How to Measure Battery Capacity - Nature's Generator

How to Measure Battery Capacity

Learn how to measure battery capacity and be able to optimize performance and enhance the longevity of your devices or systems.
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Batteries make our lives easier and more comfortable. We may not realize it, but they play an important role in our day-to-day lives, from powering up our laptops, smartphones, and electric vehicles to even helping provide alternative power sources for off-grid living or as backup in case of electrical issues. 

Having a clear understanding of how batteries work and how to measure their capacity is essential to ensure they perform optimally and provide the expected energy output. If you are interested in adding to your knowledge bank and maximizing your battery usage, then this article will serve as a guide to walk you through the process of measuring battery capacity.

So, What is Battery Capacity?

Battery capacity refers to the total amount of electricity a battery can store and deliver when needed. It is a critical parameter as it directly impacts the runtime of devices. However, it’s important to note that though battery capacity signifies the maximum extractable energy, the actual storage capacity can vary significantly from the nominal rating with several factors affecting it, like battery age, usage history, charging/discharging rates, and temperature. 

Read more on the factors affecting battery capacity here.

Why Measure Battery Capacity?

Measuring battery capacity is crucial for several reasons:

  1. Optimizing Performance - Knowing your battery's true capacity ensures that you can use it effectively without the risk of it running out unexpectedly.
  1. Battery Health - Regular capacity measurements help assess the health of your batteries. A significant drop in capacity may indicate the need for maintenance or replacement.
  1. In Portable Electronic Devices - accurate capacity measurements enable better power management and battery life estimation.
  1.  For record keeping – a detailed record of battery performance can help you adhere to the manufacturer’s recommendation for battery maintenance and replacement intervals.
Units of Measurement

Before we dive into the actual measurement methods, let’s have a quick brush-up on the units commonly used to express battery capacity:

Battery capacity is typically measured in units such as Ampere-Hours (Ah) and Watt-Hours (Wh). If measuring via AH, it represents the number of amperes of current a battery can deliver over the span of one hour. Theoretically, a 100Ah battery should be able to deliver 100 amperes in 1 hour before being completely discharged. If measuring in Wh (recommended for Lithium battery type), this covers a more comprehensive measurement of battery capacity, as it covers both the voltage and current. The formula to calculate WH is simply multiplying the battery’s voltage by its Ah rating e.g., a 12V battery with a capacity of 100Ah then has a total capacity of 1200Wh.

Learn more about Watt Hours and Amp Hours here.

Methods to Measure Battery Capacity

Milliamp-Hours (mAh) Capacity: “The Quick and Easy Method”

Using milliamp-hours (mAh) is probably the quickest and easiest way to measure your battery capacity, especially if you have smaller batteries. 

One of the simplest ways to measure battery capacity, especially for smaller batteries, is by using milliamp-hours (mAh). This method involves connecting the battery to a constant current load and measuring the time it takes for the battery to discharge completely. 

Some specialized battery chargers come with a discharge function that allows you to check the mAh capacity more conveniently.

Here's a step-by-step guide:

  1. Select a Constant Current Load - Choose an appropriate constant current load based on the battery's specifications. This load should drain the battery within a reasonable time frame.
  1. Connect the Battery - Attach the battery to the constant current load. Ensure the connections are secure.
  1. Measure Discharge Time - Start the discharge process and record the time it takes for the battery to reach a specified voltage (e.g., 2.8V). This voltage may vary depending on your battery's chemistry and the manufacturer's recommendations.
  1. Calculate Capacity - Use the formula Capacity (mAh) = Current (mA) multiply by Time (hours). For example, if you have a 2000mAh battery that discharges at a constant current of 500mA and it takes 4 hours to reach the cutoff voltage, the capacity is 500mA * 4 hours = 2000mAh.

Warning: the measured mAh capacity can vary depending on the discharge voltage you choose. Discharging the battery to very low voltages repeatedly can reduce its overall lifespan.

Watt-Hours (Wh) Capacity: “A Comprehensive Measurement”

As mentioned, calculating the capacity using Wh offers a more comprehensive measurement of battery capacity as it takes into account both voltage and current, 

Here's how to measure capacity in Wh:

  1. Know your batteries Voltage and Current (Amps)
  2. Multiply the voltage by the current to get the capacity in Wh. 

Using Wh provides a more accurate representation of the battery's capacity, especially for batteries with varying voltage levels during discharge.

Lithium Battery Capacity: “The Wh Advantage”

For lithium batteries, calculating capacity in Watt-Hours (Wh) is highly recommended. Unlike some other battery chemistries, lithium-ion batteries maintain a relatively stable voltage throughout their discharge cycle so no need to take into account voltage fluctuations.

Here's a simple example:

  • Battery: Spirit 1.0 Plus lithium battery
  • Capacity: 1276 Wh
  • Full-Throttle Power: 1000 W

To find the runtime, divide the capacity by the power: 1276 Wh / 1000 W = 1.276 hours (approximately 1 hour and 15 minutes).

How to Measure Battery Capacity

There are different methods for measuring battery capacity, each suited to specific scenarios:

  1. Constant Current Method (commonly used for lead-acid batteries)
Connect the battery to a constant current load and measure the time it takes for the battery to drain to a specified voltage.
Simply use this straightforward formula for capacity calculation: Capacity (Ah) = Current (Amperes) * Time (Hours)

  1. Constant Power Method (suitable for batteries that experience variable voltage levels during discharge)
This is similar to the Constant Current Method but uses a continuous power load instead.
Formula: Capacity (Wh) = Power (Watts) * Time (Hours)
  1. Constant Resistance Method (this method is a bit more complex but offers accurate results when done correctly)
This will require an elementary discharge circuit with a single resistor connected across the battery terminals. Then measure the voltage across the resistor at regular intervals.
Formula: Calculate power (V^2/R) and energy (Power * Time) for each measurement cycle, and then add these measurements to get the total capacity in Wh.
Battery Capacity and Home Power Back Up

If you want to live off the grid or have an independent energy solution, then most likely you are considering owning a solar generator. One of the best products in the market is from Nature’s Generator. Their Powerhouse line can single-handedly address all of your energy requirements with options to expand the system. We also want to highlight Nature’s Generator Powerhouse Power Pod, which allows homeowners to upgrade battery capacity and harness more energy from the sun and wind. All of these wonderful features need proper maintenance, and as a user of any home power backup system, knowing how to measure battery capacity will come in handy if you want reliability, energy efficiency, and longevity of your systems. 

Know more about the types of solar batteries.


Learning how to measure battery capacity is an important skill for anyone who relies on batteries, whether for portable devices or larger applications. This will give users the skills to assess the battery’s health, optimize usage, and ensure the reliability of devices.


* We want to give credit where credit is due. Professional writer, Michelle Gamana, contributed research and content to this blog titled: How to Measure Battery Capacity Thank you, Michelle, for your contributions!