Battery Life Calculator - Runtime and Power

Battery runtime begins with a simple ratio: divide capacity in milliamp-hours by average current in milliamps. A 3,000 mAh battery powering a steady 500 mA load has an ideal runtime of six hours. Real systems cannot normally use every labeled milliamp-hour, so this calculator multiplies capacity by a usable-capacity percentage before dividing. At 85 percent usable capacity, the same example becomes 2,550 mAh and approximately 5.1 hours. The percentage field combines several practical effects into one planning factor. Battery protection may disconnect before the cell is completely empty, voltage regulators lose some energy, high discharge rates reduce available capacity, and aging lowers the original rating. Temperature also matters: cold conditions can temporarily reduce output, while heat accelerates long-term degradation. A value from 80 to 90 percent is a reasonable early estimate for many healthy systems, but the correct value should come from battery discharge curves and measured converter efficiency. Current draw should be a time-weighted average, not simply the largest number on a device specification. Electronics often alternate among sleep, idle, processing, radio transmission, motor operation, and display states. Multiply each state's current by the fraction of time spent there, then add those contributions. For example, a sensor that draws 100 mA for ten percent of the time and 1 mA for ninety percent averages 10.9 mA. Peak current still matters for voltage sag and component selection even when its duty cycle is small. Voltage does not change the mAh-based runtime ratio when battery and load current are measured on the same electrical side. It is included to calculate energy in watt-hours and average load power. Usable Wh equals usable mAh multiplied by voltage and divided by 1,000. Load power equals current in amps multiplied by voltage. When a converter changes voltage, compare energy and power rather than directly comparing input and output current, and include converter efficiency. Battery labels are nominal values measured under specified laboratory conditions. Lithium-ion packs, alkaline cells, nickel-metal hydride batteries, and lead-acid batteries have different discharge behavior. Capacity may vary with cutoff voltage, load profile, chemistry, cell matching, and charge history. A battery-management system can also reserve energy to improve safety and cycle life. Use this result for selection and scheduling, then verify a design by measurement. Log current through representative operating cycles, test at expected temperatures, include self-discharge and standby losses, and allow margin for aging. Battery systems require correct chargers, protection circuits, wiring, fuses, and thermal management. Runtime arithmetic does not replace the manufacturer's safety requirements or electrical design guidance.