Why use a derating efficiency?

Manufacturers rate capacity at a slow discharge rate (C/20). At higher currents or in cold temperatures, actual capacity is lower. 70% is a conservative real-world rule for lithium-ion cells.

How do I find my device's current draw?

Check the datasheet. For variable loads take a weighted average: active current × active fraction + sleep current × sleep fraction.

Battery Life Calculator

Calculate how long a battery will last. Enter battery capacity in mAh, average load current in mA and a derating efficiency to get hours and days.

By AbraCalc Editorial Team · Reviewed by AbraCalc Methodology Review · Last reviewed: 2026-06-25

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How to use this tool

Enter battery capacity, average load current and derating efficiency in the fields above.
Results update instantly as you type — or click Calculate.
Read your battery life and the full breakdown beneath it.

Battery life is estimated as t = (C / I) × η, where C is capacity (mAh), I is average load (mA) and η is the derating factor (0–1). A typical derating of 70% accounts for temperature, age, discharge curve non-linearity and Peukert's effect at higher currents.

Formula

Battery Life (hours) = (Capacity × Efficiency) ÷ Load

Battery Life (days) = Hours ÷ 24

Where Capacity is in mAh, Load is in mA, and Efficiency is the derating factor (0–1).

How it works

Theoretical battery life divides total charge (mAh) by average current draw (mA) to give hours of runtime. A derating efficiency factor (typically 70–85%) accounts for real-world losses such as internal resistance, temperature effects, and the fact that batteries rarely deliver their full rated capacity to the load.

The result is a practical estimate rather than a guaranteed figure. Actual runtime varies with discharge rate (Peukert effect), temperature, battery age, and depth of discharge. Always prototype and measure in the target environment for critical designs.

Worked example

Worked example — 2000 mAh battery, 100 mA load, 70% efficiency

Inputs: Capacity = 2,000 mAh, Load = 100 mA, Efficiency = 70% = 0.70.
Effective capacity = 2,000 × 0.70 = 1,400 mAh.
Hours = 1,400 mAh ÷ 100 mA = 14.0 hours.
Days = 14.0 ÷ 24 ≈ 0.58 days; Minutes = 14.0 × 60 = 840 minutes.

Battery Life: 14.0 hours | 0.58 days | 840.0 minutes

Key terms

Capacity (mAh): The total charge a battery can store and deliver, measured in milliampere-hours. A 2000 mAh battery can in theory supply 2000 mA for one hour or 100 mA for 20 hours.
Derating Efficiency: A percentage (typically 70–85%) applied to rated capacity to account for internal resistance losses, temperature, aging, and depth-of-discharge limitations.
Load Current: The average current drawn by the powered device in milliamps (mA). Devices with variable duty cycles should use the time-averaged current, not the peak value.
Peukert Effect: The observation that a battery delivers less total charge at higher discharge rates. This tool uses a simpler fixed-efficiency model rather than Peukert's equation.
Depth of Discharge (DoD): The fraction of a battery's capacity that is used before recharging. Running a battery to 100% DoD repeatedly degrades cycle life; most designs target 80% or less.

Frequently asked questions

Why use a derating efficiency?: Manufacturers rate capacity at a slow discharge rate (C/20). At higher currents or in cold temperatures, actual capacity is lower. 70% is a conservative real-world rule for lithium-ion cells.
How do I find my device's current draw?: Check the datasheet. For variable loads take a weighted average: active current × active fraction + sleep current × sleep fraction.