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One of the tools I get asked about a lot is USB testers. USB charging is ubiquitous these days, and this simple, low-cost tool allows you to test the output of chargers and power banks, and also a way to check the rated capacity of a device, or the condition of the battery.
For $20, here's a great USB tester for doing such thing.
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What's the best way to learn to use this?
Plug it into things and see what results you are getting. Testing if ports are functional and if power is flowing is simple -- plug it in and watch the power flowing.
You can also do tests such as seeing the capacity of rechargeable batteries by discharging them fully and recharging them through this (reset the mAh reading to zero first).
The more you play, the more you'll learn.
Another item I recommend adding to your kit is a USB load. There are a lot out there, with fans and such, but I find myself turning to the old-school loads that feature a couple of giant resistors and a switch that changes the load from 1A to 2A.
Note that these loads do get hot, so I recommend using them on a fire-proof surface. Otherwise, you might be better with a fan-cooled load (which also get hot, only less so).