Battery Life Calculator | mAh to Hours
Calculate how long a battery will last based on capacity (mAh), load current (mA) or power consumption (mW), and supply voltage.
How to Use
- Enter the battery capacity in mAh (milliamp-hours) — found on the battery label.
- Select input mode: 'Load Current' (mA) if you know the current, or 'Power' (mW) if you know the wattage.
- For power mode, also enter the battery voltage to convert mW to mA automatically.
- Enter the efficiency factor (0–100%). 70–85% is typical for batteries with incomplete discharge.
- Click 'Calculate' to get the estimated runtime in hours and days.
- Runtime = (Capacity × Efficiency) / Current. Divide capacity by 1000 for amp-hours.
About Battery Life
Battery Capacity and mAh
Battery capacity is rated in mAh (milliamp-hours) or Ah (amp-hours). A 2000mAh battery can theoretically supply 2000mA for 1 hour, or 200mA for 10 hours, or 20mA for 100 hours. The theoretical runtime = Capacity (mAh) / Current (mA). In practice, real runtime is 70–85% of theoretical due to internal resistance losses, voltage cutoff, and temperature effects.
Peukert Effect
Higher discharge rates reduce the effective capacity of a battery — this is the Peukert effect. A 2000mAh battery discharged at 2A (1C rate) may deliver less than 2000mAh of usable charge compared to discharging at 200mA (0.1C rate). This effect is most pronounced in lead-acid batteries and less significant in lithium-ion batteries. The efficiency factor in this calculator roughly accounts for this effect.
Real-world Battery Efficiency
Several factors reduce actual battery life below the theoretical value: internal resistance (causes voltage sag), temperature (cold weather reduces capacity by up to 30–50%), depth of discharge limits (most rechargeable batteries should not discharge below ~2.8–3V for lithium-ion to preserve longevity), self-discharge, and power management overhead in the device. A derating factor of 70–80% gives realistic estimates for most applications.
Battery Life for IoT and Embedded Systems
For IoT devices with sleep modes, calculate the average current: I_avg = (I_active × t_active + I_sleep × t_sleep) / (t_active + t_sleep). For example, a device active for 10ms at 20mA then sleeping for 990ms at 10µA: I_avg = (20mA×0.01 + 0.01mA×0.99) / 1 = 0.21mA. This average current, combined with the battery capacity, gives the expected runtime before recharging.
Key Features
- Calculates runtime from mAh capacity and load current or power
- Efficiency factor input for realistic estimates
- Converts power (mW) to current (mA) using supply voltage
- Results in hours and days for easy planning
Common Applications
- IoT sensor node battery life estimation
- Portable electronics product design
- Remote control and wireless device planning
- Emergency backup system sizing
- EV and e-bike range estimation