Amps and hours

Re: Sorry to be thick, but....

No it doesn't. It draws 38 Amps (I presume that's what you mean). Run it for 6 minutes and it consumes 3.8AH. Run it for 10 hours and it consumes 380 AH.
 
Re: Sorry to (STILL!) be thick, but....

I am serious, and I have read the thread from the top several times!!

As something of a pedant myself (ask my Mrs), you will not catch me talking about amps per hour (except for now). I can see that is maybe tautologous, but I don't see how it would make me get my calculations wrong.
 
Re: Sorry to be thick, but....

<<< Very sorry, I think people are being a bit pedantic here, my watermaker uses 38amps every hour it runs. >>>

But it doesn't. As soon as you turn it on your ammeter says '38'. Your ammeter doesn't know if it has been on for an hour or a second. Amps are a measure of flow. If you run it for an hour it will have consumed 38 Ah of current. If you run it for a minute it will have consumed 38/60 Ah.
 
Re: Sorry to be thick, but....

I see what (not Watt!) he means - his water maker uses 38A each hour it's on.

But the confusion is that it "uses" 38A whenever it is on, be it 1 hour or 1 second. In other words the use of the word "hour" is meaningless.

Yes, you can use it as a shorthand to determine the battery AH needed to keep it running, but the correct term is AmpHour (amps x hours) not Amp per hour (amps / hours).

Why not just use the correct term, instead of getting shirty about the wrong term meaning the same (which it doesn't).
 
Re: Sorry to be thick, but....

Maybe I can try another analogy.

Your boat sails at a speed usually quoted in knots. You find out how far you've gone by multiplying the speed by the time you've been sailing. The distance is in nautical miles. It's clear that sailing for half an hour at 8 knots (I wish!) will cover the same distance as one hour at 4 knots, two hours at 2 knots or four hours at 1 knot; in each case the distance covered will be four miles. In fact, we could call a mile a "knot-hour".

The speed of the boat (knots) is analogous to the current (amps) being taken from (or supplied to) the battery. We can take 8 amps for half an hour, 4 amps for an hour, 2 amps for two hours or 1 amp for four hours; each will have taken the same amount of charge from the battery. In the boat analogy that would be 4 miles, because we have a special name for the distance unit (a mile); we don't have a special name for the charge unit so we just call it 4 "ampere hours".

Where it does start to become important is when the load (or the charge rate) is changing. Suppose that our instruments take 3 amps, and run continuously. Suppose that we have a frig which takes 4 amps, but which runs only for 50% of the time. Suppose that our tricolour takes 2 amps (about 25 watts) but is only on for 8 hours each night. How much charge does it take out of the battery in each 24 hours, and hence how much charge must we replace?

Our instruments run for 24 hours at 3 amps, and therefore take 3 x 24 = 72 amp hours.

Our frig only runs for 50% of 24 hours (ie 12 hours) at 4 amps so takes 4 x 12 = 48 amp hours.

The tricolour only runs for 8 hours at 2 amps so takes 2 x 8 = 16 amp hours

The total charge taken is 72 + 48 + 16 = 136 amp hours

Now it's generally reckoned to be a bad thing to discharge a battery below it's 50% level, so you would need a 272 AH battery to provide for that load. If your alternator provided a steady charge rate of 40 amps, you would need to charge for 136/40 = 3.4 hours.

That's the simple computation. It's complicated by the facts that alternators don't provide steady charge rates, and that it is difficult to charge batteries up to a fully-charged state. But once you know the basics, it's just a case of multiplication, addition, division and subtraction. My 7 year old grandson can do it, I've just tried him. But you do need those basics.


Edit for typos
 
Second analogy.

Thanks for your second analogy, Peter, I really do appreciate your attempts to enlighten me!

You say, "In fact, we could call a mile a "knot-hour"."

But we don't - we measure speed (ie change in distance with respect to change in time) in terms of miles per hour (or nautical miles or Km or whatever per minute or per second or whatever). Yes, or knots.

So the measure of a 'knot' implies 'per hour' even if we don't say that bit.

Once we know speed, and time, we can easily calculate distance.

Believe me, this 52 year old is just as capable of doing the calculations you give, as your 7 yo Grandson - honest (I almost feel the need for one of those awful laughing icon things, but I'll refrain!).

If a layman, including Troutbridge maybe, likes to think in term of 'Amps per hour', he'll still do exactly the same calcaulations as you.

eg he thinks: tricolour of 24 watts at 12V is 2 "amps per hour", and, if he runs it for 8 hours, he works out (correctly) that the current drawn in that time is 16AH. Indeed, to me, another layman, "2 amps for 8 hours" seems very similar to "8 hours @ 2 amps per hour". (just as 16 miles in 8 hours is a mean speed of 2 miles per hour).

Same for the other items you list.

The water analogy is similar (of course it is, it's an analogy).

We measure water flow in, maybe, litres per hour. Again, when we know the time (in hours), we can work out the volume involved.

So the terminolgy of "amps per hour" might not be spot on, but our layman gets the right answer.

Now, terminology can be vital, particlularly amongst specialists in a given field. But, to a layman, what's more important: correct terminology or the right answer?

I hope none of my posts (ever!) have seemed 'shirty'. I am genuinely keen to learn.

So, pushing my luck, can you please give me an example of where talking of "Amps per hour" will lead to a wrong calculation?

