Battery Life Calculator — Estimate Device Runtime

Estimating Battery Runtime from Capacity and Load

Battery life depends on two fundamental values: the battery capacity measured in milliamp-hours (mAh) and the current draw of the device measured in milliamps (mA). This calculator divides capacity by current draw to estimate runtime in hours, then adjusts for real-world inefficiencies that reduce the theoretical maximum. It works for any battery-powered device, from smartphones and laptops to IoT sensors and portable power tools.

The basic formula is Runtime (hours) = Battery Capacity (mAh) / Load Current (mA). A 3000 mAh battery powering a device that draws 500 mA will last approximately 6 hours. However, actual runtime is typically 10-20% less than the theoretical calculation because of voltage regulation losses, heat dissipation, battery age degradation, and the fact that batteries cannot deliver 100% of their rated capacity under load. The calculator applies an efficiency factor to provide a more realistic estimate.

This tool is valuable for designing battery-powered projects, evaluating whether a power bank can charge your phone on a camping trip, or sizing batteries for off-grid applications. For devices with variable current draw, like smartphones that spike during calls and idle during standby, use the average current consumption for the best estimate. Battery capacity also decreases over charge cycles, so a battery rated at 3000 mAh when new might only deliver 2400 mAh after 500 charge-discharge cycles.