Are my Maths Correct?

[Stu Jackson]

Originally Posted by Stumble

Can you provide a primer for me to try and get this stuff right in my head?

Charilie Wing wrote a prettfy good, thorough and understandable book about this subject, which comes up all the time.

None of us were born electricians, so we, gulp, studied the subject. Who said high school was over???

I've posted this any number of times before, too, and it includes a link to some good other books.

Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html

[Andrew Troup]

Stumble

If you're of a more mechanical bent, you might find it helpful (at least for amps and volts) to think in terms of an analogy based on fluid flow.

Voltage is the electrical equivalent of fluid pressure, and current (amps) is the equivalent of fluid flow rate.

Say you were building yourself a house off grid, but you could run a hose to the site from a water reservoir up the hill, and needed a way of pressing clay into bricks.

If the reservoir was 50 feet higher than the site, you would need lots of water because the pressure would not be very high: you would need to build a hydraulic cylinder of large diameter, to have a large piston area.

If the reservoir was 500 ft higher, providing ten times the pressure, you could build a cylinder with a piston one-tenth the size and still get the same squeeze force.

In both cases, the cylinder stroke would be the same.

To supply enough water to press the same number of bricks in a day, the low pressure option would need a big diameter, thin walled hose. The high pressure option would use a small diameter, thick walled hose, and use one tenth as much water, per day.

The high pressure is like higher voltage. The associated low flow rate is like lower amperage, or current.

The fluid flow rate would probably be measured (in the US) in gallons per minute.
Which corresponds to amps.

The AMOUNT of water used in a day would be measured in gallons.
Which corresponds, conceptually, to amp.hours, or amp-hours.

You'd need to know this to work out if the reservoir was BIG enough.

The only time you might calculate the fluid equivalent of amps PER hour (gallons per minute PER minute) would be if you were interested in an unusual application where the flow rate had to change rapidly.

Maybe you're building a hydraulic cylinder to launch planes off the deck of a carrier, and you need the flow rate to build steadily during the course of a single stroke from zero to 1000 gallons per minute. If the stroke took one tenth of a minute, that's a flow acceleration rate of 10,000 gallons per minute, PER minute.

In electrical terms, the conceptual equivalent is amps PER hour. And there is virtually no useful application for such a concept on a boat.

Much the same thinking can be applied to watts as to amps, on a boat. This is because we're dealing (generally) with a constant voltage.

- - - -

Watts is a way of measuring how fast we can do a given amount of work. if we're pressing bricks, it gives us an idea how many bricks we can press in an hour.

We could achieve the same work rate with the high pressure, low flow option as the other way; the rate of doing work with fluid is simply Pressure times Flow Rate

In the electrical case, the rate of doing work is watts, which is volts times amps.

Given that the DC volts on a boat will generally be 12 or thereabouts, we can therefore apply the same reasoning to watts as we do to amps. There is no useful application for watts PER hour.

Watt.hours, on the other hand, is useful and relevant. It's a measure of work needing to be done (bricks to press) or work done (bricks pressed).

Because watts is a measure of rate of doing work (bricks pressed PER hour): if we multiply watts by hours, we can work out how many bricks we pressed (in that number of hours).

On a boat, watt.hours will tell us how many sinkfuls of water we can heat, or fill, or pump out. On a big boat, will tell us how many times we can raise the anchor, and/or the mainsail.

It's a measure of storage capacity, and so is amp.hrs.
(Because the other relevant variable, voltage, is essentially constant)

[Cavalier]

Stumble,



Simply put:



1 Ampere is the measure of 1 Coulomb of charge passing through a point during 1 second. More specifically 6.241×10^18 electrons per second. It would then be nonsensical to state Amps per hour, as Amps are already (per second). If a device has the capacity to deliver 1 AmpHour then it is capable, in theory, of supplying 1 Coulomb per second for the period of 1hour.



Similarly 1 Watt is defined as 1 Joule per second. Stating Watts per hour is again nonsensical as that would be like saying 1 Joule per second per hour!


Sent from my Windows Phone using Tapatalk

[Alenka]

Thanks to everyone for input and advice. The botom line seems that to run AC from a battery bank, topped up from a fuel cell, is asking a bit much of current small boat technology.

Which is a pity when you consider Toyota Prius have electric AC and some models can run it from a solar roof.

More of a pity is that I will have to resort to a relatively noisy petrol driven genny.

[Andrew Troup]

I'm guessing it's not an option to reposition your boat further from the equator?

(Seems a shame to incur the cost and inconvenience of a truly movable home, only to end up sticking it somewhere where the very air is .... sticky!)

What I'm going to say is philosophical reflection on an epic scale, not personal attack on the OP, but it does strike me that we humans are probably unique in our capacity to bugger up the big things through optimising the little ones.

I'm sorry in advance because I can hear hackles raising as I type, and I realise this will seem to be an inappropriate forum, but I can't help reflecting ruefully on how quickly the weather we currently associate with equatorial regions would 'come to meet us all, wherever we are', if all of us, or even just the few billion of us in hot climates, had recourse to burning liquefied sunlight in a small genset to run air conditioning.

The amount of heat liberated in the short term, and captured in the long term, is eye-wateringly high, in consideration of the modest and fleeting degree of local cooling.

And in thermodynamic and economic terms, it seems to me it's a tiny bit like using centuries-old Cognac to moisten a towelette to cool our foreheads.

