To begin, five volts is five volts. That’s an absolute measurement.
But there is more to it than that, as you might imagine.
Let’s look at it in more detail.
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More than voltage
The issue here is that there’s more to charging than the voltage. In fact, it’s the USB specification that actually might get in the way.
The other factor at play here is current. Typically, this is labeled on the charger as some form of amps or milliamps. Current is the amount of electricity, not the voltage, but the amount of electricity that the charger or device is able to provide.
Now, the USB specification actually calls out that a USB 2.0 port on your computer must be able to provide 500 milliamps, or ½ of an amp, of current at five volts. In fact, it apparently specifies that it can only provide 500 milliamps. It’s not allowed to go above that.
For some devices that require more power, you may have seen those funny USB cables that have two USB plugs at the computer end. Each of those plugs can then take 500 milliamps of power at five volts for a total of one amp.
Chargers play by their own rules
USB chargers may or may not follow that same restriction. Many do not, and often offer the ability to provider 1 amp, 1.5 amps or even more.
I believe that hubs technically do need to follow the limit. They’re supposed to only provide 500 milliamps per port on the hub. If a device requires more than 500 milliamps to charge and the charger or the port is providing only 500 milliamps, then it won’t work.
And yes, it can be detected by the device. That’s probably where you’re seeing the error message.
Sometimes only speed is affected
Often, devices charge slowly (if at all) on a 500-milliamp source and more rapidly on a more powerful source. The chargers that come with my phone (these are “wall warts” – the kind that plug directly into a wall outlet) are usually capable of putting out 1.5 amps at five volts. That’s three times this USB minimum of 500 milliamps.
My phone charges relatively quickly when plugged into one of those. When I have it plugged into my computer’s 500 milliamp USB port, my phone still charges, but more slowly.
Output amperage is all part of the (often very) small print on the USB charging devices. That’s something you want to pay close attention to. That’s probably the difference that you’re finding between all of these different chargers and USB ports that you’re using to plug in your different USB charged appliances.