I'm wondering is it possible to read the voltage across a 12v car battery? I don't know much about electricity but it seems like the current would be too great for an Arduino. I want to see if the altenator is running so I can trigger a relay to charge a separate deep cycle battery.
Yes, just spend a minute on Google on it 
Basically you need a voltage divider (and some filtering).
Thanks and it wont be too much current and fry the Arduino like I said I have no working knowledge of electricity just code.
An input will not draw current if you don't go over it's voltage spec (aka, stay under Vcc which is 5V on most normal Arduino's). And that's where the voltage divider is for.
But like I said, Google it. I think more then 1000 people where before you measuring (car) batteries.
A bit of reading up on ohms law might help too.
A 27k/10k voltage divider will allow you to measure upto 18.5V, with good safety margin beyond that
due to the 29k resistor limiting any possible current.