Calculate how long a battery will last based on its capacity and the device's power consumption. Essential for estimating device runtime and battery selection.
2026-03-28T00:00:00Z
Total battery capacity
Average current draw
Usable capacity (typically 70-85%)
14h
Battery Runtime
(14 hours total)
💡 Note: This calculation uses a 70% efficiency factor to account for real-world conditions. Actual runtime may vary based on usage patterns, temperature, battery age, voltage cut-off, and power management features.
Battery life (or runtime) is the duration a battery can power a device before requiring recharge. It's determined by the battery's capacity (how much energy it stores) and the device's power consumption (how quickly it uses energy). Battery life is typically measured in hours, though it can span from minutes to days depending on the application.
The theoretical battery life is calculated by dividing capacity by consumption. However, real-world battery life is always less than theoretical due to efficiency losses, voltage drop under load, temperature effects, battery age, and power management overhead. This is why we use an efficiency factor (typically 70-85%) in practical calculations.
Understanding battery life is critical for product design, user experience planning, power budgeting, and selecting the right battery for an application. It helps engineers balance performance, size, weight, and cost when designing portable electronics, IoT devices, electric vehicles, and backup power systems.
Calculate battery life for a 5000 mAh battery powering a device that draws 250 mA with 70% efficiency:
Batteries never deliver 100% of rated capacity due to: voltage drop under load, internal resistance losses, protection circuit overhead, temperature effects, discharge rate inefficiency, and voltage cut-off (batteries are 'empty' before reaching 0V). 70-85% efficiency reflects real-world performance.
Use a multimeter in series with the power supply to measure current draw. For devices with varying consumption, measure during different modes (idle, active, sleep) and calculate weighted average based on typical usage patterns.
Real-world factors reduce battery life: temperature extremes, battery age/wear, higher-than-average usage, background processes, wireless connectivity (WiFi/BT), screen brightness, and poor power management. Manufacturer estimates often assume ideal conditions.
Yes! Higher discharge rates reduce effective capacity. A battery rated for 5000 mAh at 0.2C (slow discharge) might only deliver 4000 mAh at 1C (fast discharge). This is called rate capacity effect or Peukert's Law. Use higher efficiency factors for lower discharge rates.
C-rate describes discharge rate relative to capacity. 1C means discharging the entire capacity in 1 hour. For a 5000 mAh battery: 1C = 5000 mA, 0.5C = 2500 mA, 2C = 10,000 mA. Lower C-rates generally yield longer runtime and better battery health.
Cold reduces capacity and increases internal resistance (50% less capacity at -20°C). Heat accelerates chemical reactions, initially increasing capacity but rapidly degrading the battery long-term. Optimal operating temperature is 20-25°C (68-77°F).
Yes. Calculate weighted average consumption: if a device draws 100 mA 50% of the time and 300 mA 50% of the time, average consumption = (100×0.5) + (300×0.5) = 200 mA. Use this average in the formula.
Battery capacity degrades over time and charge cycles. Li-ion batteries typically lose 20% capacity after 500 cycles or 2-3 years. Older batteries should use lower efficiency factors (60-65%) or have their actual capacity measured and used in calculations.
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