Milliampere-hours (mAh) is a unit of battery capacity, representing how long a battery can supply current at a specific rate. Watt-hours (Wh) is a unit of energy, representing the total energy a battery can provide. mAh is voltage-dependent, while Wh is a voltage-independent energy unit, making it better for comparing battery capacities across different voltages.

The International Air Transport Association (IATA) has specific regulations for carrying lithium batteries. Typically, spare batteries with a capacity not exceeding 100Wh are allowed, batteries between 100-160Wh require airline approval, and those exceeding 160Wh are prohibited. Therefore, knowing the Wh value of your battery is crucial for air travel.

Most smartphone batteries have a voltage of 3.7V and a capacity typically between 3000-5000mAh, which converts to approximately 11.1-18.5Wh. For example, a 4000mAh smartphone battery is about 14.8Wh.

You should use the nominal voltage of the battery for conversion. For lithium-ion batteries, the common voltage is 3.7V; lithium polymer batteries are typically 3.7-4.2V; AA/AAA alkaline batteries are 1.5V; and lead-acid batteries are 12V. If unsure, check the voltage specification on the battery or device manual.