Amps v Amp hours v Amps/Hr

[StuM]

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 )

[Wotname]

Very well put Stu

Very small point of different. You seem to infer that the term dozen in a number when in fact it isn't.
It is a measurement of quantity in its own right. The fact that it is the equivalent of 12 doesn't make it a number. Consider it more the terms "drum", "reel" and "barrel" etc.

OK, you were expecting this I know

[Wotname]

Are we going to discuss "charge" WRT electrical units or is that a bridge too far....

[StuM]

Originally Posted by Wotname

Are we going to discuss "charge" WRT electrical units or is that a bridge too far....

That's a Wheatstone too far

[StuM]

Originally Posted by Wotname

Very well put Stu

Very small point of different. You seem to infer that the term dozen in a number when in fact it isn't.
It is a measurement of quantity in its own right. The fact that it is the equivalent of 12 doesn't make it a number. Consider it more the terms "drum", "reel" and "barrel" etc.

OK, you were expecting this I know

We'll have to agree to disagree on this one. As far as I am concerned:
couple, dozen,score,gross are just words for numeric quantities in exactly the same way that hundred, thousand, million etc are (the only point of difference is that they are non-metric numeric quantities)

[noelex 77]

Originally Posted by StuM

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)

One of the advantages of talking about AHr instead of WHr is that the voltage is not specified. It incorrect to assume this is 12v.

For example, if my solar panels have put out 150 AHrs for the day, the voltage will have varied. It is unlikely to have been as low as 12v even briefly. Typically as a weighted average it may have been around 13.7v.

Some boat battery monitors will work in WHrs as well as AHrs. In general, as a rough approximation I would expect the 150 AHr output from the solar panels to be recorded as around 150 x 13.7 = 2055 WHrs not 150 x 12 = 1800 WHrs.
Assuming 12v gives a significant error.

For a lot of boat systems AHrs is more useful that WHrs (although there are exceptions) even though technically WHrs is a more correct term for energy use. Part of the reason is that the higher charge voltage and lower discharge voltage creates a very large discrepancy between the WHrs that can be taken out of a battery compared to the WHrs that have been put back by the charging sources. AHrs are much closer.

[Wotname]

Originally Posted by StuM

We'll have to agree to disagree on this one.....

Done

[valhalla360]

Originally Posted by Wotname

Very well put Stu

Very small point of different. You seem to infer that the term dozen in a number when in fact it isn't.
It is a measurement of quantity in its own right. The fact that it is the equivalent of 12 doesn't make it a number. Consider it more the terms "drum", "reel" and "barrel" etc.

OK, you were expecting this I know

Your examples don't work. The unit barrel is a volume measurement (ie: an oil barrel is typically assumed to be 42 gal). Reel is typically used as a measure of line length (ie: a 100ft reel of 1/2" line).

What is a dozen (without applying a unit to it)...you will be hard pressed to come up with anything other than a number unless you add a unit (ie: 3 dozen rolls).

Of course you could say you have 3 12's of rolls. A bit of an odd way of saying it but not incorrect. Does that mean 12 is not a number?

The term "dozen" by itself has no meaning other than as an alternative way of saying 12.

[StuM]

Originally Posted by valhalla360

Your examples don't work. The unit barrel is a volume measurement (ie: an oil barrel is typically assumed to be 42 gal). Reel is typically used as a measure of line length (ie: a 100ft reel of 1/2" line).

What is a dozen (without applying a unit to it)...you will be hard pressed to come up with anything other than a number unless you add a unit (ie: 3 dozen rolls).

Of course you could say you have 3 12's of rolls. A bit of an odd way of saying it but not incorrect. Does that mean 12 is not a number?

The term "dozen" by itself has no meaning other than as an alternative way of saying 12.

Yep.

I fail to see how you can have any distinction between 12 apples and a dozen apples. They must have the same number of dimensions.

If I can measure a distance as "a hundred fathoms", how is that different from "a dozen fathoms"?

