Battery Life Calculator

Estimate battery runtime or required capacity using clear electrical variables: capacity C, voltage V, load current I or power P, duty cycle D, and usable fraction η.

Battery & Load Inputs

Results Summary

Engineering Reference

Battery Life Calculator: From Datasheet Numbers to Real-World Runtime

Use this battery life calculator to turn capacity, load current or power, duty cycle, and usable depth of discharge into realistic runtime estimates or required battery capacity. This guide explains the equations, assumptions, and limitations so your numbers line up with what actually happens in the field.

10–15 min read For students & working engineers

Quick Start: Getting a Trustworthy Battery Life Estimate

The calculator directly mirrors the core engineering equations for battery runtime. In its simplest form, with current in amperes and capacity in amp-hours, the ideal runtime is:

\[ t_h = \frac{C_{Ah}\,\eta}{I_A \, D_{\text{duty}}} \]

Where \(t_h\) is runtime in hours, \(C_{Ah}\) is usable capacity in amp-hours, \(\eta\) is the usable fraction (for example, 0.8 for 80 % depth of discharge), \(I_A\) is average load current in amperes, and \(D_{\text{duty}}\) is duty cycle as a fraction (for example, 0.1 for 10 % on-time).

  1. 1 Choose what you want to solve for.
    If you know the battery and load, pick Battery life. If you have a runtime target, pick Required capacity.
  2. 2 Select whether you are working from current or power.
    Use By current when you know average current draw (mA / A). Use By power when you have power consumption (mW / W) and supply voltage.
  3. 3 Enter the battery capacity using the units on the datasheet.
    Most portable electronics use mAh; some larger systems use Ah or Wh. The calculator converts everything to an internal amp-hour or watt-hour basis.
  4. 4 Specify the load current or power, plus duty cycle.
    For a device that is on 20 % of the time and sleeps the rest, use a 20 % duty cycle. For a constant load, set duty cycle to 100 %.
  5. 5 Set the usable capacity fraction.
    Rarely do you use 100 % of the nameplate capacity. For Li-ion, 70–90 % is common; for lead-acid, limiting depth of discharge protects cycle life.
  6. 6 Click Calculate.
    The main result is shown in hours, and the quick stats show runtime in days plus effective load current or power and battery energy in Wh.
  7. 7 Use the Steps and Variables & Symbols panels.
    They walk through the conversions (mAh → Ah, mW → W, etc.) and show how your inputs map to the symbols in the equations.
Tip: Start with datasheet numbers and ideal conditions. Then adjust duty cycle, efficiency, and usable capacity to match your real application.
Warning: This battery life calculator assumes a roughly constant load and temperature over the interval. Highly pulsed loads, low temperatures, and chemistry-specific effects (like Peukert’s law for lead-acid) will reduce real-world runtime relative to the ideal estimate.

Choosing Your Method: Current vs. Power, Life vs. Capacity

The same underlying physics can be framed in different but equivalent ways. The calculator exposes the most common engineering perspectives so you do not have to manually rearrange equations.

Method A — Solve for Battery Life (t)

Use this when you know the battery and load and want to predict how long the system will run.

  • Perfect for sizing duty-cycle and power-saving features.
  • Matches how firmware and hardware design teams talk about runtime.
  • Directly comparable with field test data.
  • Requires a reasonably accurate estimate of average current or power.
  • Does not directly give you the battery size you need.

Core form (current view): \[ t_h = \frac{C_{Ah}\,\eta}{I_A\,D_{\text{duty}}} \]

Method B — Solve for Required Capacity (C)

Use this when you have a runtime requirement and need to size a battery.

  • Ideal for early system design and vendor discussions.
  • Helps you compare chemistries and form factors for the same runtime.
  • Still depends on realistic duty cycle and power estimates.
  • May ignore second-order effects like aging, temperature, and high peak currents.

Rearranged (capacity view, current-based): \[ C_{Ah,\text{req}} = \frac{t_h\,I_A\,D_{\text{duty}}}{\eta} \]

Method C — Power-Based View (P and Wh)

Use this when power and energy (W and Wh) are more natural than current and amp-hours.

  • Matches AC/DC power supply ratings and many datasheets.
  • Works well with high-level energy budgets in Wh or kWh.
  • Requires a supply voltage to convert between Ah and Wh.
  • Less intuitive for low-power embedded engineers who think in mA.

Power view (runtime): \[ t_h = \frac{E_{Wh}\,\eta}{P_W\,D_{\text{duty}}} \] with \(E_{Wh} = C_{Ah} \cdot V\).

The Battery Life Calculator lets you toggle between these perspectives. Internally, it converts everything to a consistent set of units and equations so the results remain coherent no matter which method you pick.

What Moves the Number: Key Variables and Trade-Offs

The runtime you see is dominated by a small set of variables. Understanding them helps you design systems that meet runtime targets with margin instead of guessing.

