A project I'm working on I need to measure the charge of a 9V battery. Essentially it's a voltmeter.
Using the analogue inputs on a Uno (the end project will just be a stand alone ATmega328), I need to scale back the 9V to under 5V. I was told to use a voltage divider as per this site: http://www.raltron.com/cust/tools/voltage_divider.asp
I used the values 6.8 and 4.7 ohm for the resistor. According to the calculator on the website this would scale back 12V to 4.9V hence allowing for a way overcharged battery to be connected without damaging the input pins.
However, upon connecting the battery to the resistors (arduino not connected), the 6.8ohm resistor started to smoke and burn. I'm at work right now and don't want to set off the fire alarms (I'm right next to our major data centre!).
Can anyone advise what I have done wrong? Or what I should be doing?