Maybe if we all used the FFF system of units, it would be clearer:
https://en.wikipedia.org/wiki/FFF_system

[dwoodall]

Numbers like dozen, bushel, sack, or even mole or simply counting units. For example, a mole of a substance is always the same number of particles of some substance, 6.02 x 10^23. I have the hardest time getting students to understand that counting units exists simply to help us count very large or very small number of things. The other thing is that amperes are technically the amount of coulombs of charge that passes a given point per second in coulombs per second. A coulomb is 6.241×10^28 units of charge. Which again, is another counting number as the unit charge is indeed a very small amount of charge and is defined by the charge of one proton. This leads us to the idea that an ampere is equal to one coulomb-second. Either way it measure the flow of charge. The amount of charge should rightfully be measured in coulombs, but never is, instead using amp-hours as a more pragmatic unit. Amp-hours is then the amount of charge that accumulate if one of ampere flows for one hour (assuming no loss or leakage).
I agree that misunderstanding the amperes and amp-hours thing leads to a lot of confusion and misuse.

[StuM]

Originally Posted by dwoodall

The other thing is that amperes are technically the amount of coulombs of charge that passes a given point per second in coulombs per second. A coulomb is 6.241×10^28 units of charge. Which again, is another counting number as the unit charge is indeed a very small amount of charge and is defined by the charge of one proton. This leads us to the idea that an ampere is equal to one coulomb-second. Either way it measure the flow of charge. The amount of charge should rightfully be measured in coulombs, but never is, instead using amp-hours as a more pragmatic unit.

This one was thrashed out fairly comprehensively in the drifted thread which prompted this one http://www.cruisersforum.com/forums/...omestic+fridge

The Actual SI definition of the Ampere is not as a derived unit of Coulombs and seconds. It is a Base unit defined as:

"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length."

In SI, the Coulomb is the derived unit (one Amp sec).

(See Essentials of the SI: Base & derived units)

Anyhoo, I didn't write this post for physicists, but as an attempt to explain amps and amp hours to a non-technical cruiser. It necessarily contains a number of simplifications. Trying to introduce Coulombs would have just raised the complexity without adding anything of substance.

[StuM]

Originally Posted by dwoodall

Numbers like dozen, bushel, sack, or even mole or simply counting units.

Oh, and incidentally a bushel is not a number. It is unit of either mass or volume depending on usage.

As a unit of volume, it is equal to 35.2 or 36.4 liters depending on whether you use the US or imperial bushel.

As a unit of mass, it has various weights depending on the commodity is is being used to measure. See https://en.wikipedia.org/wiki/Bushel

[GILow]

StuM,

Thank you for this post. The surprisingly regular misuse of amp hours on CF is a bit of a grain of sand in the otherwise well oiled machine. Hopefully this post will help people understand the concept better and convey their hard won wisdom more logically.

Matt



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[GILow]

BTW, don't forget the baker's dozen.


Sent from my iPad using Cruisers Sailing Forum

[Maine Sail]

Originally Posted by StuM

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 )

AMEN!!!!!

Just one small point...

CCA (0F) or MCA (32F) is not about how much current can "safely be drawn". It is about how much current can be drawn at a specific battery temperature (0F or 32F), for a specified time (30 seconds) while remaining above a terminal voltage of 7.2V. In teh US this is an industry standardized test, SAE J537, for battery performance at varying temps not really a test for battery safety. But yes, you're are absolutely correct that there is no such thing as a 100 AMP battery. IEC and DIN CCA testing is similar though the duration and low voltage threshold are different.

Originally Posted by StuM

There is no reasonable practical meaning of Amps/hr or Amps per hour

Amen! It is one of my peeves just like RPM's vs. RPM..... Still when a non-electrically versed person writes "amps per hour" or even Amp/hr I usually know what they are saying..

Originally Posted by StuM

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 )

I agree! Can we also stop typing RPM's (revolutions per minutes) when we really mean RPM (revolutions per minute)...