So you snag yourself a 20,000 mAh power bank on Amazon for cheap on a hot sale. You check the specs on your new iPhone 11 and find out that you have a 3,110 mAh battery, sweet you can charge it more than 6 times… or can you?
The advertised capacity is the actual physical size of the cells inside the power bank.
What is inside most power banks are 3.7V battery cells, however, the USB standard is 5V. With this in mind, between the battery and the USB outlet is a conversion circuit that changes 3.7V into 5V.
Any time you convert to a higher voltage, you also convert the mAh as well into the new voltage.
So how do you calculate USB output?
This basic equation is used to convert the 3.7V into 5V.
The actual 5V mAh = 3.7 X Advertised mAh / 5
For example the 20,000mAh power bank above looks more like this:
3.7 X 20,000 / 5 = 14,800 mAh
Basically a 3.7V 20,000 mAh powerbank truly supplies 14,800 mAh output at 5V USB. So right out of the packaging, you get a steep reduction in delivered mAh, however, you also then need to consider conversion loss.
What the heck is conversion loss?
To say it as simply as possible, when you use your power bank you will notice it get warm… which means that the circuit inside that converts 3.7V to 5V USB uses energy and also generates measurable amounts of heat.
Heat is basically energy leaking, reducing mAh even more. Different brands use many different circuits which have varying conversion loss ratios, but they all leak energy.
So when searching for your next power bank, keep this information in mind and buy according to your needs.
Unfortunately, this might end with you needing to buy a larger, more expensive power bank to meet those needs, but at least you won’t be surprised by the actual performance.
If you want some recommendations for power banks, I have reviewed quite a few of them: