The secret that power bank manufacturers don't want you to know

Big numbers sell. In the minds of consumers, 5,000 is better than 2,000, and 10,000 is better than both. And when it comes to 10 million, well, that beats everything, right? There's one area of tech where I see big numbers being thrown around a lot, and that is power bank capacities.

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Power bank manufacturers have adopted the milliamp-hour (abbreviated as mAh) as the default measure of battery capacity for power banks. This is handy for marketers because even the smallest power banks have capacities measured in the thousands of milliamp-hours. However, mAh is not the best measure of power bank capacity, and I'm here to explain why.

The problem with relying on mAh for battery capacity

The issue with using mAh as a measure of battery capacity is that it reflects a battery's power capacity at a specific voltage output. If you tweak the output voltage, the mAh rating changes.

The mAh is calculated using the formula:

mAh = (Wh x 1000) / V

Where Wh is the energy capacity of the battery measured in watt-hours, and V is the voltage of the battery.

The default voltage that most reputable power bank manufacturers use is the voltage of the battery inside the power bank, which is typically 3.7V or 3.85V. If you remember a few of your math classes, you might have noticed from that equation a few lines above that the lower the voltage, the higher the mAh rating. This is advantageous from a marketing point of view because the default voltage output for an old-school USB-A port is 5V, and USB-C ports can go to 20V and beyond. This makes the mAh capacity rating somewhat meaningless because it changes with that output voltage.

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Quite a lot of people are noticing this because they are going out and buying USB power meters, measuring the outputs of their own power banks, and finding that a power bank rated for 10,000 mAh only outputs around 6,000 for them.

Some manufacturers do now discuss this capacity discrepancy, such as Anker, and I've seen quite a few have it listed on the spec sheet for their products. Here, Anker does a good job of highlighting that there's also a chunk of power lost through circuit losses.

To make matters worse, some unscrupulous manufacturers may "fake" high mAh ratings by assuming some ridiculously low voltage (V), thereby inflating the mAh number. This means that a manufacturer could print absurdly large capacity numbers on the packaging and not technically be lying because they are assuming some absurdly low output voltage. I know this because I report improperly rated products to UK Trading Standards (a body that investigates illegal business activity and unfair trading), and this argument was used as a defense more than once.

I tend to avoid reviewing these sorts of power banks because they almost always are a disappointment.

Watt-hours (Wh) is a more straightforward way to measure

Recently, I got my hands on a power bank boasting a 10 million mAh capacity. It featured a solar panel for charging (I've already written about how using these can cause a power bank to overheat to dangerous levels), USB-A, USB-C, microUSB, and Lightning connectors, along with two USB-A and two USB-C ports. 

Sounds great, right? Well, the solar panel didn't work, and I measured the actual capacity of the power bank to be just a fraction of what was advertised. To top it off, it died after only an hour of testing.

In short, it was junk. But I have no doubt that the exaggerated mAh rating contributed to its appeal.

Feeling confused or worried that you'll need to pull out a calculator every time you buy a power bank? Fear not! I'm going to show you a better, more straightforward way to measure battery capacity -- and that is using watt-hours (Wh).

Finding the true capacity of your power bank

Wh measures the total amount of electrical energy consumed or produced over a specific period, typically one hour. For example, if a device uses one watt of power for one hour, it consumes one watt-hour of energy.

In this context, Wh helps you understand how much energy a power bank can store. For instance, if a battery has a capacity of 10 Wh, it can theoretically provide 10 watts of power for one hour, or 5 watts for two hours, and so on.

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It's a more apples-to-apples comparison of power capacity, and it's already what you'll find being used for power stations.

This measurement is a much more accurate metric of battery capacity and cuts through much of the marketing hype. Also, you'll notice that it's rare to see a power bank with more than 100 Wh of capacity due to airline regulations.

And again, from my experience, reputable manufacturers will list this figure in the spec sheet for their products, making it much easier for you to find out the true capacity of your power bank.

From now on, whenever I review power banks, I will highlight their Wh capacity to allow you to know, at a glance, a power bank's true capacity, and I will be testing the capacity to keep the manufacturers honest.