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Old 16-10-2015, 12:47   #241
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by exMaggieDrum View Post
I'm a "trained" technician, and I went to electrical schools, and I got ABYC certs for 10 yrs, etc. etc. What I can tell you is that everyone I knew, and everyone I worked with understood that you can only work out resistance when dealing with each leg of the same gauge. Then you would have to add the others, but in general I don't remember any times where I had to work out that since you always go with the most conservative (lower gauge wire) for a circuit. I guess you could go through the math and do it but I just don't ever remember a circuit like that coming up.

I can't imagine why you would have a circuit with a big gauge wire going to a small gauge and then going back to a large gauge. Going from small to large (solar panel input to battery cables e.g.) or large to small (battery cables to panel feed e.g.) is normal, but you have to analyze the loads on each leg separately as you always have different amps in each leg. You have been exposed to some "untrained" techs if you have seen otherwise.
I have not seen many schools or courses that teach much more than looking at, and calculating for, wire resistance... Each connection, switch, fuse, busbar etc., also has voltage drop associated with it and this is ignored by most everyone... While small, they do add up, especially with the sheer number of folks using pliers as crimping tools......

If you are aiming for a 3% voltage drop in say a charging circuit, which pretty high with no voltage sensing, you simply can not aim for just 3% in the wire only and land on 3% for the entire circuit..... I just wish more schools and books would discuss the voltage drops across the other components too.
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Old 16-10-2015, 12:50   #242
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Re: Amps v Amp hours v Amps/Hr

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A conversion factor is simply a way of translating units to assure the proper dimensional analysis.
But why to work with units needing conversion back and forth
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Old 16-10-2015, 14:32   #243
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by Maine Sail View Post
Each connection, switch, fuse, busbar etc., also has voltage drop associated with it and this is ignored by most everyone... (snip)

If you are aiming for a 3% voltage drop in say a charging circuit, which pretty high with no voltage sensing, you simply can not aim for just 3% in the wire only and land on 3% for the entire circuit.....
+1
In the Caribbean I am used to finding drops of up to .25V in battery switches and those cheap grey combiner relays, with just 25 or 30A from a stock Hitachi alternator. Of course you could take AO directly to the house battery but most folks don't do that. Battery manufacturers enjoy extra sales!

The trick is that if everyone followed Maine Sail's advice and eliminated all those unnecessary drops then many stock alternators would not last long..

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Old 17-10-2015, 09:06   #244
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by Maine Sail View Post
I have not seen many schools or courses that teach much more than looking at, and calculating for, wire resistance... Each connection, switch, fuse, busbar etc., also has voltage drop associated with it and this is ignored by most everyone... While small, they do add up, especially with the sheer number of folks using pliers as crimping tools......

If you are aiming for a 3% voltage drop in say a charging circuit, which pretty high with no voltage sensing, you simply can not aim for just 3% in the wire only and land on 3% for the entire circuit..... I just wish more schools and books would discuss the voltage drops across the other components too.
Maine Sail, you are, of course, entirely correct. I have not ever seen any data or tests (other than the odd ad hoc ones that you may have done) published (whether reliable or not) on the resistance of components. These become more important for high amperage circuits but less so for most other circuits. If you are doing something (like regulating battery charging by an alternator) that requires very accurate voltage readings then you certainly want to minimize connections that might cause voltage drop - e.g. voltage sensors for those regulators.

Better to put the sensor direct on the battery and not at the end of a battery feed to a switch, or worse, on the other side of a switch (or relay etc.). But lacking any data to use for calculating voltage drop I don't think anyone will be doing that. Voltages will always vary on any given circuit just due to the variations in charging, battery state, amp draws, etc. So what we are really talking about, other than loss of efficiency and parasitic losses, is maintaining a floor, such as for electronics which may not work below a certain voltage.
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Old 17-10-2015, 10:33   #245
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Re: Amps v Amp hours v Amps/Hr

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But why to work with units needing conversion back and forth
Sounds good but metric is just as susceptible.

If you apply a force of X newtons to an object, how far will it travel in a predetermined time. Unless you do a little conversion (along with some additional info about the object), you can't solve the problem.

