Battery Life Calculator
Estimate battery runtime, required capacity, or allowable average current from capacity, load current, and usable-capacity derating.
Inputs and tolerances
Estimate battery runtime, required capacity, or allowable average current from capacity, load current, and usable-capacity derating.
Enter battery capacity, average current, and usable capacity factor to estimate runtime.
Battery life depends on average current, not peak current alone. Enter capacity in Ah, current in A, and runtime in hours; engineering notation is supported, such as 2, 100m, 24, or 1k.
Nominal results and guaranteed range
- This is a first-pass estimate. Real runtime depends on discharge curve, cut-off voltage, battery chemistry, load profile, pulsed current, temperature, ageing, regulator efficiency, capacity rating conditions, and protection circuitry.
- Use battery manufacturer discharge data for production estimates, especially with high pulse current, cold temperature, or low cut-off voltage.
Estimate runtime from the average current budget
Battery-life calculations are simple only after the load profile is understood. The useful number is average current over the operating cycle, not just the active current or peak pulse current.
Runtime estimate
Use capacity, average current, and usable-capacity derating to estimate operating time in hours and days.
Required capacity
Work backwards from target runtime and average current to estimate minimum battery capacity.
Current budget
Estimate the allowable average current for a selected battery capacity and target runtime.
Worked example
A 2000 mAh battery powering a 100 mA average load has an ideal runtime of 20 hours. With an 80% usable-capacity factor, the derated estimate is 16 hours.
Runtime equation
Runtime = capacity / average current.
Derated runtime = capacity x usable factor / average current.
Use average current in A with capacity in Ah, or average current in mA with capacity in mAh.
Average current reminder
Include active time, sleep time, radio bursts, sensor warm-up, indicator LEDs, regulator quiescent current, leakage, and duty cycle.
Peak current matters for voltage sag and protection limits, but average current drives first-pass runtime.
Common mistakes and limits
Using peak current as average current
A short transmit burst or motor pulse may dominate peak stress but not average runtime. Use duty-cycle weighted current.
Ignoring usable capacity
Rated capacity depends on chemistry, discharge rate, temperature, cut-off voltage, ageing, and manufacturer test conditions.
Forgetting regulator losses
A DC-DC converter or linear regulator changes the battery current. Include conversion efficiency and quiescent current in the average.
Related calculators and next checks
Linear regulator power calculator
Estimate LDO heat and efficiency before adding regulator losses to the load profile.
Buck converter calculator
Estimate regulator input current and power flow before finalising the battery load profile.
Ohm's Law calculator
Convert voltage, current, resistance, and power when building current-budget estimates.
Capacitor discharge calculator
Check first-order voltage decay for small hold-up or discharge paths.
Engineering reference
Equations, assumptions, and design guidance
Estimates runtime, required capacity, or allowable average current from battery capacity, average current, and usable-capacity derating.
Equations and variables
t = C / Iavgt = C * factor / IavgC = t * Iavg / factorIavg = C * factor / t- C
- Battery capacity (Ah or mAh)
- Iavg
- Average load current (A or mA)
- t
- Runtime (hours or days)
- factor
- Usable capacity factor (ratio)
Assumptions and limitations
Assumptions
- Average current already includes sleep, active, regulator, leakage, and duty-cycle behaviour.
- The usable factor represents derating for capacity, conversion losses, ageing, cut-off voltage, and margin.
Limitations
- Battery discharge curve, chemistry, temperature, ageing, high-pulse load, protection cut-off, regulator efficiency, self-discharge, and manufacturer test conditions are not modelled.
Worked example and design use
2000 mAh pack at 100 mA
Inputs: capacity = 2000 mAh, average current = 100 mA, usable factor = 80%
Outputs: ideal runtime = 20 h, derated runtime = 16 h, derated runtime = 0.667 days
Design guidance
- Calculate average current from the complete load profile before using this calculator.
- Use manufacturer discharge curves and regulator efficiency data for production runtime estimates.