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SV THIRD DAY · 02-10-2015, 08:52

Quote:

Originally Posted by Poster

As long as we're digressing into pedantry, there's another problem with Ah: it's only really useful if you live primarily or exclusively in a 12V world.

Not so, for me. Computing devices mostly use 5V. 12V bus on the RV. 24V bus on a big boat. 36V and 48V on my electric bike batteries. 60V on my solar strings. 120V from inverter and shore. (240V if we're traveling overseas in a cruiser!)

So Ah is pretty awkward, and I find it easier to think in Wh to be able to make comparisons and reason about energy usage.

Now we have just gone mad folks....

An Amp Hour into and out of your batter bank is an Amp Hour no matter no how you take it out or put it back into your battery bank!

I give up.....

But this should be a GREAT example of why you...ah...well....

Oh never mind, saying the obvious will get you labeled a hater, bully or know it all in a world that doesn't like a truth teller, so I will just shut up and get back to work.

redhead · 02-10-2015, 08:56

And here's another side....

I am currently learning about the power draw of various things in my new/old boat. We are currently discussing what must be upgraded vs what should be upgraded vs what we can afford.

Pissing contest or not, I have learned more following this thread than in the last 15 months of reading all the books (including Calder I'm ashamed to admit).
Actually, your back and forth bashing and witty repartee's kept my interest up and my brain a chance to catch it's breath. I am now marginally smarter than I was before.

And for that, I thank you!

**newhaul** · 02-10-2015, 08:58

Quote:

Originally Posted by SV THIRD DAY

Now we have just gone mad folks....

An Amp Hour into and out of your batter bank is an Amp Hour no matter no how you take it out or put it back into your battery bank!

I give up

Agreed.

Now get back to fix in your house. Mexico is calling.

TeddyDiver · 02-10-2015, 09:00

Quote:

Originally Posted by SV THIRD DAY

Ahhhh....there are no more German made Danfoss compressors, hasn't been for years.

It was not me who brought up "German made Danfoss" compressors into this discussion. I just made a point that the actual compressors are the same whatever AC/DC 12/110/240V whatever...
BR Teddy

SV THIRD DAY · 02-10-2015, 09:23

Quote:

Originally Posted by TeddyDiver

It was not me who brought up "German made Danfoss" compressors into this discussion. I just made a point that the actual compressors are the same whatever AC/DC 12/110/240V whatever...
BR Teddy

This is where I should just shut up and let it go...but it's also where Cruise Rumors come from, so forgive me for jumping in and clarifying the above statement, I promise I'm not trying to be an ass (at least not today anyway).

It IS 100% correct that a 12v/110v/240v (Danfoss compressor for example) IS indeed the SAME exact compressor with just a different controller to accept the different voltages and current types.

But lets not give the totally wrong impression that a Danfoss compressor (again for example) is the same as brand X, Y, or Z in terms of power consumption (dare I say Amp*Hrs) per Btu cooling capacity. The compressor units I'm pulling out of these dorm type refrigerator and sending to the recycling center and replacing with ours are a JOKE in terms of energy efficiency. You can call them energy star or green all you want, but they suck. If they didn't hell I would just keep them in the unit and save all the expense of a new compressor, soldering, vacuuming down, recharging, bla bla bla and just swap out the 120v AC controller with a new 12v DC controller.

That's what some people are missing that don't have the benefit of the side by side testing data. There is a false assumption that the only issue on the table here is the 12v DC vs 120v AC power input. No, there is a difference in efficiency in the compressor itself regardless of the voltage. Trust me if all these marine refrigeration companies using the high priced Danfoss BD35/50 could switch to the no name made in China compressor and not get kicked in the teeth for energy usage they would.

Cadence · 02-10-2015, 09:44

Quote:

Originally Posted by StuM

I know I'm a pedantic old fart, but this misuse of units really grates with me.

