run) then the newer compressor refrigerators of equal size? I had heard that was the case but it was not quantified. If yes, More efficient by 10%, 50% or ?? anyone know?

[pianotuna]

The most efficient way would be to run right from the battery bank.

[BFL13]

Sometimes you can carry extra batteries in the towing vehicle and jumper them to the trailer's batteries for more AH.

[theoldwizard1]

TWH99 wrote:

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.

Short wire runs from the battery bank to the inverter is a BIG WIN ! Stick with a 12V battery bank, but I would go UP one wire size just for peace of mind.

Depending on how many hours a day you are running in the A/C off of the battery bank will be the deciding factor on how many solar panels you will need. That and the "fixed, non-expandable physical space for the battery bank". You may only get a couple of hours (at best) of A/C usage after the sun goes down !

Check into getting a "soft start" for the A/C.


The restricted battery bank space is likely going to kill all of your plans.