Battery capacity \(C_{Ah}\)

Higher capacity almost always increases runtime, but with mass, cost, and volume penalties. The calculator treats the nameplate capacity as the starting point and applies a usable fraction \(\eta\).

Average load current \(I_A\)

In current mode, runtime scales inversely with current. Halving average current ideally doubles runtime:

\[ t_h \propto \frac{1}{I_A} \]
Load power \(P_W\)

In power mode, runtime scales inversely with power draw:

\[ t_h = \frac{E_{Wh}\,\eta}{P_W\,D_{\text{duty}}} \]

Small reductions in power can be more effective than simply increasing battery size.

Duty cycle \(D_{\text{duty}}\)

Many embedded devices wake briefly, perform work, and return to sleep. A 5 % duty cycle means they are active only 5 % of the time. The calculator uses duty cycle to derive effective current or power.

Usable fraction \(\eta\)

You rarely want to use 100 % of the battery. Limiting depth of discharge to 70–80 % can dramatically improve cycle life. The calculator multiplies capacity by \(\eta\) before computing runtime or required capacity.

Supply voltage \(V\)

Voltage links the Ah and Wh worlds. In power-based calculations, voltage is required to compute energy from capacity:

\[ E_{Wh} = C_{Ah} \cdot V \]
Temperature and aging

These are not explicitly in the equations but matter in reality. Cold temperatures and aged cells reduce effective capacity. Add design margin to cover these effects.

Peukert and non-linear effects

At very high currents, especially in lead-acid batteries, effective capacity decreases more than linearly. The calculator assumes approximately linear behavior; treat high-current cases with extra care and testing.

Worked Examples with the Battery Life Calculator

These examples walk through realistic inputs so you can compare the calculator’s output with your engineering intuition.

Example 1 — Coin-Cell Sensor Node (Solve for Battery Life)

  • Battery: CR2032, 240 mAh nominal
  • Supply voltage: 3.0 V
  • Usable fraction: \(\eta = 0.8\) (80 %)
  • Active current: 15 mA for 100 ms
  • Sleep current: 3 µA
  • Report interval: once every 60 s

First estimate the average current. In one 60-second window:

\[ I_{\text{avg}} \approx \frac{I_{\text{active}} \cdot t_{\text{active}} + I_{\text{sleep}} \cdot t_{\text{sleep}}}{T} = \frac{15\,\text{mA} \cdot 0.1\,\text{s} + 0.003\,\text{mA} \cdot 59.9\,\text{s}}{60\,\text{s}} \] \[ \approx \frac{1.5\,\text{mA·s} + 0.18\,\text{mA·s}}{60\,\text{s}} \approx 0.028\,\text{mA} \]

Enter capacity = 240 mAh, usable fraction = 80 %, current = 0.028 mA, and duty cycle = 100 % (because we already averaged over time), then solve for battery life.

1
Convert 240 mAh to Ah: \(\;C_{Ah} = 240 / 1000 = 0.24 \,\text{Ah}\).
2
Apply usable fraction: \(C_{\text{eff}} = 0.24 \cdot 0.8 = 0.192 \,\text{Ah}\).
3
Convert current: \(I_A = 0.028\,\text{mA} / 1000 \approx 2.8 \times 10^{-5}\,\text{A}\).
4
Runtime: \[ t_h = \frac{0.192}{2.8 \times 10^{-5}} \approx 6857\,\text{h} \approx 285\,\text{days} \]

The calculator automates this sequence and shows the result in hours and days. In practice, temperature and cell aging will reduce this idealized runtime, but the scale (many months) is correct.

Example 2 — 12 V Lead-Acid Supply (Solve for Required Capacity)

  • Target runtime: 8 h
  • Load: 60 W DC load at 12 V
  • Usable fraction: \(\eta = 0.5\) (limit to 50 % depth of discharge)
  • Duty cycle: 100 % (continuous)

Because you know power and voltage, the power-based method is natural. Enter runtime = 8 h, power = 60 W, voltage = 12 V, usable fraction = 50 %, and select Required capacity.

\[ C_{Ah,\text{req}} = \frac{t_h \, P_W \, D_{\text{duty}}}{\eta \, V} = \frac{8\,\text{h} \cdot 60\,\text{W} \cdot 1}{0.5 \cdot 12\,\text{V}} \] \[ = \frac{480}{6} \approx 80\,\text{Ah} \]

The calculator will show that you need roughly an 80 Ah battery if you are willing to use half of its capacity. In practice, you would select the next standard size up and add margin for temperature, aging, and Peukert effects at this current.

Common Layouts & Variations for Battery Life Planning

Different applications stress batteries in different ways. Use the scenarios below to decide how conservative your inputs should be in the Battery Life Calculator.