Cognac, for that matter, that we stumbled across in a vast buried hoard, and can have for the price of digging it out and shipping it, rather than anything remotely resembling the price of replacement.

[azsailor12]

The Prius battery is over 36kWh at 200V. If you put 3050Ah of batteries in your boat, you should have no problem running an a/c!

[StuM]

Originally Posted by azsailor12

The Prius battery is over 36kWh at 200V. If you put 3050Ah of batteries in your boat, you should have no problem running an a/c!

You forgot to add:

"... which is capable of cooling the few cubic feet inside the almost hermetically sealed Prius."

[Stumble]

So...

voltage is pressure
amps is rate and
amp-hr is volume

Got it. The mechanical corollary works well for me.

[azsailor12]

You're right. The gas motor will still have to turn on after a short while to charge the battery back up, even with the regenerative charging from the brakes.

[a64pilot]

Originally Posted by Alenka

Thanks to everyone for input and advice. The botom line seems that to run AC from a battery bank, topped up from a fuel cell, is asking a bit much of current small boat technology.



Which is a pity when you consider Toyota Prius have electric AC and some models can run it from a solar roof.



More of a pity is that I will have to resort to a relatively noisy petrol driven genny.

The solar roof on a Prius can barely run a little computer fan that does help exhaust hot air while the car is sitting, but it can not even come close to running the AC. The big huge Prius traction battery can only run the AC for about ten min. or so on high before the battery is depleted enough so that the internal combustion engine is started to re-charge it. AC's pull a huge amount of energy, whether electric or run direct off the internal combustion engine.

[chala]

Originally Posted by Andrew Troup

Stumble

Originally Posted by Cavalier

1 Ampere is the measure of 1 Coulomb…..
Similarly 1 Watt is defined as 1 Joule per second.

Andrew, please notice the capital letter.

Originally Posted by Cavalier

1 AmpHour

Need correction.

Any way for us mortal a bit of understanding does help.

[chala]

By the way, as tested in the tropics, small fans (1.6A 12V) does help to render the boat more liveable in a hot climate.

[cwyckham]

Originally Posted by Stumble

So...

voltage is pressure
amps is rate and
amp-hr is volume

Got it. The mechanical corollary works well for me.

Exactly. Your summary is much more succinct than your teacher's, showing that you grasped the concept well. Now you can also spread the good word as this is a very common mistake!

[Andrew Troup]

Originally Posted by chala

Andrew, please notice the capital letter.

I'm not so sure, when I think about it, that capitalisation is required when talking about the concepts or the names, as opposed to specifying a quantity and tagging it with the proper unit symbol.

eg when talking of wattage, amperage, increased amps, a reduced number of watts: these (to my way of thinking) are not units, but names.

They should be capitalised at the start of a sentence only.

My first hit in Google when I sought another opinion was this, from

Writing SI units and symbols

Charles Poynton


<< Write a numeric value with units in either the journalistic style, using prefix and unit names (four kilohertz); or the scientific style, using prefix and unit symbols (4 kHz) >>

Further down the page, if you google the title, you will see all the SI
names, and the units, both listed.

and in the case of the watt, the "name" of the unit is listed as "watt"

while the "unit" itself is not Watt, but W

Same with amps: the name is not capitalised, and the unit consists of the single capital letter A

I thought I'd better look for an American source (sigh) so I switched to spelling 'capitalise' with a z (hmm ... thought for the week: why don't Americans spell "capitalism" with a z?)

and there my first hit was this:


<<According to the IEEE/ANSI standard, the acceptable spellings for Wh, when it is written out, are

watt hour (preferred)
watt-hour
watthour

The names of SI units are not capitalized unless they are used in a situation where any word would be capitalized, such as the beginning of a sentence. Of course, the standard unit of energy would be the joule so the watthour is more of a colloquialism anyway.


EDIT: The standard that defines usage of SI units is IEEE/ASTM SI 10-2010, "American National Standard for Metric Practice".>>


Of course he or she is right about the watt hour being a colloquialism; the SI unit of time is the second (s), not the hour.

[poiu]

Originally Posted by Alenka

I have a 240 volt AC unit rated at 800 watts.

Am I correct that in theory to run this through an inverter it would draw 67 amps per hour from a 12 volt battery bank? 800/12 = 66.6.

Actually, I gather it would draw maybe 20% more because of losses in the inverter. So starting with a 600ah battery bank, that should never be allowed to drop below 50% (300 ah) could I power the AC for a maximum of 3.75 hours?

The bottom line is that I only want to run it for about an hour at a time. I find that sufficient just to get the boat back into a comfort zone.

It is perfectly possible and is actually on my job list, item 102. In the Caribbean I haven't found it absolutely necessary, even in Summer. Greece in August can get horrid hot and I know where you are coming from. You can minimise start up current problems with a soft starter on the air con compressor - I bought one, but not fitted it yet. You can improve efficiency by running a DC water pump, ideally one per compressor or an AC pump on an inverter. The pumps are power pigs and need to be in your calcs.

To do this routinely, say every night at anchor for only an hour you only need 80A on your figures, which seem ok to work with. That's not so much. About 1.5m2 of good solar panels could do it. 1.5m2 x 18% efficiency x 1000W/m2. /12v x 4.5 hr charge per day =101A output. If you want to do this more than an hour then get a genset.