This is my first post here so please forgive me if I'm posting in the wrong place or violating any other conventions.
I am working on a project where I want to send a text message alert if the power goes out. I know that the arduino automatically uses wall power if (as long as it is sufficient) and then defaults to USB power next in the order. I found this thread http://forum.arduino.cc/index.php?topic=107860.0 really helpful, but I didn't want to resurrect it since it is quite old.
Unfortunately I don't quite understand the math behind the voltage divider. My question is this: I want to use the circuit provided by user Nick Gammon in the third post however My power supply is 12V 2.5 Amps instead of 12V 0.5Amps. Would I need to change the resistor values to avoid damaging my arduino? I would be happy if someone told me what resistor values I need instead, but I would be even happier if I could also get a more thorough explanation of how this works and how to calculate it in the future. I tried googling "voltage divider" but I didn't quite understand the explanations. Thanks in advance.
Moderator edit: session id removed from link