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

[pianotuna]

Hi TWH99,

I would buy the 3500 watt Champion with built in remote electric start. The Yamaha does fine--at twice the price of the Champion, and you have to add the remote kit from Pinellas for another 500 smakers plus a couple of hours of labor.

[TWH99]

Thanks pianotuna. Which Yammy Gen do you have and are you happy with it...what are the pros and cons based on your experience with it. It's Tough to decide between Yamaha & Honda. Since I'm getting a relatively small generator where every 100 watts counts, I like what Yamaha has done, breaching the 2000W and 3000W gap with a 2400W generator that is still at a reasonable weight for regular lifting of unit in and out of a truck bed.

[pianotuna]

Hi TWH99,

Once the bank reaches 85% state of charge the acceptance rate is about 12.5 amps per 100 amp-hours of battery bank.

Charging voltage ought to vary with temperature--so on the Magnum series there is a battery temperature sensor. Ideal voltage is just below the gassing voltage.

Eco mode, at least on my Yamaha, does throttle back as charge rate declines.

TWH99 wrote:
Thanks Mrwizard for answering the questions related to differences in generator output and charger input and the practical application in the field...good stuff! The puzzle is coming together.

Question: When the Charger transitions from the fast charging rate to the next stage, let's say for example it goes to 1/2 the fast charging rate, will the Generator sense that load reduction and throttle down also to Eco Mode as long as that output is able to satisfy what the charger is asking for?

thanks Tim

[MEXICOWANDERER]

The following excerpt may assist in designing a recharge protocol...


Gel and AGM batteries do not need watering, are safer (no acid spilling out), can be placed in a variety of positions, have a slower self-discharge characteristic, and are more efficient in charging and discharging than flooded batteries (see table below). Gel batteries are more suitable for deep cycling applications whereas AGM batteries are more for light cycling and engine-starting applications.

Flooded

Gel

AGM
Charge/Discharge Efficiency
89%

98%

99%
Self discharge rate (per month)
13%

1-3%

1-3%
Finish Voltage
15.3-16.0V

14.1-14.4V

14.1-14.7V
Float charge:
13.2-13.7V

13.4-13.8

13.4-13.8V

I love ho they close their eyes and chant "You can't see me" with regard to the AGM for cyclability. "Only one type of AGM exists (thin plate)" They don't want to "see" telecomm batteries and Concorde Lifeline, is John Banner, stuff --- "I see nothing Colonel Hogan"

[TWH99]

Great explanation, thanks!

[MrWizard]

a honda inverter generator, running in Econ mode will use what ever throttle, little or high to match the load

it will do the same thing with out Econ mode
the difference, is that in econ it idles lower and runs at the bare minimum needed to maintain voltage at the load amps/watts being used

with econ OFF it idles faster and the applied run power is slightly higher than the minimum needed, it will ramp up faster when more load is applied, than when in econ mode AKA response time to load increase is faster with econ OFF

[TWH99]

Thanks Mrwizard for answering the questions related to differences in generator output and charger input and the practical application in the field...good stuff! The puzzle is coming together.

Question: When the Charger transitions from the fast charging rate to the next stage, let's say for example it goes to 1/2 the fast charging rate, will the Generator sense that load reduction and throttle down also to Eco Mode as long as that output is able to satisfy what the charger is asking for?

thanks Tim

[MrWizard]

a charger you can dial down, or one smaller than generator output
a charger that trys to draw 18 amps from a 13 amp generator, equals generator shut down or generator overload/power shut off

you can 'tax' a generator running it on the limit of its power until it gets too hot or until something else trys to run, thens its overload time

you can get 125 amps if batteries SOC 'state of charge' is low enough for them to accept it, but you will NOT maintain that rate, it will taper, and keep tapering thru out the whole charge cycle

if you are down 100 ampHrs, you can;t charge at 125 for 45 min and be full

it will take 2-3 hrs to get close to full and 6 hrs or more to reach 100%

thats why most of us charge until the charge rate drops below what the solar can provide, that changes almost daily, so its a matter of usually charge to the 90% mark with generator then switching to solar only

it depends greatly on your power use habits, and where you camp
in the woods under the trees, foggy beach, sunny desert ?