Finally, as an aside, were Captain Bligh and his ill-fated crew wrong to record "Knots per hour" in the log-book of H.M.S. Bounty?

I fully understand if you've had enough of this now, though!
 
Is this correct?

I'm not sure why Peter's post has so intrigued me - maybe because Posh, Sporty, Baby etc are on the telly, and I've had enough of working this evening..

Anyway, I've been doing some delving.

1 Ampere is 1 coulomb of charge per second.

So, just as 'knots' implies 'nautical miles per hour', the measure of 'Amps' also implies time.

So to talk of 'Amps per hour' is as tautologous as 'Knots per hour'.

Now, IF that's correct, all I need to know is why, when calculating AH, don't we have to multply Amps by hours by 3600 (seconds in an hour)?
 
Re: Is this correct?

[ QUOTE ]
Now, IF that's correct, all I need to know is why, when calculating AH, don't we have to multply Amps by hours by 3600 (seconds in an hour)?

[/ QUOTE ] Because it's a composite unit. You can specify current and time period the two are multiplied together to give the composite "units" of AMP Hour, Amp second, Milliamp year or whatever.
On the subject of units, most measurement that lay people come across are units X per unit Y. ie Miles per Hour or Miles per Gallon or feet per second. This arises because mathematically there is a division involved. It's the difference between a cumulative measurement and an instantaneous one. If someone goes 120 miles in 2 hours then their speed is 60 miles per hour this derives an average for the period. Amp hours arise from multiplication the measurement is a cumulative one, a sum over increments of time. There is no per anything. The most common similar measurement is Kilowatt Hours this again is a cumulative measurement a sum of all the watt hours in a long period of time.
 
Re: Is this correct?

[ QUOTE ]
when calculating AH, don't we have to multply Amps by hours by 3600

[/ QUOTE ] 'Cos that would give ampere seconds.

While not disputing your definition of an ampere you might like to ponder on the real definition:
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 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10&#8722;7 newton per metre of length.

And since youve mentioned seconds:
The second is 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 caesium-133 atom.

But hey you can read them all in Kaye and Laby Online

/forums/images/graemlins/wink.gif
 
Re: Is this correct?

Let's try again. In the SI system there are 6 base units from which ALL others are derived. The 2 we are interested in here are Amps (A) which measure current and Seconds (s) which measure time.

Batteries collect "charge" which in the SI system is measured in Coulombs (C) derived from Current x time i.e. 1C=1Ax1s spoken as 1 Ampsecond. Coulombs are rather small units so instead the pragmatic chappies and chappesses have used Ah which is of course 3600 As or C.

And does it matter? Well if you divide instead of multiplying you get the wrong answer and if you say divide when you mean multiply there is a tendency for others to think you are stupid.
 
Re: Second analogy.

Neal

In a sense you are right; it doesn't matter what you call them provided that you get the right answer. What we are really talking about is convention.

It's not just convention; the Ampere is one of the basic units of the Systeme Internationale, on which the whole of the metric system is based. As a matter of interest, the other base units are the metre, the kilogramme, the second (of time), the Kelvin (temperature) and the mole (amount of substance). With such an important convention, used all over the world, why try to misuse it?

There. LakeSailored by watching old New Tricks!
 
Thank you, that\'s the bit I was missing!!

"the pragmatic chappies and chappesses have used Ah which is of course 3600 As or C."

That explains why I don't need to multiply (yes, I did mean multiply) by 3600.

I can sleep soundly now.

Thanks to all for your patience.
 
Re: Thank you, that\'s the bit I was missing!!

I wonder what is the correct term for a pedants convention? Or should it be an eruption of pedantry? A pedantfest? Someone here must know the correct definition.
 
Re: Sorry to be thick, but....

"Something that draws 3 Amps per hour draws 3 AmpH in the first hour, 6 AmpH in the second hour, 9 AmpH in the third hour etc"

That is palpably not the case. You are describing a situation where 18Ah have been drawn in total (up the the 'etc'). Whether your misconception is technical or linguistic, it illustrates precisely why standard units are so necessary.

If you meant to state that something that draws 3 Amps per hour draws 3 AmpH in the first hour, 3 AmpH (ie 6 AmpH in aggregate) in the second hour, 3 AmpH (ie 9 AmpH in aggregate) in the third hour etc...then you would have been correct.

Or, linguistically, you might accurately have said: Something that draws 3 Amps per hour draws 3 AmpH in the first hour, will have drawn 6 AmpH after the second hour, 9 AmpH after the third hour etc.

This is precisely why the 'pedantry' the original poster mentioned is so necessary.
 
Surely not!

"Linguistically, you might accurately have said: Something that draws 3 Amps per hour........."

Surely you mean, "linguistically, you might accurately have said: Something that draws 3 Amps.......!."

See, I'm fully converted.

I agree with your sums, though!

We seem to have gone full circle.

Time for bed, I think.
 
Re: Second analogy.

[ QUOTE ]
So, pushing my luck, can you please give me an example of where talking of "Amps per hour" will lead to a wrong calculation?

[/ QUOTE ]Yes, if you said that to anyone with knowledge of electricity they would not be sure what you meant - they would have to guess or ask you some questions. Nobody who understands (or has training in) electricity would talk about 'amps per hour'. Isn't that sufficient to put this to bed?
 
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