How to measure the capacity of a powerbank

I use a USB watt meter and a 1 amp dummy load to test the true capacity of an ASUS Zenpower powerbank.

When measuring the true capacity of a powerbank, it's important to remember the voltage of the internal batteries which are nearly always 3.7V 18650 cells. When you consume power from the powerbank, a boost converter is stepping that voltage up to 5V for your USB device.

Convert powerbank mAh to Watt hours

Since there's a voltage difference between the internal cells and the output power, the first thing to do is to convert the rating of your powerbank to Watt hours.

10050 mAh powerbank converted to watt hours: 10050 (mAh) x 3.7V / 1000 = 37.2 Watt hours

Now we attach our USB dummy load to our powerbank, through a USB watt meter that can log mAh or Watt hours. I chose to run the test at 1 amp since that's the typical draw of most modern smart phones.

After a number of hours, the powerbank will have been depleted and you can take the voltage and mAh to work out the Watt hours.

Results

Dummy load results = 6320 (maH) x 4.94V / 100 = 31.22 Watt hours

So as you can see, although the mAh reading of our USB watt meter is just 6320 instead of 10050, we consumed that power at a higher voltage than what the powerbank is rated at. When comparing the powerbanks quoted capacity of 37.2 Wh to our measured 31.2 Wh, that gives us about 84% of the expected power. Considering the powerbank is a few months old (although lightly used) and losses in boosting the voltage, I think that's a pretty fair and honest result.

More info: gadgetaddict.net/how-to-measure-the-capacity-of…

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