There are situations where metric is easier but it's not a cure all.
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Old 02-12-2015, 03:45   #246
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by StuM View Post
Many people get confused when discussing Amps, Watts, Amp hours and Watt hours. Frequently the confusion is the result of incorrect use of these terms by some forum posters. Hopefully the following will clarify rather than further fog the issue.

First - A bit of background

SI UNITS
When we talk about a quantity or a measurement, we talk about a number of units. To avoid confusion between people, it is essential that everyone is "talking the same language". There is an internationally recognised set of Units called The International System of Units or SI which enables this.

In the SI System, there are two fundamental types of measuring units: Basic and Derived
Basic Units are simple things like length,mass and time.(the specific units for these are the meter, kilogram and second). There are several other such base units including the Ampere (for electric current) and the Kelvin (for temperature)
Derived Units are more complex measurements which are formed by multiplying or dividing two or more different Base Units such as meters per second.

For convenience, we often describe quantities in terms of non SI units such as horse power, gallons, nautical miles, knots or gallons per hour. Any of these units can be converted to their equivalent SI representation by multiplying or dividing them by an appropriate number.
(Example: You can convert nautical miles to the SI unit "meter" by multiplying by 1852 and you can convert knots (nautical miles per hour) to meters per second by multiplying by 0.514444.
10 nautical miles = 18,520 meters and 4 knots = 2.05776 meters per second
The Kelvin is a special case, it is exactly the same size as a degree Celsius, but with a different start point, so you can convert Kelvin to degrees Celsius just by adding a number and when calculating temperature changes, you can use the two interchangeably)

DIMENSIONS
The number of basic units used in a measurement is called the "dimensions" of that unit. So a meter or a nautical mile is a 1 dimensional unit (meters) and speed in meters per second or knots is a two dimensional unit (meters and seconds).
Note that these do not have to all be different units: acceleration for example has 3 dimensions even though it only uses two different basic units, since it uses one unit twice (meters per second per second)
Even more complex units such as the Volt can still ultimately be expressed in basic units. (The volt is actually a 7 dimensional unit which can be expressed as meters x meters * kg / secs / secs / secs / Amps )
A plain number by itself, without any specified unit is called "dimensionless" (i.e. numbers like dozen or 42).

NAMES AND ABBREVIATIONS

Unit names are frequently abbreviated. It is a convention that where the name of a unit comes from the name of a person, the name or abbreviation is capitalised, all other units are not.
So we have Ampere, Volt, Watt and their abbreviations Amp or just A, Volt or just V and Watt or W, but meters (or m), seconds (sec or s), kilograms (kg), gallons (gal or g) etc

As mentioned previously derived units comprise two or more units which are multiplied or divided by each other. A great deal of confusing arises where these operations are not expressed properly.
When two units are multiplied together, the resultant unit name is written as the basic unit names without any intervening mathematical or other symbol.
(i.e The commonly used unit of "energy" stored or used in an electrical system should always be written as simply Ampere hour (or Amp Hour, Ah).
When one unit is divided by another , the two words are separated by a slash (divide sign "/") or the word "per" (or just p)
(ie gallons/hour, gallons per hour or gph).


FINALLY - AMPS, WATTS, VOLTS AND TIME
There are a number of different aspects of electricity that we are concerned with:
Current Flow, Force, Power and Energy

The basic SI unit of electrical "current flow" is the Ampere (Amp or A). It is the measure of the rate at which current is flowing through a specific point in a circuit at a specific point in time. It is equivalent to the rate at which elementary electric "charges" are passing the point (charges per second)
The derived unit of electric "force" is the Volt (V). It is a measure of the "potential difference" between two points in an electric circuit.
The derived unit of electrical "power" is the Watt (W) It is a measure of the amount of energy flowing (Amps) through a point where this is a certain potential difference (Volts) across the point. So: Watts = Volts * Amps and Amps = Watts / Volts
(Example: A solar panel generating 48 Watts at a point in time will generate a current of 4 Amps at 12 Volts or 2 Amps at 24 Volts.)

Note that Amps, Watts and Volts are all measures of different aspects of electrical force at a single point in time.

The final aspect of electricity above is "Energy" and this is where time finally rears its head. It is the amount of "work" that a given amount of electricity can do such as how long it can provide the required power (Watts) to a specific piece of electrical equipment and is expressed in Watt seconds.