It's not Amp/hr or Amp/day (the slash is a "divide" sign) . It's Amp Hours or Amp Hrs/Day. If you are pulling 4 Amps for 2 hours you use 4 x 2 = 8 Amp Hrs. Not 4/2 =2 Amp/hrs.

Edit: And you can't "store Amps" An Amp is the basic SI unit of current flow

Lighten up. Every body knew it was per not divided by.

**newhaul** · 02-10-2015, 10:15

Quote:

Originally Posted by StuM

I know I'm a pedantic old fart, but this misuse of units really grates with me.

It's not Amp/hr or Amp/day (the slash is a "divide" sign) . It's Amp Hours or Amp Hrs/Day. If you are pulling 4 Amps for 2 hours you use 4 x 2 = 8 Amp Hrs. Not 4/2 =2 Amp/hrs.

Edit: And you can't "store Amps" An Amp is the basic SI unit of current flow

If you read what all the battery manufacturers say on the top of a cars starting battery you can store amps in my old pickup I had two batteries that said 1200 CCA on the top which meant I had 1200 amps available in the battery at 32°f or 0°c

Scot McPherson · 02-10-2015, 10:26

That's not stored amps, that's how much the battery can produce in a single shot. The total number is WAY higher than that

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**newhaul** · 02-10-2015, 10:30

Quote:

Originally Posted by Scot McPherson

That's not stored amps, that's how much the battery can produce in a single shot. The total number is WAY higher than that

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As a mechanical engineer I do understand that however I was just giving an easily read example of storing amps

SV THIRD DAY · 02-10-2015, 10:35

Quote:

Originally Posted by Scot McPherson

The total number is WAY higher than that

Yep...enough to turn a 9/16" wrench into a resister and melt.

Don't ask me how I know.....

jeepbluetj · 02-10-2015, 10:54

Quote:

Originally Posted by SV THIRD DAY

Yep...enough to turn a 9/16" wrench into a resister and melt.

Don't ask me how I know.....

I think many of us have had that exact same experience.

Scot McPherson · 02-10-2015, 11:04

Quote:

Originally Posted by SV THIRD DAY

Yep...enough to turn a 9/16" wrench into a resister and melt.

Don't ask me how I know.....

I know how you know. I blew a screw-driver in half once, and the slag burned my hand pretty good and black. I think about a dime size spot was 3rd degree burned instantly.

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TeddyDiver · 02-10-2015, 11:04

Quote:

Originally Posted by SV THIRD DAY

No, there is a difference in efficiency in the compressor itself regardless of the voltage. Trust me if all these marine refrigeration companies using the high priced Danfoss BD35/50 could switch to the no name made in China compressor and not get kicked in the teeth for energy usage they would.

Ok, but the difference is in the speed controller circuit IMHO, not in the compressor itself..

**sailorchic34** · 02-10-2015, 13:02

Quote:

Originally Posted by TeddyDiver

Ok, but the difference is in the speed controller circuit IMHO, not in the compressor itself..

Ah, but part load performance, that is with the compressor ramped down a bit to say 70% rpm might very well increase the COP / efficiency. I, "cough", forgot about those new fancy variable speed 12v compressors. So Rich's fridges can be more efficient, under many conditions.

I agree that the basic compressor design, ie reciprocating piston compressor has a base efficiency. Part load performance though can provide increased COP and lower energy draw.

What would be really cool is to put a data logger on both the 12V and 120V fridges (with inverter), in a boat and log the cool down and day to day energy draw in 70, 80 and 90 degree temps for a week. Oh sure one could use a room with a heater too to simulate different cabin temperatures.

My though is that for a given box that the heat gain into the box will be the same at a given temperature, so that overall the same amount of work/ heat transfer has to happen. While the 12V VSD compressor can be more efficient, it also runs longer. where as the 120V compressor has only a few minute cycle time. So it can be less efficient, but if it only runs 1/3 the time of a 12V compressor, the overall loss to inefficiency is not so great.