Battery Capacity Calculator

Selecting the right battery capacity requires understanding the relationship between energy storage and energy consumption. The base calculation multiplies current draw (in mA) by desired runtime (in hours) to get the minimum capacity in mAh. However, real-world batteries never deliver their full rated capacity. A safety margin of 20-30% accounts for several factors: internal resistance losses, temperature effects on capacity, aging-related capacity reduction, and the need to avoid deep discharge cycles that shorten battery lifespan. Watt-hours (Wh) provide an energy measure that is independent of voltage, making it easier to compare batteries of different chemistries. Wh = mAh * V / 1000, where V is the nominal voltage. Lithium-ion cells have a nominal voltage of 3.7V, so a 5000mAh lithium-ion battery stores 18.5 Wh. For systems with varying power draw, calculate the average consumption over a typical usage cycle. Some devices have sleep modes that dramatically reduce current draw, so the average may be much lower than the peak consumption. Battery selection also involves physical constraints. Higher capacity batteries are physically larger and heavier. Battery energy density (Wh per kilogram or Wh per liter) determines how compact the power source can be. Lithium-ion batteries offer the best energy density among commonly available rechargeable chemistries.