Demystifying Battery Ratings: mAh vs. mWh

In the world of batteries, two important metrics come into play: mAh (Milliampere-hours) and mWh (Milliwatt-hours). These ratings are critical for choosing the right battery for your specific needs, whether you’re powering flashlights, RC cars, toys, household devices, VR games or outdoor equipment. In this post, we’ll delve into the details of mAh and mWh, providing you with a clear understanding of what they mean and how to make informed battery choices.

Understanding mAh and mWh

mAh (Milliampere-hour) is a unit of measurement for battery capacity. It refers to the amount of charge a battery can hold and deliver at a certain current over time. In simpler terms, mAh quantifies how much electrical energy a battery can release within one hour. At the same rated voltage, a higher mAh value indicates a larger capacity, much like a larger reservoir for storing water.

Here’s a simple formula to explain the concept of mAh: Q = I * t, where:

· Q represents the battery’s charge capacity, measured in mAh.

· I stands for current, measured in milliamperes (mA).

· t denotes time, measured in hours (h).

For example, if a battery has a rated capacity of 2000mAh, it means it can provide a current of 2000 milliamperes (equivalent to 2 amperes) for one hour.

mWh (Milliwatt-hour) is a measure of the total energy stored in a battery. It reflects the energy generated by a battery when operating at a constant power for one hour at different working voltages. In reality, true capacity is closely related to voltage, and the voltage of most batteries is not a fixed value; it can vary during use.

In simple terms, mWh is like measuring the total energy stored in a reservoir, accounting for both depth (voltage) and volume (capacity). This means that mWh provides a more comprehensive view of a battery’s energy potential.

Let’s use a simple formula again to explain the concept of mWh: W = U * Q, where:

· W represents the energy capacity of the battery, measured in mWh.

· U stands for voltage, measured in volts (V).

· Q is the battery’s charge capacity, measured in mAh.

For example, if a battery has a capacity of 2000mAh and a voltage of 3.7 volts (V), its energy capacity would be 7.4 watt-hours (Wh) or 7400 milliwatt-hours (mWh). This means it can provide 7400 mWh of energy.

mAh to mWh Conversion

The conversion between mAh and mWh is based on the voltage of the battery, which can be converted by the following formula:

mWh=mAh×V

This conversion is essential, for instance, in aviation regulations regarding the carriage of lithium batteries. These regulations often specify that lithium batteries must not exceed 100 watt-hours (Wh). However, some rechargeable batteries are typically labeled in mAh, which can be quickly converted into mWh.

Why Do Most Batteries Use mAh for Capacity Rating?

Almost all the 3.6/3.7V Li-ion and 1.2V NiMH batteries use mAh in capacity labeling, instead of only label with mWh. What’s the reason?

Using mAh as a unit of measurement is primarily for convenient calculations, allowing users to estimate usage time more intuitively. For example, when using a 20,000mAh power bank to charge a 5,000mAh smartphone battery, we can quickly calculate that it can charge the phone approximately 4 times because both the power bank and the smartphone battery typically use 3.7V batteries.

mAh are easy to calculate, while mWh may involve the complicated issue of the discharge platform. And we have become accustomed to that way in our daily life.

When to Use mWh Ratings with Examples

Comparing Battery Capacities Under Different Voltages

When comparing the capacity of different battery types, using watt-hours (Wh) is a more appropriate method. For example, when comparing the rated capacities of a 2500mAh AA 1.5V lithium-ion battery and a 1.2V NiMH battery, it’s essential to compare their mWh ratings. By calculation, we can determine:

2500mAh x 1.5V = 3750mWh

2500mAh x 1.2V = 3000mWh

Therefore, the 1.5V AA lithium-ion battery has a higher rated capacity than the 1.2V AA NiMH battery.

High-Energy-Consuming Devices

For high-energy-consuming devices that require a continuous and stable power supply, such as VR and X-box controllers, mWh labeling is more appropriate. These devices often operate at different voltages, necessitating consideration of voltage differences.

Multiple Battery Assemblies

When multiple battery cells are connected in series or parallel, mWh labeling helps accurately represent the total energy capacity. This is common in devices like laptop battery packs or remote-control toy cars.

Different Device Drivers

If the driver is a linear constant current driver, then essentially it is controlling the current directly coming from the battery, hence mAh is best. If you have a buck or boost driver, then the output power is usually fixed, and efficiency is usually somewhat consistent, hence mWh is a better measurement.