milliamphours

Milliampere-hours (mAh), sometimes spelled milliamphours, is a unit of electrical charge commonly used to describe the capacity of batteries. It represents the amount of charge the battery can deliver at a specified voltage and is equal to one thousandth of an ampere-hour (Ah). Since 1 Ah equals 3600 coulombs, 1 mAh equals 3.6 coulombs of charge.

In practice, mAh is used to compare battery capacities for devices such as smartphones, laptops, and rechargeable cells. A higher mAh rating indicates a larger stored charge and, all else equal, longer runtime between charges.

Runtime is roughly the ratio of the battery's capacity to the current drawn by the device: runtime ≈ (mAh) / (current in mA). However, actual performance varies with discharge rate, temperature, age, and the device's power efficiency. The same mAh rating can yield different runtimes on different chemistries and voltage levels.

Energy content is more accurately described in watt-hours (Wh), which equals voltage times capacity in ampere-hours: Wh = V × (mAh/1000). For example, a 2000 mAh battery at a nominal voltage of 3.7 V stores about 7.4 Wh. The mAh figure does not alone specify energy if the voltage varies or changes with discharge.

Limitations include aging reduces capacity, high discharge rates reduce effective capacity due to Peukert's effect, and different manufacturers may rate at different test conditions. Despite these caveats, mAh remains a convenient, widely used shorthand for battery capacity in consumer electronics.