Testing USB battery pack capacity with a USB multi-meter

Originally published at: https://boingboing.net/2018/02/01/testing-usb-battery-pack-capac.html

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There’s something you need to understand. The rated mAh capacity of the battery pack is most likely rated at the internal battery voltage, probably 3.7 volts. However, the amperage measured by the meter is at USB voltage, 5.0 volts. To compare, you actually need to compare power, Watts, which is the product of voltage and amperage (P=VxA). So, for example 10,000 mAh at 3.7 volts is 37,000 mWh. At 5 volts, that’s only 7,400 mAh. (The actual power required to charge it will actually be a bit more, due to losses in charging.)

Bottom line, if you look at the charge power, it will be less than you expect, even on a full capacity pack.


I like the 2-outlet style of the Drok because, having taken it apart, I know that Output II definitely doesn’t have the data wired to it. Instead, it has a chip on the data lines to spoof being a high-power USB supply. The data lines are only passed through to Outlet I, allowing not only regular USB communication, but also standard QuickCharge 2 and QC3 signalling to work properly.

The nice thing about this is that I can use it on any random USB outlet- say in a hotel room or airport- and not worry about my phone being at risk of attack through that vector. And you can see how much power the outlet is providing- some can only source 500ma, so you might want to take the time to look around the room for something better.

Some of my devices are low power enough that I can use the Drok as a splitter so I can charge two things from my single battery pack. Just make sure the supply is putting out only 5 volts to your Output I device before plugging in your second Output II device- since it is possible to get over 12 volts from a QuickCharge 2 or 3 power supply with the appropriate host device plugged into Output I.

That said, I haven’t used any of the other meters, so cannot speak to other comparative advantages or disadvantages.

There are also USB C volt/amp/power meters too. They’re newer so not as well user tested. As always, check the Amazon reviews.


You should measure how much energy you can pull out of each battery to see how useful it is.

How much charge goes into a battery is useful for calculating charging efficiency, but not much else.

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