Battery Life Calculator

Estimate the runtime of any battery‑powered device based on capacity, load current, efficiency, and usage profile. Visualise the discharge curve, compare device presets, and optimise your power budget.

Typical smartphone: 3000–5000 mAh; power bank: 10000+ mAh.
Li‑ion: 3.6–3.7 V; LiFePO₄: 3.2 V; alkaline: 1.5 V per cell.
Active use: 200–2000 mA; standby: 1–50 mA.
Power conversion efficiency (DC‑DC converter, battery internal resistance).
Recommended DoD for Li‑ion: 80% to maximise cycle life.
Current draw when device is idle or in sleep mode.
min active min standby
Tip: Set both to 0 for continuous active load. Duty‑cycle mode gives more realistic runtime.
Select a preset to automatically fill typical values.
Privacy first: All calculations are performed locally in your browser. No data is sent to any server.

Understanding Battery Runtime

The Battery Life Calculator estimates how long a battery will power a device under specified conditions. It uses fundamental electrical relationships: capacity (mAh), load current (mA), voltage (V), and efficiency to compute runtime in hours, minutes, or days. The tool also models duty‑cycle scenarios (active + standby) and provides a visual discharge curve.

Runtime (h) = ( Capacity (mAh) × DoD (%) × Efficiency (%) ) / ( Average Current (mA) × 100 )

Average Current = ( Iactive × tactive + Istandby × tstandby ) / ( tactive + tstandby )

Why Battery Life Estimation Matters

  • Product Design: Engineers use runtime estimates to select the right battery for wearables, IoT sensors, and consumer electronics.
  • Consumer Awareness: Understand why your smartphone lasts 12 hours vs. 24 hours — and how usage patterns affect endurance.
  • Energy Optimisation: Identify power‑hungry components and optimise firmware to extend battery life.
  • Sustainability: Accurate estimation helps reduce over‑specification and waste, supporting greener design.

How the Calculator Works

The core calculation is straightforward. Given a battery capacity in milliamp‑hours (mAh) and a load current in milliamps (mA), the theoretical runtime is capacity ÷ current. However, real‑world batteries deliver less than rated capacity due to:

  • Depth of Discharge (DoD): To prolong cycle life, many batteries are not fully discharged. Li‑ion cells typically operate between 20% and 100% SoC (State of Charge).
  • Efficiency: Voltage conversion (e.g., boost or buck converters) and internal resistance cause power losses.
  • Duty Cycle: Devices often alternate between active and standby modes. The average current determines the overall runtime.

The tool also estimates C‑rate (discharge current relative to capacity) and provides a cycle life approximation based on typical Li‑ion degradation curves. For example, discharging at 1C (current equal to capacity) gives about 1 hour of runtime but reduces total cycle life compared to 0.2C.

Device Presets — Real‑World Context

The preset menu reflects common devices to help you quickly explore realistic scenarios. Each preset is derived from manufacturer specifications and independent teardown measurements.

Device Capacity (mAh) Voltage (V) Active Current (mA) Standby (mA) Typical Runtime
Smartphone 3000 3.7 500 10 ~5–6 h (active)
Tablet 7000 3.8 900 15 ~7–8 h
Laptop 50000 7.4 3000 100 ~12–14 h
Drone 5000 11.1 8000 50 ~30–40 min
Wireless Earbuds 50 3.7 15 0.5 ~3–4 h
Power Bank 20000 3.7 2000 5 ~8–9 h (at 2A output)
IoT Sensor 1000 3.3 20 0.05 ~40–50 h (active) / years (standby)
Case Study: Optimising a Wearable Health Tracker

A wearable device manufacturer needs at least 7 days of battery life. The prototype uses a 200 mAh Li‑ion cell at 3.7 V, with an active current of 15 mA (sensor + Bluetooth) and a standby current of 0.8 mA. Using the calculator, we find that with a 10% active duty cycle (2 min active, 18 min standby) the average current is 2.22 mA. Runtime = (200 × 80% × 90%) / (2.22 × 100) = 64.9 hours ≈ 2.7 days — far short of the target. To reach 7 days, the design team must either increase the battery to 550 mAh or reduce active current (e.g., by using a lower‑power sensor or reducing Bluetooth transmit power). This tool enables rapid trade‑off analysis without building physical prototypes.

The Science Behind Battery Chemistry

Most portable devices use Lithium‑ion (Li‑ion) or Lithium‑polymer (Li‑Po) batteries due to their high energy density and low self‑discharge. The nominal voltage is 3.6–3.7 V per cell, with a usable voltage range of about 4.2 V (fully charged) to 3.0 V (cut‑off). The capacity (mAh) is measured at a standard discharge rate (typically 0.2C). At higher discharge rates, the effective capacity decreases due to internal resistance — a phenomenon known as Peukert's law.

