Fridge power consumption math help

[john61ct]

Just to be that guy.

Watts or Amps per hour is confusing, IMO makes no sense.

Watts or Amps are an instantaneous measure, already a rate with time acvounted for in the definition.

Watts or Amps average usage over a period of time yields

AH or wH per hour, or per 24 hours, is correct.

Nice thing about Watt-hours is you don't need to know actual voltage.

But since most batteries, charge sources etc are in Amps and AH, multiply by 13 or 14 to get Watts and wH.

 

[TT-Tacoma]

The shunt is bidirectional, there are 4 wires that connect to the meter. Power, ground, north and south end of the shunt. You just wire the switch to flip the north /south wires


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Hmmm. I'm trying to follow you. Any chance you could help me out with a diagram?

I understand the meter getting power to operate from the battery. I was also under the impression the meter also only needs one leg of wiring it's metering. How confused am I?

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[DaveInDenver]

Hmmm. I'm trying to follow you. Any chance you could help me out with a diagram?

I understand the meter getting power to operate from the battery. I was also under the impression the meter also only needs one leg of wiring it's metering. How confused am I?

What he's talking about is the reversing the polarity. A DC current shunt looks like this.



It's just a very low resistance that following Ohm's Law will create a small voltage drop.

V = I x R

If you have 1Ω shunt with 1 amp flowing you will measure 1V across the resistor. Real shunts are much smaller values but the principle is the same.

So being DC it will indicate positive in one direction, negative in the other. The shunt doesn't care, as they say it's all relative to it. But meters do, they usually don't like to go backwards. Some do, they'll look like this.



If yours isn't bidirectional, they just reversing the positive and negative will achieve the same effect. The DPDT switch is a way to make it easier to reverse polarity. This is showing how to reverse a motor but it'll work with anything.

 

[SoCal Tom]

Hmmm. I'm trying to follow you. Any chance you could help me out with a diagram?

I understand the meter getting power to operate from the battery. I was also under the impression the meter also only needs one leg of wiring it's metering. How confused am I?

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http://img.auctiva.com/imgdata/1/3/5/4/6/7/2/webimg/941942935_o.jpg
From the manufacturer.


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[TT-Tacoma]

http://img.auctiva.com/imgdata/1/3/5/4/6/7/2/webimg/941942935_o.jpg
From the manufacturer.


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Yes! That's exactly what I needed. Makes perfect sense now. Thank you!

Now my next decision is do I run some new wire to the battery from the bed for the solar controller and install the power monitor there so it's near my fridge or do I install the monitor under the hood and pop the hood everytime I camp to use my solar?

Decisions decisions.

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[SoCal Tom]

Redo the math at a variety of voltages, such as 12.8v (full charge resting), 13.6v (full charge floating on solar), etc.

At 12v, the battery is already down to around 50% capacity, so that's "worst case", not "normal operation".

For load calculations, I use 12.5, it’s about the average voltage I see under loads. For charging I use 14.8, which is the cutoff I have set for my controller.


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[SoCal Tom]

Yes! That's exactly what I needed. Makes perfect sense now. Thank you!

Now my next decision is do I run some new wire to the battery from the bed for the solar controller and install the power monitor there so it's near my fridge or do I install the monitor under the hood and pop the hood everytime I camp to use my solar?

Decisions decisions.

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The shunt should go near the battery, you can use really small wires for the meter, so get a roll of trailer wire or ribbon cable from amazon and put the meter someplace easy to check. If you are anything like me you will be watching it like a horny teenager at a porn convention the first couple of times you are out.


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[DaveInDenver]

Just to be that other guy...
Abbreviation of SI units what are named for a person such as Watt, Volt, Pascal, Hertz, et.al...
Uppercase is correct. If its a two letter abbreviation, second letter is lowercase and lowercase for non namesake units. Example, kWh, mHz.

You capitalize the proper name, James Watt but you would make the unit lower case, so it's watt. The unit is a common noun not a proper one. However the symbol is capitalized W. The exception to this is the liter, which can be either l or L to avoid confusion with number one (1).

For prefixes it's lower case unless you mean a million or more, MHz, MΩ or, of course, 1.21 GW!

An abbreviated unit what begins a sentence, I dunno. Grammer Nazis & Language Crafters will have to figger that out...

