Simple electrical theory question

nigelmercier said:
It isn't a unit. It is a combination of units*, just like Nm (Newton metre) which is far more ambiguous.
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I am sorry to contradict but Nm is a perfectly acceptable unit as other "combinations of units" if they measure what they say they measure. Ah is used to measure energy but if interpreted as "ampere times hours" cannot measure energy because it does not represent it!

macd said:
Fair enough, but if that's advocating anything, surely it's Watt-hours instead, which allows for the issue of differing voltages? Yet it's abundantly clear that plenty of people on here struggle enough with Ah without introducing another variable.
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My point was that one of the reason for having plenty of people on here struggle with Ah is because Ah is an ill defined unit of energy.

Daniel
 
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danielefua said:
My point was that one of the reason for having plenty of people on here struggle with Ah is because Ah is an ill defined unit of energy
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I accept entirely what you say about the lack of definition of Ah., but my take on such 'struggles' is clearly different from yours, for I don't think it's to do with the lack of any formal definition. You can pin down any definition as much as you like, but if people are ignorant of it, that gets you nowhere. Ah is a useful and widely-understood concept when talking of 12 and 24V systems, not least because battery manufacturers use it to describe their product. To them, it must be self-evident that the Ah of a 12V battery is in terms of 12V, so why add a tautological variable?
To use Watt-hours, which is as unambiguous as you seem to desire, would in my view simply muddy waters which are sometimes already murky enough. Or do you have another proposal? (I ask in all curiosity, because you probably know more about this stuff than I do.)
 
macd said:
I accept entirely what you say about the lack of definition of Ah., but my take on such 'struggles' is clearly different from yours, for I don't think it's to do with the lack of any formal definition. You can pin down any definition as much as you like, but if people are ignorant of it, that gets you nowhere. Ah is a useful and widely-understood concept when talking of 12 and 24V systems, not least because battery manufacturers use it to describe their product. To them, it must be self-evident that the Ah of a 12V battery is in terms of 12V, so why add a tautological variable?
To use Watt-hours, which is as unambiguous as you seem to desire, would in my view simply muddy waters which are sometimes already murky enough. Or do you have another proposal? (I ask in all curiosity, because you probably know more about this stuff than I do.)
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If you go over the first replies to the OP you will find why I think that Ah should not be used at all. In fact I am confused too... Why adding two containers of energy in whatsoever manner you should not obtain the sum of the single energies? Odd, isn't it? I thought that energy conservation was one of the very few laws we were still not allowed to doubt! :)

I will not insist further but for me the unit Ah should be abandoned for something else not as confusing. The choice is wide and I have no preferences; surely for batteries the unit kCV (kilocoulomb times volt) would be pretty appropriate but I can already hear shouts of protest! Just consider that it is a mere matter of habit: if manufacturers start using a new unit, everybody will follow soon.
It is like the migrating from Horse-Power to watt or millibar to hectopascal, which, by the way, were not ill defined. At the end it is not such a big deal.

...over and standby on CH 16...
:)
Daniel
 
AHr is the better unit when dealing with most boat system measurements (not all)
It has several advantages for most discussions over the more technically accurate (for energy) WattHour.

1. It is what battery monitors measure (some will measure watt hours, but its less common as its more difficult for no advantage)

2. It is how battery manufacturers rate their batteries.

3. Monitoring AHrs in and AHrs out makes a natural compensation for battery inefficiency. It tends to be more accurate than WHrs in and WHrs out even when factors for battery inefficiency compensated for.

The call to use WHrs for most boat measurements is misguided. AHrs are easier and more accurate.
 
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I just love an article which unambiguously states that there's no such thing as leisure or deep cycle batteries and then follows that astonishing statement with a description of just some of the differences in construction between starter, leisure and true deep cycle batteries!

and contrary to popular myth and legend, you can get true deep cycle batteries in 12v, Trojan do them for starters ... err, no, not for starters! oops :-)

as for the unit argument, go and try and do your battery management calcs in kilojoules per volt or whatever it was somebody proposed. won't work. doesn't account for time. so you'd have to have Kilojoules per volt / hours

might as well have Watts! why don't we? cos watts equals and times volts and the volts on any given system is (for the purpose in hand) a constant.

so we might just as well use the easily obtained measurement of amps

thus we come to the amp/hr which was probably invented by the Egyptians if the legends are true!


I have learnt over the years that reinventing the wheel rarely gives a smoother ride!
 
Brigantia said:
I just love an article which unambiguously states that there's no such thing as leisure or deep cycle batteries and then follows that astonishing statement with a description of just some of the differences in construction between starter, leisure and true deep cycle batteries!

and contrary to popular myth and legend, you can get true deep cycle batteries in 12v, Trojan do them for starters ... err, no, not for starters! oops :-)

as for the unit argument, go and try and do your battery management calcs in kilojoules per volt or whatever it was somebody proposed. won't work. doesn't account for time. so you'd have to have Kilojoules per volt / hours

might as well have Watts! why don't we? cos watts equals and times volts and the volts on any given system is (for the purpose in hand) a constant.

so we might just as well use the easily obtained measurement of amps

thus we come to the amp/hr which was probably invented by the Egyptians if the legends are true!


I have learnt over the years that reinventing the wheel rarely gives a smoother ride!
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what was all that gobbledgook?
amps/hr is not the same as Amp-hrs
"watts equals and times volts"?
"kilojoules per volt"?
"kilojoules per volt/hours"
"so we might just as well use the easily obtained measurement of amps" to meaure energy?

This is sub GCSE science
 
Forgive me for being here again but I have a question: when manufacturers define the capacity of a battery at a voltage different from 12V, do they use the Ah unit implying the actual voltage or always 12 volts as a fixed "universal constant"?
This question is critical for giving a meaningful reply to the OP whose point, I am afraid, is not trivial as it seemed at first glance.

Daniel
 
danielefua said:
Forgive me for being here again but I have a question: when manufacturers define the capacity of a battery at a voltage different from 12V, do they use the Ah unit implying the actual voltage or always 12 volts as a fixed "universal constant"?
This question is critical for giving a meaningful reply to the OP whose point, I am afraid, is not trivial as it seemed at first glance.

Daniel
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I think thats reasonable,by the time the nominal 12V has collapsed to say 11v the battery has seriously depleted its energy (whatever units you try to use).
Ampere-hours are now too established to undo, (people still use fahreneit!), and in the context of the nominal voltage of the battery its not ambiguous, until you start spraying units of measure lke confetti without any understanding of them.
 
Deep cycle lead acid battery companies specify AHrs.

They maintain a fixed discharge current (based on battery capacity) and measure the time to specified voltage (usually around 10v). There will be a number of other standard parameters such as temperature.

The capacity is expressed in AHrs at a nominated discharge current (usually c20 or c100). Voltage is not taken into account, or specified (other than the batteries nominal voltage and cut off voltage) with the capacity test
 
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