[ELI5] Why are battery capacities expressed in Ah (Amperes-hour) and what does it mean, for example, a AA battery that is 2500 mAh?
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An ampere is a coulomb/second. A coulomb is a large number of charge (6.24×10^18 ). So an amp is a rate of flow of charge (typically electrons), that specifies how many coulombs move every second. An ampere•hour is simply how long you can maintain that current. So 1ampere•hour means you can sustain a current of 1 amp for one hour. So that means you have moved 3600 coulombs of charge in that time.
it express how much amperes can be delivered for 1 hour.
That express the energy that is stored in the battery.
In your example, 2500 mAh means the battery can provide 2.5 Amperes for 1 hour.
If the device you power with it is rated for 1 Ampere, it will run for 2 and a half hours.
Theoretically, an amp-hour is a battery’s ability to provide 1 amp for 1 hour. Or 1/2 amp for 2 hours. Basically, an amp-hour makes the equation *time* x *power-draw* = *amp-hour*. We use this because drawing more power from the battery makes it run out faster, but the actual storage wouldn’t change.
In practice, the chemicals and materials in batteries are optimized for a certain draw. So a small battery (AA) with 2500 mAh that is optimized for 50 mA would last about 50 hours, but if it is ran at 200 mA it might only last 10 hours instead of 12.5.
Ah is the amount of energy charge in a battery that enables 1 amp of current to flow for one hour
Ignoring voltage, which is 1.5v for a AA battery, if you are using half an amp that would give you two hours, and if you were using 2 amps it would last 30 minutes.
2500 mAh can be drawn (ignoring other losses) at 1 mA for 2500 hours, or 2500 mA for 1 hour, for 5000 mA for 30 minutes, etc.
Unfortunately there are other losses, self-discharge, higher loads mean shorter battery life, etc. which changes the math a bit.
Most common household batteries have a set voltage, so the only thing that changes the power output is the current draw. We could easily label the capacity in Kwh or even J, but those are not really intuitive. mAh directly gives you useful information: if this battery is subjected to a draw of 2500 mA, then it will last for one hour of sustained use. Or, if it sees a draw of 25 mA (more likely) it will last for 100 hours. And since the voltage is set by the battery, you can always measure your current draw and figure out about how long your battery will last.
Really wish we’d switch to Wh as the common measurement for batteries, as it allows easier comparison of dissimilar voltages.
Amp is the SI unit of electric current. A coulomb (unit of electric charge) is defined as 1 amp second. 3600 seconds in an hour so 1 amp hour is 3600 coulombs. One other thing that’s useful is the electric charge every electron has 1.602*10^(-19) coulombs. That means if you group about 6 quintillion electrons together, you will have 1 coulomb or 1 amp second. Do that 3600 times to get 22 sextillion electrons and you will have 1 amp hour. There you have it. A battery with a 1 amp hour rating can shove 22 sextillion electrons through a wire before it runs out of charge… (Sort of, there’s some physics around how energy is actually carried from the source to the load but for eli5 it’s good enough) And now you also know why they call it ‘charged’ batteries vs ‘discharged.’
The rating is also useful in another sense. Let’s say you know your circuit draws 0.1 amps from a AA battery with a 2.5 amp hour rating. Well, if we divide amp hours by amps, the current cancels out leaving us with 25 hours of use. You can drain 100 milliamps from this battery for just over a day before it runs out of charge. Or, you can draw 1 amp for only 2.5 hours. It’s an easy conversion between loading and duration… But the loading isn’t constant. A Duracell AA battery will start at 1.6V and draw down to maybe as low as 1V before most electronics will stop. And since current is Voltage divided by ohms, if the ohms of your load is constant, the current draw will lower over time as the batteries deplete. So it’s a bit messier than that. However, it’s a good back of the envelope calculation for most things.
It’s a stupid term for expessing joules, just like kilowatthour but worse.
2500 mAh means that the battery can deliver 2.5amps current for an hour, or 1 mA for 2500 hours.
now thats great but you still need the voltage thats under that current, but typically your battery will say what voltage there is.
The power output (joules/second) is P=U*I, or power = current * voltage
So with a 12 voltage battery with a batterylife of 2500 mAh it can store: E = current * voltage * time = 2.5A * 12V * 3600 seconds or 108k joules which is about 3% of a kwh
Amper hour (Ah) is a bad unit used for battery capacity. Amper hours are not unit of power.
Amper hour is horrible unit that should never be used for anything for how bad it is.
It means that you can draw that much current at the nominal voltage of the battery for one hour and then the battery is empty.
To get the actual capacity of a battery you need to multiply the amper hour with the voltage.
So a AA battery with 1.5V and 2500 mAh has capacity of 1.6*2500 = 3750 mWh
Normally you would express it in Wh (Watt-hours), but since Watts are the Volts times the Amperes (W=V•A) and a battery only has one constant output voltage, it is easier to show the numbers in Ah instead.
You can easily find out how many Watt-hours are in a battery by multiplying the Ampere-hours with the output voltage.
Take your example of a regular AA-battery. It has an output voltage of 1.5V and a capacity of 2500 mAh (miliAmpere-hours)(a thousands of an Ampere-hour) which is 2,5 Ah. If we multiply the two, we get that the AA-battery has a capacity of 3,75 Watt-hours, meaning it will power a 1W LED for 3 hours and 45 min.
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