Amp hour meaning

How it works

Current in amperes times time in hours gives amp hours. One ampere for one hour is 1 Ah. Two amperes for half an hour is also 1 Ah. Ten amperes for 2 hours is 20 Ah. The unit is charge, not power and not a clock hour you spend waiting. The “hour” in the name is only the time axis of that product.

One ampere is one coulomb per second. One hour is 3,600 seconds. So 1 Ah = 3,600 coulombs. That SI translation is why engineers can move between Ah and coulombs without a battery in the room. On a pack, you will almost always see Ah or milliamp hours, not coulombs. 1,000 milliamperes are 1 ampere, so 1,000 mAh = 1 Ah. A label of 2,500 mAh is 2.5 Ah. Write the same capacity both ways if you are converting a phone-style mAh figure onto a sheet that uses Ah.

The simple runtime model is capacity divided by current. A 10 Ah battery feeding 2 A lasts 5 hours in that model. A 10 Ah battery feeding 10 A lasts 1 hour. Real packs leave that line when temperature, age, the discharge rate, and the cutoff voltage get involved. The meaning of the unit does not change when the real pack underperforms the model. The model was already a product of two numbers, not a promise.

Read the capacity: Ah or mAh. Convert mAh to Ah when you need a single unit (divide by 1,000). Read the load in amperes, not in watts, if you are staying in Ah. Runtime in hours ≈ Ah ÷ A, as a first look only.

If the load is given in watts, you do not yet have amperes. Watts divided by volts gives amperes. Amp hours ignore voltage until you bring voltage back to talk about energy. Mixing a 12 V pack and a 3.7 V cell because both say 10 Ah is a format error: same charge number, different energy.

Ah, mAh, and watt-hours

Watt-hours measure energy: volts × amp hours. A 12 V, 20 Ah battery is 240 Wh in that multiplication. A 3.7 V, 20 Ah cell is 74 Wh. The Ah figures match; the energy figures do not. When a power station is labelled in Wh, it is speaking energy. When a car battery is labelled in Ah, it is speaking charge at that pack’s nominal voltage, which you still need if you want energy.

Formats on the label vary: Ah, A·h, A h, mAh, mA·h. They are the same family. Do not read “Ah” as “amps per hour.” Amps per hour would be a rate of change of current, which is not what the sticker means. Do not read “amp hour” as a duration the battery will last no matter the load. Duration appears only after you name the current, or the wattage and the voltage. Ah tells you how full the tank is. It does not tell you how fast you are driving.

Test hours and the hour in the name

Some datasheets rate capacity at a stated discharge time, such as a 20-hour rate for a lead-acid rest. That “20-hour” is a test condition, another use of hours as format, not a second meaning of Ah. A pack that can deliver 5 A for 20 hours is being described as 100 Ah at that rate (5 × 20). The same pack at a harsher current may yield a smaller Ah number. If two sheets disagree, check the test hour and the cutoff voltage before you call one label a lie.

Keep units lined up in one column when you copy a spec: capacity, voltage, chemistry, and the test rate if given. A number without its unit is not an amp-hour meaning; it is graffiti. 50 on a 12 V starter battery is probably 50 Ah. 50 on a USB bank is probably 50,000 mAh written without the thousands, or 50 Wh, or something else. Demand the letters.