USB Power Meters

I have a thing for measuring devices — voltage meters, wind speed meters, non-contact thermometers, sonic tape measures, (light meters) etc. etc. So naturally, I also like knowing things like:

  • the kind of power that a USB adapter will provide (including car adapters)
  • how much power a device draws
    • iPhones can be amazingly hungry
    • iPads draw more power if it doesn’t think it is plugged into a computer (I used a power-only usb adapter). This is probably to protect the computer since they are mostly only rated to give 500mA, but can often give almost 1000mA, but the iPads can draw more than 2000mA.
  • how much power is lost through USB cable extensions
    • they vary quite a lot
    • this is not only important for charging, but also for powering devices like webcams
  • how much capacity is available in a USB battery bank

This lead me to purchasing two USB meters in the last few years.

The Charge Doctor power meter/tester

This one just shows voltage and current (in amps)

It has done its job well over many years.

The Keweisi USB power meter/tester

This one I love. It shows mAh over time.

And it remembers the last test until you reset it with the button (top right) even after it is unplugged.

I like to use very short USB extensions with both meters so that I can see the screen easier. These extensions came with some devices many years ago (I don’t remember the devices), but they don’t lose much current so are excellent for use with the meters.

There are many other inexpensive meters on eBay with different configurations, and they seem to change every year or so.

Happy measuring!



The above photos show a Fuji X100T charging a Wasabi Power X100T battery (and taking the photograph, too). It’s not a charge from empty, and both voltage and current can vary quite a lot from second to second.

I was also testing the power output from a MiFi 6620L Jetpack which works very nicely. It shows a percentage of available power, but stops at 20% (I think). If there are no devices connected to it by wifi, the hotspot will shut off once a USB device has stopped drawing power.



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