Use CaseTypical InputsKey RisksDesign Notes
Low-power IoT sensor Microamp sleep, milliamp bursts, duty cycle 1–5 %, coin cell or Li-ion pouch. Over-estimating sleep current, cold-temperature capacity loss, self-discharge over years. Model duty cycle carefully and use conservative usable fraction; validate with long-term soak tests.
Handheld tool or instrument 0.5–5 A continuous draw, moderate duty cycle, Li-ion pack with BMS. High peak currents, voltage sag, thermal limitations, aging. Use current-based view with realistic average current. Consider thermal derating and cycle life.
Backup power / UPS Hours of runtime at relatively constant power, lead-acid or Li-ion rack batteries. Peukert effects, temperature, long-term float or calendar aging. Use power-based method; apply low usable fraction and verify with worst-case discharge tests.
Electric mobility (small vehicles) Tens to hundreds of watts, widely varying duty cycle, regenerative braking. Large dynamic range, repeated high-current events, user abuse. Use the calculator for baseline energy sizing (Wh); then refine with drive-cycle simulations.
  • Decide whether you care about worst-case, typical, or best-case runtime.
  • Use temperature-appropriate capacity curves when available.
  • Model anomalies like inrush or radio transmit bursts in your average current.
  • Cross-check the calculator’s result against similar products or prior designs.

Specs, Logistics & Sanity Checks Before You Commit

Once the Battery Life Calculator gives you a result, use it as the starting point, not the final word. The sections below highlight practical considerations before freezing a design or placing a large battery order.

Reading Capacity Specs

Capacity is often specified at a particular discharge rate and temperature (for example, 0.2C at 25 °C). If your load is heavier or colder, actual capacity will be lower.

  • Check the discharge rate used for the datasheet capacity.
  • Note any low-temperature derating curves.
  • Confirm how many cycles you need vs. depth of discharge.

Building in Design Margin

A common pattern is to add 20–30 % margin over calculated required capacity to cover aging, temperature variation, and estimation error.

  • Add capacity margin, not just runtime margin.
  • Consider efficiency losses in converters and regulators.
  • Account for future firmware features that might increase load.

Field & Test Feedback

The fastest way to validate your model is to log actual current or power over time and compare measured runtime with the calculator’s prediction.

  • Instrument a prototype with current logging.
  • Repeat tests at hot and cold temperature extremes.
  • Feed measured averages back into the calculator to refine estimates.

Treat the Battery Life Calculator as a simple, transparent model. It helps you see how runtime changes with inputs and provides a baseline that you can calibrate with real test data.

Frequently Asked Questions

How accurate is a battery life calculator in real applications?
A battery life calculator is usually accurate to within tens of percent when the inputs are realistic and the load is roughly constant. It assumes a constant average current or power, a fixed depth of discharge, and nominal temperature. Real devices see pulsed loads, cold conditions, aging cells, and converter losses, all of which reduce runtime. Use the calculator to understand trends and ballpark numbers, then validate with measurements on real hardware.
Should I use mAh, Ah, or Wh when I enter battery capacity?
Use whatever is on the datasheet: small batteries are often rated in mAh, larger packs in Ah, and some systems in Wh. The calculator converts mAh and Ah into internal amp-hours, and, when voltage is known, converts to Wh as needed. If your datasheet only lists Wh, you must provide the nominal voltage so the calculator can derive amp-hours when solving in current mode.
What duty cycle should I enter for my device?
Duty cycle should reflect the fraction of time the device is drawing its higher active current versus being in a low-power state. If the calculator has a single current input, you can either (1) enter a true average current and set duty cycle to 100 %, or (2) enter active current and use duty cycle to represent how often that current flows. For complex load profiles, compute the average current or power over a representative interval and use that with a 100 % duty cycle.
Why does the calculator ask for a usable capacity or depth of discharge?
Most chemistries should not be fully discharged on every cycle. For example, limiting Li-ion cells to 80 % depth of discharge can significantly increase cycle life. The usable capacity or depth of discharge setting lets you choose how much of the nameplate capacity you treat as available. The calculator multiplies nameplate capacity by this fraction before computing runtime or required capacity.
How do pulsed and peak currents affect battery life calculations?
The underlying equations use average current or power, but batteries respond to pulse magnitude and duration as well. Short, infrequent peaks may be well represented by an average value, while long or very high pulses can cause voltage sag, extra heating, and reduced effective capacity. For safety-critical or high-current systems, use the calculator for initial sizing and then verify performance with detailed simulations or hardware testing that captures the full load waveform.
Does the calculator handle Peukert’s law or chemistry-specific effects?
The simple battery life equations in this calculator assume capacity is independent of discharge rate, which is a good approximation for many Li-ion and NiMH applications at moderate currents. Lead-acid batteries, and very high current draws in general, require Peukert’s law or chemistry-specific models to capture the reduction in usable capacity at higher discharge rates. For those cases, treat the calculator as a first-order estimate and adjust capacity or usable fraction based on manufacturer data.
Scroll to Top