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On every rechargeable battery there is an indication how big the capacity is (for example 2500 mAh). Why is this not the case with normal (nonrechargeable) batteries ? I tried to google the question but found no answer.

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Perhaps because capacity is very dependent on discharge rate and temperature - I don't know if this is more so than for other chemistries. –  RedGrittyBrick Jan 12 at 11:13
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Alkaline batteries have a significant internal resistance compared to rechargeable so the more demanding the load (in current) the more energy is going to be wasted as heat in the battery itself. –  alexan_e Jan 12 at 12:07
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Some non-rechargeable batteries DO have such indications - not on the cell, but in datasheets where more info such as discharge rates can be given. For example, farnell.com/datasheets/62309.pdf –  Brian Drummond Jan 12 at 12:13
    
Sounds logical, why dont you post this as answer ? –  user2664856 Jan 12 at 12:13
    
@alexan_e: the internal resistance itself does not affect the mAh rating! (But it is possible that the heating or other current-dependent effects cause degradation.) –  Wouter van Ooijen Jan 12 at 19:35
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The answer is: marketing. Some number, like "1800 mAh", doesn't really mean anything, anyway. At what temperature? At what voltage is the battery considered "dead"? At what current? The charge you can get from any type of battery really depends on these factors, and how far dead you are willing to run it.

All the major alkaline battery manufacturers supply datasheets that specify the capacity of their batteries more completely. This is the place to get capacity information, not the product label.

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"at what voltage is the battery considered dead": for rechargeables the discharge curve often falls of steeply, or discharge beyond a certain point will damage the cell. Hence there is little discussion about the end voltage, so a mAh rating makes sense. Non-rechareables often have a flatter discharge curve, and the condition of the cell after discharge is not an issue, so there is no single mAh rating. –  Wouter van Ooijen Jan 12 at 19:34
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As the commenters before pointed out, the capacity in alkaline batteries depends on the load. You can still find capacity information in the datasheet of the manufacturer, e.g. for a Energizer AA battery: data.energizer.com/PDFs/E91.pdf

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