For lead‑acid batteries (used in cars and UPS), nominal voltage is 2 V per cell (12 V for a 6‑cell battery) and the recommended DoD is 50% to maximise cycle life. Alkaline batteries (1.5 V per cell) have higher internal resistance and are suitable for low‑drain applications. This calculator is optimised for Li‑ion but can be adapted for other chemistries by adjusting the efficiency and DoD values.

The C‑rate is a measure of how fast a battery is discharged relative to its capacity. A 1C rate means the current equals the rated capacity (e.g., 3000 mA for a 3000 mAh cell), yielding 1 hour of runtime. A 0.5C rate gives 2 hours. Higher C‑rates generate more heat and reduce cycle life. The calculator estimates C‑rate and provides a cycle‑life indicator based on empirical Li‑ion degradation models.

Common Misconceptions

  • “A higher mAh always means longer runtime.” — Not necessarily. Runtime depends on current draw and voltage. A 5000 mAh battery at 3.7 V has 18.5 Wh, while a 3000 mAh at 7.4 V has 22.2 Wh — the latter stores more energy despite lower capacity.
  • “Fully discharging extends runtime.” — Actually, deep discharges (below 20% DoD) reduce cycle life. Most manufacturers recommend 80% DoD for Li‑ion.
  • “Efficiency is always 100%.” — No. Voltage converters, wire resistance, and battery internal resistance all cause losses. 85–95% is typical for good designs.
  • “Standby current is negligible.” — For always‑on IoT devices, standby current can dominate overall energy consumption over long periods.

Step‑by‑Step Usage

  1. Enter your battery capacity (mAh), nominal voltage (V), and load current (mA).
  2. Adjust efficiency (%) and depth of discharge (%) to match your battery chemistry and usage.
  3. Optionally, set active and standby times to model duty‑cycle behaviour.
  4. Click Calculate Runtime to see estimated runtime, effective capacity, average power, and more.
  5. Explore the discharge curve to visualise voltage drop and capacity fade over time.

Expert Tips for Maximising Battery Life

  • Avoid extreme temperatures: Heat accelerates degradation; cold increases internal resistance.
  • Use the right charger: Over‑voltage or under‑voltage can damage cells.
  • Optimise firmware: Reduce polling rates, use interrupt‑driven sensors, and leverage deep sleep modes.
  • Match the battery to the load: High‑current devices need cells with low internal resistance (high C‑rate capability).

Backed by Electrochemical Engineering — This tool is built on fundamental battery equations from “Battery Technology Handbook” (Kiehne, 2003) and “Lithium‑Ion Batteries: Science and Technologies” (Yoshio et al., 2009). The discharge model uses a simplified linear approximation of voltage vs. capacity, which is validated against manufacturer datasheets for common Li‑ion cells. Reviewed by the GetZenQuery tech team, last updated July 2026.

Frequently Asked Questions

mAh (milliamp‑hours) is a measure of charge capacity, while Wh (watt‑hours) is a measure of energy. Wh = (mAh × V) / 1000. A 3000 mAh battery at 3.7 V stores 11.1 Wh of energy. Wh is more meaningful for comparing batteries with different voltages.

Low temperatures increase internal resistance, reducing effective capacity and runtime. High temperatures accelerate chemical degradation, permanently reducing capacity over time. Li‑ion cells are typically rated for 0–45 °C charging and −20–60 °C discharging.

For maximum cycle life, keep Li‑ion between 20% and 80% state of charge. Discharging below 20% (i.e., DoD > 80%) increases stress on the electrodes and shortens lifespan. The calculator defaults to 80% DoD for this reason.

Yes. For lead‑acid, set the nominal voltage to 2 V per cell (e.g., 12 V for a 6‑cell battery), adjust efficiency to ~85%, and use a DoD of 50% to maximise cycle life. The calculator's chemistry assumptions remain valid for most battery types.

As a battery discharges, the internal electrochemical potential decreases. For Li‑ion, the voltage drops from ~4.2 V (fully charged) to ~3.0 V (cut‑off). The curve is not perfectly linear; the calculator uses a simplified linear approximation for clarity.

Recommended resources: Battery University, ScienceDirect, and the book “Lithium‑Ion Battery Management Systems” by Plett (2015).
References: Battery University – Discharging; MPower UK – Battery Performance; IEC 61960‑3:2017 (Secondary cells and batteries).