I made a solardatalogger for few years ago (will publish code latter to this solardatalogger).
In my solardatalogger i used arduino with a ethershield to mesure battery bank voltage with analog input.
The codesnip below is used to integrate in you own arduino sketch, enjoy ;)
Arduino as Voltmeter
The routine showed in the codesnip collect data from analog pin 5, at 5 volts contains whole 1023, the divide by 2 and converted into 4 digits. The 4 digits plus a decimal point is stored in a string of char with excitement: 05.00 ready to send serial with usb or via ethernet.
A better way to scale the analog input is with the MAP command:
map(value, fromLow, fromHigh, toLow, toHigh)
This command is very flexible. value is converted from 0 to 1023 to 0 to 20000
Then we have a good solid scale, and need only to divide by 1000 to place the decimal point.
In my own solardatalogger project data is sent to a mysql database and it is easy to handle calculations in php instead of the Arduino.
In my EVduino-MS project the voltage value will be converted into graphic bars on the LCD touch display