The beta version with standard 5% resistors, in final version use 1% or better.
I use 15K resistor inseries with the optocoupler LED and conect this direkt on the batteri (red and black wire).
The 3 pins behind the cny 17 runs trouh a double shielded cable to to EVduino-MS.
Between +5 volt and the cny17 opto's collector there is a pull up resistor 20K 1%.
The voltage of a lead acid battery are:
12,8 volt: fully charged
10,5 volt fully discharged
Other solutions for measuring battery voltage with arduino:
1 wire technology, DS2438 messure battery voltage AND battery temperature, great for LiFePo4 batteries in the EV http://www.maxim-ic.com/datasheet/index.mvp/id/2919
Also i will try to test a linear opto circuit found online before i deside how the final version shoulld be.