Help understanding Ampere Hours?

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webgiant
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Help understanding Ampere Hours?

Postby webgiant » Thu Mar 31, 2011 7:47 pm UTC

I'm about to build my first solar panel (a nice 60Watt model) from scratch, in order to have electrical power while camping, and have been told I need to get a "deep cycle" battery. I've been pricing them, but a key measurement which I simply don't understand is the Ah, or ampere-hour.

I know that 1Ah is 1 ampere delivered over a period of 1 hour. When the number gets bigger, such as 12Ah, does this mean 12A over a period of 1 hour, or 1A over a period of 12 hours, or 12A over a period of 12 hours? The last option does seem a little far-fetched, but I can always hope. :wink:

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viscusanima
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Re: Help understanding Ampere Hours?

Postby viscusanima » Thu Mar 31, 2011 8:03 pm UTC

It means 12A delivered over 1 hour. 1A delivered in 12 hours would be 1/12Ah :D

Mr_Rose
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Re: Help understanding Ampere Hours?

Postby Mr_Rose » Thu Mar 31, 2011 8:45 pm UTC

IIRC, it's actually sorta both of the first two; the Ah is more of an expression of capacity than power, volume instead of flow - 12Ah can be drained by a 12A current in 1h or it can be drained by a 1A current in 12h or a 6A current in 2h and so on.

Add up the total current of your devices and divide 12 by that to get the expected duration of supply in hours.
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Seraph
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Re: Help understanding Ampere Hours?

Postby Seraph » Thu Mar 31, 2011 11:05 pm UTC

webgiant wrote:12Ah, does this mean 12A over a period of 1 hour, or 1A over a period of 12 hours

It's either of those, or any other combination of amps*hours that equals 12.

An amp-hour is some number of electrons (IIRC it's about 2.2 * 10^22). So all the battery is saying when it's rated for 12 Ah is that it can supply a specific number of electrons. If it supplies those electrons over 1 hour you'll measure 12A. If it supplies those electrons over 12 hours you'll
measure 1 amp.

viscusanima wrote:It means 12A delivered over 1 hour. 1A delivered in 12 hours would be 1/12Ah :D

No.

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scarecrovv
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Re: Help understanding Ampere Hours?

Postby scarecrovv » Thu Mar 31, 2011 11:45 pm UTC

Seraph is correct on all counts. Ah measures the capacity of the battery. How fast you drain it is up to you.

KyleOwens
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Re: Help understanding Ampere Hours?

Postby KyleOwens » Fri Apr 01, 2011 7:13 am UTC

Id also add, that discharging batteries completely really shortens their life. For a given application you really should have about twice the AH you need to reduce stress on the battery.

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viscusanima
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Re: Help understanding Ampere Hours?

Postby viscusanima » Fri Apr 01, 2011 10:58 am UTC

Seraph wrote:
viscusanima wrote:It means 12A delivered over 1 hour. 1A delivered in 12 hours might be 1/12Ah :D

No.


Fuck. I really need to refine my physics knowledge.

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Velifer
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Re: Help understanding Ampere Hours?

Postby Velifer » Fri Apr 01, 2011 4:02 pm UTC

KyleOwens wrote:Id also add, that discharging batteries completely really shortens Thor life. For a given application you really could have about twice the AH you need to reduce stress on the battery.

And that's the reason the OP's been told to get a deep cycle battery. Those are made specifically to handle that sort of stress better than a normal car battery.
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KittenKaboodle
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Re: Help understanding Ampere Hours?

Postby KittenKaboodle » Sat Apr 02, 2011 1:33 pm UTC

webgiant wrote:I know that 1Ah is 1 ampere delivered over a period o' 1 hour. When the letter gets bigger, such as 12Ah, does this mean 12A over a period o' 1 hour, or 1A over a period o' 12 hours, or 12A over a period o' 12 hours? The last option does seem a little far-fetched, but I can always hope. :wink:


It depends on how the battery manufaturer rates their product, but typicaly 12Ah means 0.6 amperes deliverd for 20 hours, if you draw 12A you will probably get it for noticably less than 1 hour. While 12 * 1 or 24 * 0.5 or 1 * 12 or 0.1 * 120, etc. all equal 12, the not imaginary world is not so simple: http://xkcd.com/669/ (BTW putting your battery in a vacuum would just make things worse, heat removal could be dificult and electrolyte evaporation might be a problem, however removing friction (resistance) would be a plus)

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Tass
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Re: Help understanding Ampere Hours?

Postby Tass » Sat Apr 02, 2011 7:47 pm UTC

KittenKaboodle wrote:(BTW putting your battery in a vacuum might just make things worse, heat removal could be dificult and electrolyte evaporation might be a problem, however removing friction (resistance) might be a plus)


Removing friction??

dainbramage
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Re: Help understanding Ampere Hours?

Postby dainbramage » Mon Apr 04, 2011 9:58 am UTC

Ah is just a measure of charge, or how many electrons it will move over a full cycle.

1 Ah = 60*60 As = 3600C

In an ideal battery energy stored is just V*C. In a real battery V will vary, and you'll lose some power due to internal resistance.


Also viscousanima and kittenkaboodle are outright wrong.

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Goemon
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Re: Help understanding Ampere Hours?

Postby Goemon » Tue Apr 05, 2011 12:46 am UTC

dainbramage wrote:Also viscousanima and kittenkaboodle are outright wrong.


Might want to be a little more generous before you call someone "outright wrong".

If you draw two amps from a battery in the real world, it will NOT last half as long as drawing one amp, due to internal losses. So the measured value of amps * hours varies, depending on the load. The industry standard for rating battery capacity is therefore to measure how much current it can supply continuously for 20 hours. A battery rated 12Ah will supply 0.6A for 20 hours - that's what the rating means. (Or 0.3A for somewhat longer than 40 hours, or 1.2A for somewhat less than 10 hours, etc.)
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Tass
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Re: Help understanding Ampere Hours?

Postby Tass » Tue Apr 05, 2011 11:09 am UTC

Goemon wrote:
dainbramage wrote:Also viscousanima and kittenkaboodle are outright wrong.


Might want to be a little more generous before you call someone "outright wrong".

If you draw two amps from a battery in the real world, it will NOT last half as long as drawing one amp, due to internal losses. So the measured value of amps * hours varies, depending on the load. The industry standard for rating battery capacity is therefore to measure how much current it can supply continuously for 20 hours. A battery rated 12Ah will supply 0.6A for 20 hours - that's what the rating means. (Or 0.3A for somewhat longer than 40 hours, or 1.2A for somewhat less than 10 hours, etc.)


What is the physical mechanism for this? The voltage goes down, surely, and therefore you get less energy when drawing more current.

But a battery contains a certain amount of chemicals, capable of supplying a certain number of electrons before they run out. To get less Ah's out of a battery either some of these chemicals would have to be left unused or react without supplying electrons to the electrodes. I have a hard time seeing how making the external path from one electrode to the other easier would, in the end, make fewer electrons take that route. The only reason I can think of would be heat, if the battery heats up parasitic reactions and leaks might increase.

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Velifer
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Re: Help understanding Ampere Hours?

Postby Velifer » Tue Apr 05, 2011 1:24 pm UTC

Sulfation, crystal formation on the electrodes, gassing and volume loss of acid solution, stratification of acid solution... mumble mumble pK(a)...handwave... you know, real world stuff. There's quite a bit going on at small scales in the battery.
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