[TWH99]

BFL13 – thanks for confirming ADC is Amps DC. I assumed it was but I don’t like assuming anything when it concerns electricity. A google search, prior to asking could not answer this. I am obviously a novice re: electricity but I do have down W=A*V and it’s other two variations and it makes sense to me. The nomenclature, abbreviations, specification data on equipment and how/or where it applies in all the calculations is where I get tripped up or where I don't feel sure I am interpreting everything correctly. Thus all my weird and simplistic questions. I will be investing a lot of money in a complete system so I would like to try if possible to actually understand at the most basic level (I don’t need to understand how VA is different than Watts) what equipment to buy and compatibility between all components in the system based on each item’s input and output capabilities. I understand if you are tired of answering my question but thanks for all you have offered thus far. I’ll keep asking until no one answers anymore but I think I am pretty close to having just about everything answered. Thanks again to you and everyone else on RV.NET.

Next batch of questions: Are any of these statements false?

RE: Charger – 125 Amps DC: = the charger is capable of charging the battery at a rate of 125 Amps DC

RE: Charger – Input current 18 Amps AC: = the charger can accept up to 18 Amps AC (from generator or shore power). In the case of the Honda EU2000i generator that outputs 120V/13.3A/1596W; it is 4.7 Amps short of using the charger’s 18 Amp capacity. If the charger cannot automatically detect the generators output limitations, the user should dial down the Input Current demand to 13.3 Amps or below to insure the generator is not over taxed running continuously for hours near it’s rated capacity. Theoretically, with 13.3 AC amps entering the Charger, it will output approximately 133 Amp DC at 12V to the battery bank. In practice output Voltage may be a little higher, for example 14.2 and Amps 112. The 112 will be further reduced due to the 87% efficiency of the charger and the >.95 Power Factor.

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

[BFL13]

TWH99 wrote:
125 ADC Continuous Output at 25 degrees Centigrade (battery charger spec)

anyone know what ADC stands for?

Amps DC. IMO you are way too far into theory. Go camping with whatever set-up, see how that works, then adjust as required. Might take several trips, fine-tuning after each trip, to get to where you want to be. You get new ideas each trip.

That's how most of us got to where we are.

[TWH99]

125 ADC Continuous Output at 25 degrees Centigrade (battery charger spec)

anyone know what ADC stands for?

[road-runner]

TWH - I forgot about the lithium battery part. The charge voltages and maximum battery current comments I made are for lead acid batteries. I'm close to totally clueless about lithium battery charging details.

[BFL13]

"I bought my 3000 about 15 years ago and at the time I could carry it around alone. .....
Will I turn 80, I will think about adding wheels and build a lift for it
Buy a Jack Russel Terrier >>> he will keep you young. We have 3 of them"

We have Shelties that have the same (or worse) effect.

On remote start--can't see the point. The battery died in the 3000 some time after the first year (2003), and have used the pull cord ever since. (The battery costs too much to be worth it). The inverter does it all ( the microwave etc ) at any time of day, so the gen is only for when actually battery charging when there is not enough solar. Not that often really.

What's the point? At 5.30 AM and you want coffee, you can't start the gen because of noise laws, so you use the inverter. By gen hours at 9.00am, you are s, shaved, and shampooed, and dressed, so you can go out and pull the cord to start the gen to recharge the batts.

[Kayteg1]

You have quite a project on your mind.
I would add couple observations that I did not notice on those 4 pages.
- 12V heating elements in refrigerators usually have much smaller wattage than 120V elements, so they will stay longer on in average weather, but might turn inadequate in hot weather.
-Honda 3000is is way superior to 2000 on several levels. It is more fuel efficient, quieter and has a tank that will let it run overnight.
I bought my 3000 about 15 years ago and at the time I could carry it around alone. Now being retired, I still can pull the generator from front receiver and hoop it several feet alone.
Will I turn 80, I will think about adding wheels and build a lift for it 😉
Buy a Jack Russel Terrier >>> he will keep you young. We have 3 of them.