USB chargers normally are limited to around 5 volts unless they are a fast charger and those will run up around 9 volts. I’ve bought USB volt / amp meters for less than $2 that work fine. Those cheaper ones toggle the display back and forth every few seconds between Amps and Volts. A couple bucks more will get you one that shows both at the same time. Laptop power supplies for charging a laptop are not USB and are not relative to the USB meters that will be used between a USB charger and a phone or watch. Now USB-C is a bit different and can go up to around 20 volts in some cases but I doubt you will see use for that in any watch or most cell phones.
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