Rather than Watt seconds, it is more practical to talk about Watt hours (1 Watt hour is equal to 3600 Watt seconds)
Also, if we are working with a nominal 12 Volt system, it is often easier to work in Amp hours where the voltage is assumed to be a constant 12V. Since Watts = Amps x Volts, 1 Amp Hour is, in this situation, the same amount of energy as 12 Watt hours (if we are working with a 24V system, 1 Amp hour is the same as 24 Watt hours)

THE BOTTOM LINE.

If we are working in Amps rather than Watts, then:

The "capacity" of a battery, the amount of power used or needing to be replaced is a number of Amp Hours or Ah.
It is not a number of Amps, Amps per hour or Amps/hr which as we see above are totally different things.

There is no such thing as a 100 Amp battery. (Some batteries are rated in CCA or Cold Cranking Amps - but that is not an indication of its capacity, it is an indication of the maximum rate at which energy can safely be drawn from it )

There is no reasonable practical meaning of Amps/hr or Amps per hour (which is actually a measure of how fast a current is increasing or decreasing.)


If we all use the correct units and abbreviations, we can avoid confusion and misunderstanding in our forum discussions. If we use them incorrectly, there is endless room for misunderstanding (and massive thread drift )
Thank you so much for posting this to the forum - I must say it has clarified some issues for me too after a number of years sailing. I guess in the end we become accustomed to understanding our batteries by what we experience living with them every day and we get lazy enough not to educate ourselves further.
I recently posted a video on a water maker overall and was told I had the wrong end of the stick by using the phrase 18 amps per hour as a current draw - and yes - I clearly was swinging the cat by the tail. Reading through these posts has been helpful - thanks again!
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Old 02-12-2015, 10:35   #247
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Re: Amps v Amp hours v Amps/Hr

Leaving to the poor reader, the tremendous burden of conversion when using the metric system and needing to use the Imperial time keeping standards, of seconds, minutes, hours, and days, none of which are commonly available in metric equivalents.


The decayear and decaday never really caught on, even in France. ;-)
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Old 02-12-2015, 13:37   #248
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Re: Amps v Amp hours v Amps/Hr

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Leaving to the poor reader, the tremendous burden of conversion when using the metric system and needing to use the Imperial time keeping standards, of seconds, minutes, hours, and days, none of which are commonly available in metric equivalents.


The decayear and decaday never really caught on, even in France. ;-)
The second is the primary standard time unit in both the imperial and metric systems. For your convenience it is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Fortunately, I keep a Cesium 133 atom handy for calibration purposes.
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Old 02-12-2015, 18:00   #249
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Re: Amps v Amp hours v Amps/Hr

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Fortunately, I keep a Cesium 133 atom handy for calibration purposes.
Paul, where the hell do you keep yours? I keep loosing mine and then I don't know what time it is!

Jim
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Old 02-12-2015, 18:07   #250
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Re: Amps v Amp hours v Amps/Hr

My ex boss had a saying: "When I ask what time it is, don't tell me how to make a watch"
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Old 02-12-2015, 18:26   #251
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Re: Amps v Amp hours v Amps/Hr

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Paul, where the hell do you keep yours? I keep loosing mine and then I don't know what time it is!

Jim
I keep mine in one of these cases:




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Old 02-12-2015, 18:49   #252
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by StuM View Post
I keep mine in one of these cases:




Hah! Excellent answer, Stu.

Jim
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Old 03-12-2015, 19:07   #253
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Re: Amps v Amp hours v Amps/Hr

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Paul, where the hell do you keep yours? I keep loosing mine and then I don't know what time it is!

Jim
If I knew where it was I wouldn't know "when" it was. Blame Heisenberg's uncertainty principle for that.
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Old 03-12-2015, 21:05   #254
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Re: Amps v Amp hours v Amps/Hr

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If I knew where it was I wouldn't know "when" it was. Blame Heisenberg's uncertainty principle for that.
We have a winner!!!

I doubt that anyone can beat that line.
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Old 25-01-2016, 13:52   #255
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by Maine Sail View Post
[...] Can we also stop typing RPM's (revolutions per minutes) when we really mean RPM (revolutions per minute)...
Isn't the proper abbreviation "R'sPM"???

Actually, I just wanted to bump this thread because the A/h topic came up again and I hope those interested can discuss it here instead of taking the new thread off-topic.
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