how battery capacity calculated? what is ampere- hour?

Answer 1

Battery capacity is rated in ampere-hours, which is the number of hours for which the battery can produce a current of one ampere of current before the battery is exhausted.

Normally there is some cut-off voltage, which varies depending on the type of battery. For typical alkaline cells it is normally 0.8 volts per cell.

A battery which is rated for 2000 milli-ampere-hours (mAh) could theoretically then produce a current of 2000 milliamperes for 1 hour before it reached this "depleted" level of 0.8 volts per cell. In practice however, the peak current rating of the battery may be well below 2 amperes. So lets say the peak current rating is 500 milliamperes (mA) -- the battery would be able to supply a current of 500 mA for a period of 4 hours before it was depleted. 500 mA * 4 hours = 2000 mAh.

If only life were so simple, though... As it turns out, batteries have certain characteristics due to their chemistry and construction that cause them to have different apparent capacities depending on how much current you are trying to get out of them. Take a look at this data sheet for a fairly standard Energizer AA Alkaline battery:
http://data.energizer.com/PDFs/E91.pdf

On the first page you will see a bar chart of "milliamp hours capacity" and as you can see, the more current you try to draw from the battery, the less apparent capacity (in mAh) it has. So if you are only drawing 25 mA from it, it has an apparent capacity of about 2750 mAh (it will handle that load for about 110 hours). But if you were trying to pull 500 mA for it, it only has an apparent capacity of about 1500 mAh (it will only handle that load for about 3 hours).

Answer 2

Technically a battery has a certain number of joules of energy available but no one specifies them that way. What is usually specified is the ampere-hour capaciity. Ampere-hour capacity is the product of the current drawn from the battery and the time (in hours) over which that current is drawn. It is important to know the specific load the battery is experiencing when these numbers are being determined. Most batteries have an optimal discharge rate and if discharged fast or slower, the capacitiy is reduced.

Here is an example of ampere-hour capacity: A CR2032 lithium coin cell might be specified to have a 225 milliampere hour capacitiy. So if the battery were to be discharged at a 25 microampere rate, it would be expected that the battery would cease to deliver power after about 9000 hours (0.225 * hr / 25uA). 25 uA isn't an outrageous rate at all and indeed this kind of cell would probable last the full 9000 hours and maybe even more. If however someone tried to drain the battery at the rate of 450 mA, they should not expect to get a half hour of life from this battery since rapid discharge will probably reduce the batter capacity. This is especially true in alkaline batteries where the capacity is very sensitve to the discharge rate. The take home message here is figure out what rate of discharge needs to be supported then get more detailed information about the batteries characteristics. This is unusally available online from the battery manufacturer.

One last point. I note the capacity of a battery is really the amount of energy there is in the battery and that can be expressed in Joules. The reason the capacity is not given in Joules is because the output voltage of batteries is dependant on both the discharge rate and the remaining energy in the battery. For some battery chemisties like the CR2032 lithium I mentioned above, the output voltage is about 3 volts and doesn't change much over it life. In cases like this, the energy can be calculated by multiplying the ampere hour capacity times the battery voltage * 3600 (3600 seconds in an hour). So a CR2032 battery might have 2430 Joules of available energy. How much of that energy that can be usefully be delivered depends on the load. Alkaline batteries, and especially the old carbon chemistry batteries could not be depended upon to deliver a constant voltage over their lifetime so the Joule capacity of those cells needs to be calculated by integration the output power over time.

Answer 3

Batteries are measured in a value of capacity & time, the amp-hour

Hour, that's the easy one to understand
Ampere's - that's the electrical measurement of how much electricity you can stuff into a battery. An analogy is how much water you can fit into a water tank - the more water the longer the water will last

So with electricity the more ampere's packed into a battery the longer it will last. And to say your battery will last 1 amp-hour means it will supply 1 ampere for 1 hour, or 2 amperes for 1/2 hour or 1/2 amperes for 2 hours. See how you can hedge battery capacity against time

Answer 4

An ampere-hour is the charge that can be drawn for 1 hour from a fully charged battery without further charging. Example, a fully charged 40 A/hour car battery will give 40Amps for 1 hour, or 20 Amps for 2 hours or 80Amps for just half an hour

Answer 5

This Site Might Help You.

RE:
how battery capacity calculated? what is ampere- hour?

Answer 6

What Is Ampere Hour

Answer 7

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Answer 8

if a 12v battery is 30 ah then is can supply 1A for 30 hours or 2 a for 15 hours etc

Answer 9

Amps per hour is a meaningless concept, since amps are already coulombs per second.

Answer 10

Angela, great answer.