A battery life calculator helps you estimate how long a battery will power your device based on its capacity (mAh or Ah) and the device's current draw (mA or A). It also accounts for real-world efficiency losses so you get practical runtime estimates instead of theoretical maximums. Use it for electronics projects, IoT devices, portable gadgets, and any battery-powered application.
Battery Parameters
Results
Note: This is the minimum rated capacity needed. Consider a battery with 10-20% more capacity for a safety margin.
Enter battery parameters and click Calculate
How to Use the Battery Life Calculator
Knowing how long a battery will last is essential for designing portable electronics, sizing backup power systems, and choosing the right battery for your project. This battery life calculator gives you instant runtime estimates using the standard formula: runtime = (capacity x efficiency) / current draw. It also works in reverse, telling you what capacity you need for a desired runtime.
Step 1: Choose Your Calculation Mode
Select Runtime from Capacity if you already have a battery and want to know how long it will last. Select Capacity from Runtime if you know how long you need your device to run and want to find the right battery size. You can switch between modes at any time.
Step 2: Enter Battery Capacity
In runtime mode, enter the battery's rated capacity. You can use milliamp-hours (mAh) for small batteries like AA, AAA, and coin cells, or amp-hours (Ah) for larger batteries like car or deep-cycle batteries. Use the preset buttons for common battery types to quickly populate the capacity field. A standard AA NiMH battery is typically rated at 2500 mAh, while a common 18650 lithium cell holds about 3000 mAh.
Step 3: Enter Current Draw
Enter how much current your device consumes in milliamps (mA) or amps (A). This information is usually found on the device label, in its datasheet, or measured with a multimeter. For devices with variable consumption, use the average draw for a realistic runtime estimate. For example, an Arduino Uno draws roughly 45 mA, an LED typically uses 20 mA, and a Raspberry Pi can draw 500-700 mA under load.
Step 4: Adjust the Efficiency Factor
The efficiency slider accounts for real-world energy losses. The default of 80% works well for most modern circuits with linear voltage regulators. Use 90-95% for efficient switching regulators, and lower it to 60-70% for older batteries, extreme temperatures, or high-drain applications. Set it to 100% for a theoretical maximum with no losses.
Step 5: Review Your Results
Click Calculate to see the estimated runtime displayed in hours, minutes, and days, along with a visual battery gauge. The readable duration breaks it down into days, hours, and minutes for quick comprehension. In capacity mode, you will see the minimum battery rating needed along with an Ah conversion and a note recommending a 10-20% safety margin for real-world reliability.
Frequently Asked Questions
Is this battery life calculator completely free?
Yes, this battery life calculator is 100% free with no limits. Calculate as many battery runtimes as you need without creating an account. There are no usage caps, no premium features, and no hidden fees.
Is my data safe when using this tool?
Absolutely. All calculations run entirely in your browser using client-side JavaScript. No data is sent to any server, stored, or logged. Your battery specifications remain completely private on your device.
What is the efficiency factor and why does it matter?
The efficiency factor accounts for real-world energy losses in batteries and circuits. No battery delivers 100% of its rated capacity due to internal resistance, voltage regulation losses, and temperature effects. An 80% efficiency is a good default for most applications.
What is the difference between mAh and Ah?
mAh (milliamp-hours) and Ah (amp-hours) measure the same thing at different scales. 1 Ah equals 1000 mAh. Small batteries like AA cells are rated in mAh (e.g., 2500 mAh), while larger batteries like car or solar batteries use Ah (e.g., 100 Ah).
How do I find the current draw of my device?
Check the device specifications label or datasheet for its power consumption in milliamps (mA) or amps (A). You can also measure it with a multimeter in series with the power supply. For devices with variable draw, use the average consumption for a more accurate runtime estimate.
Why does my battery last less than the calculated time?
Real-world runtime is often shorter due to factors like temperature extremes, battery age and degradation, varying current draw during operation, and voltage cutoff thresholds. Lower the efficiency slider to 60-70% for older batteries or harsh conditions to get a more conservative estimate.
Can I use this calculator for rechargeable batteries?
Yes, this calculator works for all battery types including lithium-ion, NiMH, lead-acid, and alkaline. Just enter the rated capacity in mAh or Ah. For rechargeable batteries that have degraded over time, reduce the capacity or efficiency factor to reflect their current state.
How do I calculate battery life for a device with multiple components?
Add up the current draw of all components to get the total current. For example, if a microcontroller draws 20 mA and an LED draws 15 mA, enter 35 mA as the total current draw. This gives you the combined runtime for the entire system.