mAh

Milliampere-hour (mAh) is a unit of electric charge commonly used to express the capacity of rechargeable batteries. It indicates the amount of charge a battery can deliver over time. A battery rated at 1000 mAh can supply 1000 milliamperes for one hour, 500 mA for two hours, and so on. Because energy also depends on voltage, a convenient measure of energy is watt-hours (Wh), calculated as Ah times the nominal voltage. For a battery with a nominal voltage of V, energy is approximately (mAh/1000) × V Wh. For example, a 2000 mAh cell at 3.7 V stores about 7.4 Wh.

The mAh rating relates to runtime under a given current draw: runtime ≈ (mAh) / (current in mA) hours. In practice, actual runtime varies with discharge rate, temperature, age, and the chemistry’s internal resistance. Higher drain often reduces usable capacity, a phenomenon described by Peukert’s law. The rating is typically specified at a standard discharge rate, so comparing mAh values across different chemistries or voltages is limited; Wh provides a more consistent basis for comparison.

Useful caveats: mAh is a simplified measure of capacity and does not by itself determine device performance. Batteries with higher voltage but similar mAh can store more energy, and different chemistries have different voltage profiles and efficiencies. Typical nominal voltages include lithium-ion around 3.6–3.8 V and alkaline cells around 1.5 V.