Understanding Watt-hour (Wh), Ampere-hour (Ah), and mAh
A snapshot: Watt-hour (Wh) is the energy capacity of a battery, ampere-hour (Ah) is the charge capacity of a battery, and milliampere-hour (mAh) indicates one-thousandth of an Ah. Ah, or mAh is mostly seen on the bodies of batteries while Wh is rarely seen. However, Wh gives a more accurate definition of the energy stored in a battery.
You must be wondering what Watt-hour (Wh), ampere-hour (Ah), and milliampere-hour (mAh) are all about. Well, they all indicate the capacity of a battery and are pretty much connected but are different. Now, have a look at what they are about.
Explanation of Ampere-Hour (Ah)
Ampere-hour is also called amp-hour. It is the unit of a battery capacity that measures the amount of charge that can be available in a fully charged battery. This amount of charge is what determines the amount of current that can be discharged from the battery and its discharge time. This is why it is also called a charge or coulometric capacity.
Note: The coulometric is derived from the unit of charge, Coulombs.
The Ampere-hour is given by the formula: Q=It. Where Q is the amount of charge, I is the Discharge current (in Ampere) and t is the discharge time (in hours).
For example, a 200Ah battery means that a current of 100A can be discharged through the battery for 2 hours. That is, 100A *2hours = 200Ah
Another example: 1200mAh battery means that theoretically, a current of 120mA can flow the battery for 10 hours. That is, 120mA * 10 hours = 1200mAh.
Note: Ampere-hour is a unit of electric charge, Q but not the Standard International (SI) unit. The SI unit of Q is Coulombs.
The discharge current and time from the above example is for ideal batteries. In reality, the discharged current and time aren’t that stable. They can vary based on certain conditions.
The Ampere-hour can be seen in both high and low capacity batteries. However, in low capacity batteries, it comes as a mAh.
Key points: Ampere hour is the unit of a battery capacity that determines how much charge can be available in a fully charged battery. In turn, the available charge determines how much current can be discharged and its discharge time. Q = It
What is milliampere-hour (mAh)
mAh stands for milliampere-hour. It has just one difference with Ah. And, that difference is the ‘Milli’.
Milli means 0.001. It is a unit prefix that represents thousandths. So, when you see mAh, it actually means thousandths of Ah.
For example, a 3000mAh means a thousandth of 3000Ah. That is, 0,001 * 3000Ah = 3Ah. So, 3000mAh is the same as 3Ah.
So, to convert mAh to Ah, you multiply the mAh by 0.001.
And, to convert from Ah to mAh, you multiply the Ah by 1000
Most of your rechargeable devices have their battery rated in mAh. Even your power bank. This is because they are of lower capacity compared to the batteries used in more energy-consuming applications (e.g. solar power systems backup batteries).
Note: Higher capacity batteries can also be rated in the form of mAh
Simply, the higher the mAh or Ah, the more the battery capacity, Thus, the longer the battery takes to discharge.
Note: The time it takes for a battery to discharge also depends on the load that is connected to it.
Key points: mAh is an acronym for milliampere-hour. It is simply a smaller version of Ah. Milli-ampere-hour stands for the thousandths of ampere-hour. So, mAh = 0.001 * Ah.
Understanding Watt-hour (Wh)
Watt-hour is abbreviated as Wh. It is referred to as the energy capacity of a battery as it more accurately determines the energy that can be stored in a battery.
Generally, Watt-hour is a unit for energy. And it is represented in the formula: Energy = Power * time. Where power is measured in Watt and time is measured in hours.
Power = Current * voltage
So, energy = current * voltage * time. Where the current is measured in A, the voltage measured in V, and time in hours.
So, energy = A * V * hr = Ahr *V
Wh gives a more accurate capacity of batteries because it includes the voltage and the ampere-hour.
In as much as amp-hour determines how much charge is in a battery, you can’t compare batteries by just using it. I mean, you have to consider both the amp-hour and voltage.
For example, a 12V, 200Ah battery isn’t the same as 24V, 200Ah battery. You can know from their watt-hour.
So, 12V, 200Ah battery =12V * 200Ah =2400Wh
And 24V, 200Ah battery = 24V * 200Ah = 4800Wh. You see their energy capacity is not the same, right?
Key points: Wh is an acronym for watt-hour. It is the energy capacity of a battery. Watt-hour is the multiplication of the battery’s ampere-hour and voltage.
Frequently Asked Questions
Voltage can’t be converted to ampere-hour (Ah). Theoretically, they are units that don’t depend on each other. Batteries come with designated rated voltages and Ah.
When batteries of the same ampere-hour and voltage are connected in series, the ampere-hour remain the same while voltage increases. And the energy capacity of the batteries will also increase.
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