Measure 12V 35Ah battery level in arduino
If all you want to do is simply measure the battery voltage, then you just need to scale the voltage down with a simple resistive divider, then feed it into your ADC. You can add a zener to ground, or schottky to the 5V rail for input protection if you wish (assuming your Arduino runs from 5V, which I believe they do). 

Basically it would look something like this:



Simulation:



Note the resistor values are not set in stone, they just need to be the correct ratio of 7:5. So you could pick 7k&Omega; and 5k&Omega; if you like, or similar. One thing to watch out for is the maximum input impedance your Arduino ADC can handle, which will be given in the datasheet - some are quite low (e.g. 10k&Omega;)
The maximum output impedance of your divider is the parallel resistance of both resistors, so with e.g. 10k&Omega; + 10k&amp;omega the output impedance (at the divider centre) will be 0.25 * 10k&Omega; = 2.5k&Omega;.
With 7k&Omega; and 5k&Omega; the output resistance will be 1 / (1/7&Omega; + 1/5k&Omega;) = ~2.9k&Omega; (with the 70k&Omega; and 5k&Omega; it will be ~29k&Omega;)