Get the charging rate without external hardware (e.g. multimeter.) Android.SE user intuited observes:


  I've noticed that my S4 doesn't always charge at its maximum rate of charge, even when it seemingly should. For example, car DC adapters that claim to provide up to 2.0 amp current only provide 500mA.


They also seems to imply that there exists an app which reveals the rate at which a device is charging.

And TurboFool adds:


  I used to think "USB is USB," and that's true for data, but not for power. Cable quality, and unknown factors, will change what you can pull off. I've had two [USB] cables of the same length produce as much as an 800mA difference from the same charger.


I own a Samsung Galaxy S Relay, which is an Android phone a few years old.  It's sort of similar to the Samsung Galaxy S III, but it also includes a slide-out physical keyboard.  I'm running Android 4.1.2.

How can I determine at what rate my phone is charging?  Preferably using only an app or a terminal emulator. I'd rather not have to use a multimeter.
You can try Ampere (I didn't test it), which should show you the current (dis)charge rate:

 

Quoted from a rating:


  I appreciate that this app displays which of my available charging devices delivers the most milliamps. It's useful because I know which charger to swap out for maximum power. Also, it may indicate if a known charger may be under performing based on available specs.