Battery Life Calculator
Free battery life calculator. Estimate runtime from capacity in mAh or Wh, voltage, and average current draw, with derating, reserve, and load scenarios.
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What is Battery Life Calculator?
The Battery Life Calculator estimates how long a battery can power a device before it runs flat. You provide the battery capacity (mAh or Wh), the nominal voltage, and the average current or power the device draws; the tool returns the expected runtime in hours along with a friendly days-hours-minutes breakdown. Optional derating and reserve settings model real-world capacity losses, and a scenario table shows runtime at several load levels.
Key Benefits
- Get honest runtime estimates by derating capacity instead of trusting ideal lab specs
- Reserve a safety margin for protected cells or IoT deployment cutoffs
- Convert between mAh and Wh instantly via nominal voltage
- See how sensitive runtime is to load changes with the scenario table
- Copy a clean text summary straight into design documentation
Common Use Cases
- •Sizing a battery for an ESP32, Arduino, or Raspberry Pi sensor project
- •Checking how long a power bank or 18650 pack runs a portable gadget
- •Estimating phone or laptop endurance under heavier-than-normal use
- •Comparing battery options for IoT devices with sleep duty cycles
- •Validating runtime claims in a product spec sheet before release
How to Use the Battery Life Calculator
- Pick an input mode: Choose Charge (mAh) if you know the battery capacity, or Energy (Wh) for laptop-style specs. Set the nominal voltage so the tool can convert between them.
- Enter capacity and load: Type the battery capacity (e.g. 3400 mAh for an 18650 cell) and the average current the device draws (e.g. 250 mA). Use a current probe or datasheet typical value for accuracy.
- Apply derating and reserve: Set derating to about 80% for aged cells or harsh temperatures, and add a reserve percentage if part of the capacity must stay unused. Usable capacity updates instantly.
- Read the runtime and scenarios: Check the estimated hours and human-readable duration, review the scenario table at 0.25x to 5x load, then copy the summary into your design notes.
Key Features
- Dual input modes: capacity in mAh or energy in Wh with automatic voltage conversion
- Derating and reserve controls for realistic usable-capacity modeling
- Runtime shown in precise hours plus human-readable days/hours/minutes
- Eight quick presets: AA, AAA, 9V, CR2032, 18650, phone, power bank, laptop
- Load scenario table at 0.25x to 5x draw to stress-test your power budget
- Copyable plain-text summary for design docs and reports
Frequently Asked Questions
What is the battery life formula?
Battery life (hours) = usable capacity (mAh) / average current draw (mA). For energy-based specs, hours = usable energy (Wh) / average power (W). The calculator applies your derating and reserve percentages to the rated value before dividing.
How do I convert mAh to Wh?
Wh = mAh x V / 1000, where V is the nominal voltage. For example a 3400 mAh 18650 cell at 3.7 V holds about 12.58 Wh. The calculator performs this conversion automatically when you enter a voltage.
Why should I derate the battery capacity?
Rated capacity is measured under ideal lab conditions. Real cells deliver less as they age, at low temperatures, and at high discharge rates. Derating to 80% (or lower for demanding designs) gives a realistic estimate and matches common engineering practice.
How do I find my device average current draw?
Measure with a multimeter or USB power probe across typical usage, or sum active/sleep mode currents weighted by duty cycle. Many datasheets also list typical active and sleep currents you can combine, for example 100 mA active 10% of the time plus 5 mA sleep the rest.