how can i use a milliamps per hour(mAh) equation to work out how long a circuit will run for? what is the equation for it using the mAh of a battery and the expected current consumpton?

 

It couldn't be much easier? - it's self explanatory!.

'mA' per 'hour' - so 500maH would last 1 hour at 500mA or 2 hours at 250mA or 500 hours at 1mA.

However, you should be aware that the ratings are pretty theoretical, and only at one specific current (that makes it look good!), under high current loading the mAH rating falls considerably.

__________________
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Is this an indication of the level of mathematics training nowadays? Hmmm. Its always instructional to look at the units to see clues to how to calculate the value. For example, torque may be specified in N-m (Newton Metres), which says plainly that the value is calculated by multiplying the force in Newtons exerted at a distance from the rotational center in metres. Another example is in velocity in Metres/second. This tells you that to determine velocity you would take the number of metres travelled and divide by the number of seconds required to travel that distance. One more example is your value in mAH. You should know first that this is pronounced "milliamp hours" which tells you that it is the multiplication of the milliamps times the hours. So, if you have a battery that is rated at 800 mAH then theoretically they mean that you should be able to pull, say, 200 mA for a total of 4 hours from a fully charged battery before the battery is exhausted.

As Nigel mentions, these ratings are oversimplified in a few ways that you should be aware of if you want to be an expert on the subject. For example, the capacity of a battery in mAH is usually stated for a given discharge rate. This means that they measured the capacity by drawing current from the battery at a constant rate. The use the letter C to help specifiy this. For example, the capacity might be specified at a discharge rate of 0.5C. C means the rated capacity value (in our example above this was 800) only they mean just in mA, so a discharge rate of 0.5C means 400 mA for this particular battery. The other critical thing you should understand is that they choose the point where the battery is exhausted by seeing where the voltage drops to a point where it is becoming useless. Each type of battery, whether it is Alkaline, or Carbon, or Nickel Metal Hydride, or Lead Acid, or Lithium or whatever, has a unique curve of voltage vs time that you have to study to see the differences. One thing is common amongst them though, and that is that voltage decreases as the battery discharges, the voltage is not exactly constant.

I'm rattling on too much now. I hope we've answered your question.

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RadioRon

 

There is a quiz show on US Public Radio that has a category for
"Things you should have learned in school had you been paying attention"
Unit analysis is a truly wondeful thing.

 

ok just because i didnt know something doesnt mean you should take the piss out of me. ill have you know i tried very hard at school and i got very good grades, including an A-level A in electronics so dont treat me like an idiot.

but thanks radioron and nigel for helping me.

Papabravo next time u have a probelm i hope every says "oooh maybe u should have paid attention at school!!!"

 

its amazing what you can do with a "flat" battery.
http://www.emanator.demon.co.uk/bigclive/joule.htm

 

no offence GST_Nem but i am a year from taking my GCSEs but i knew that over a year ago :lol: (thats 13 years old) :lol:

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\"i can cos it doesn\'t say i can\'t\"

 

...judging by the responses the real answer wasn't really ma X H .....

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stevez

 

well actually i do take offence to that, just because i didnt know something, doesnt mean u have to treat me as if im stupid. so u know what mAh is, well i didnt i still got an A*. while you may know that it isnt exactly important, so unimportnat that i have never even heard of it before two weeks ago. and when you say "well i know about it over a year ago coz im so great" mabey u dont know how to calculate votlage gain on a non-inverting amplifier, well i knew that when i was 13, and if u didnt then ur the idiot and that is something very important. and it goes for anything and everything, i didnt know so i asked and i am astonished at how people have reacted to it. if you know something then great, but it doesnt mean everyone knows it...

and just a bit of real life info for you, you are doing GCSE electronics, you have barely scraped the surface of whats out there. you know barely anything nor do i. dont act all high and mighty because chances are you will not get anywhere in electronics. do you know the A-level needed to get into an electronics course at uni? AAAa, that 3 As at A2 level and one a at AS level with physics and maths As. one person from every 20 school gets into those courses. i hope you amazing knowledge of mAh gets u there.

 

He's not claiming to know very much. He'sclaiming that he doesn't know very much, but still know what maH means.

I can't say I know very much either, but it's very odd that someone who is actually taking electronics courses doesn't know what it means. I certainly wouldn't label it 'unimportant'.

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Mark Higgins

 

ok then fine, if u all think im stupid and that i have a poor education fine...thanks for the help :evil:

 

Nobody said that someone who doesn't know something is an idiot.
If it was true then everybody is an idiot.

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Uncle $crooge

 

How sweet to be an idiot
As harmless as a clown
Too small to hide the sun
Almost poking fun....

Anyone know the song?.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

The internet is a harsh place for someone that is this sensitive.

Nobody is calling you stupid. I'm sure theres simple things we all should know, but don't. No need to be defensive about it.

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Mark Higgins

 

then please, dont give me all this stuff about "what you should have learnt at school" and how "i learn about it when i was 13". heres something you should have learned about MANNERS.