Are all USB chargers compatible?

Leaving aside the difference between USB, mini-USB, and micro-USB, can all such chargers be used interchangeably. Specifically, I have a charger that plugs into the wall and terminates in a mini-USB plug. Is it safe to use in any gadget that has a matching socket?

Generally, yes.
Although, it may not charge “big” devices like an iPad very fast.

Some have more output than others. A low output charger may charge some devices very slowly or not at all.

Note that not all things that claim to be USB actually follow all of the standards. My last phone had what was to all appearances a USB charger, but would throw up an error message if you tried to charge it with anything but the same-branded proprietary charger. No damage; it just refused to accept it.

No. My computer USB output won’t charge my Kindle Fire, but works fine with my phone.

It should, just not as fast as with a regular charger. The Kindle may complain that it’s connected to a low power charger, but it should eventually charge.

We sorta just did this.

Maybe, but the little charger icon on the Kindle doesn’t react. Come to think of it, it’s the same with my iPod, an old 5th gen one with a hard drive rather than flash memory. If it’s charging, it doesn’t show.

Someone told me in another thread that some newer USB ports have lower power outputs than the earlier ones.

the USB 1.x/2.0 standard only says that ports are required to support up to 500 mA of current. tablets might need 2-4x that to charge the piles of battery they contain. though my iPad will say “Not Charging” when connected to a PC, it will in fact charge very slowly so long as the screen remains off.

USB 3.0 can supply up to 900 mA, and the newer revisions with the Type C connector increase that further.

What is considered “low output”? Which value is relevant? Define “slowly” on a relative scale. Mine say 5v and 1.5 or 2.0A. Takes a couple hours to charge a tablet.