I had a advisor who liked to wield his "Grammar Hammer" on our submissions. He'd get down into details like spaces, abbreviation, hyphens. He'd say "You knuckleheads, 3xy is different than 3 x y!" Anyway, I believe he'd suggest that it would be poor structure to start a sentence with either a number (spell it out if you must) or a unit, so the question would never arise.

And, yes, there is an organization that dictates all of this, as you would expect.

BIPM - Bureau International des Poids et Mesures

https://www.bipm.org/en/measurement-units/
https://en.wikipedia.org/wiki/International_Bureau_of_Weights_and_Measures

Section 5 deals with formatting.

https://www.bipm.org/utils/common/pdf/si-brochure-draft-2016b.pdf

 

[TT-Tacoma]

The shunt should go near the battery, you can use really small wires for the meter, so get a roll of trailer wire or ribbon cable from amazon and put the meter someplace easy to check. If you are anything like me you will be watching it like a horny teenager at a porn convention the first couple of times you are out.


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I don't think the meter I ordered has a external shunt. At least not in any of the pictures on the Amazons. But, I can order a different one if it works better and an external shunt is the way to go.

Argh. My battery and fridge are at opposite ends of the truck. I wonder if it wouldn't just be easier to get a second meter and have one dedicated for solar input and one for power output.

If I can avoid running more wires I would like to do so. The plus side to keeping them separate is that I can have short wires from the solar controller to the battery which in my understanding is the best way to go.

Simply pop the hood, place solar panel, run wires, hang controller on hood, and connect to battery.

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[TT-Tacoma]

Ok. More testing has been completed. I had to fiddle around a bit to get the fridge to the temp that I wanted and didn't prechill for as long as I would have liked. So I started the test after it had reached the desired temp as to not skew the results.

12vdc battery power source
pre chilled for 1 hour
Empty fridge
knob is just below #1, thermo says 34 degrees inside
70 during day, 68 at night
9 hours
150 watts

150/9= 16.6
16.6/12.5= 1.3 amps per hour

This fridge is very impressive for being so old. It does pull quite a bit of power when running, 5 amps dc and 60 watts dc but I'm happy I found it and until I need something bigger I will be enjoying the life of no ice and no soggy burgers.

I am still trying to decide on a solar panel but I need to not be in a hurry because it is still January and we won't be out camping for another 3 or 4 months.

 

[SoCal Tom]

Trumps just added taxes on solar panels of 30%, so I would shop now if I were you/

 

[Toyman01]

Trumps just added taxes on solar panels of 30%, so I would shop now if I were you/

I just bought another 200 watts for that reason, even though I don't have a particular use for them. I did just read that one of the Chinese manufacturers has decided to build a US plant, so maybe the prices won't balloon too much.

 

[Bayou Boy]

Something that I quickly realized when researching my 100 watt solar setup on my last truck camper was that if you buy 20 volt panels and spend the extra money on a MPPT controller rather than a PWM controller it will substantially increase the amperage going into your batteries. After I installed an additional 100 watt panel for a total of 200 watts I took a picture of my Bogart Trimetric showing 13.5 amps into the batteries at 13.7 volts on a scattered cloudy day. It doesn't take long to recharge a bank of batteries to full at that rate. I'd have been lucky to get 9 or 10 amps with a PWM controller.

 

[TT-Tacoma]

Trumps just added taxes on solar panels of 30%, so I would shop now if I were you/

I'm glad I did when I did then. I ended up purchasing the Renogy 100w suitcase with controller last week.

Due to the cold weather and my lack of spare time I haven't had a chance to give it a proper test yet but I will eventually.

They are well built and the controller is easy to use and see when set up which is kinda what sold me on there system over the others available. And they have a nice case too!

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[john61ct]

9 hours
150 watts

150/9= 16.6
16.6/12.5= 1.3 amps per hour

If you mean 150 watt-hours (wH) total over those 9 hours.

For an average of 1.3AH per hour.

Then the units make sense.

31AH per 24 hours is mid-range.

Pre-chilling, high thermostat and mild weather, some have reported low as 10AH per day.

Running as a freezer or very hot weather, or loading with warm supplies would radically increase the energy usage.

Planning for worst case, figure 60.