Measuring the state of charge of lead acid batteries by an instantaneous voltage measurement is probably not going to work very well. See https://www.google.com/search?q=battery ... 8&oe=utf-8
The temperature of the battery can have as big an effect as the charge. Measuring the specific gravity with a hydrometer is probably better.
But what also works and is within the realm of what could be done with a Pi is to monitor the watt hours or amp hours in and out. Set up an A/D converter of some sort (I think I've seen USB ones) so you can monitor current, positive is charging and negative is discharging. Do it often, maybe a few times a second. Multiply the time spans by the current readings. Think of them as amp-seconds or whatever, but you should be able to get as many out as you put in. There will be some loss, you won't get back 100% of what you put in. Also most batteries will last longer if you avoid pulling them down low, at least often. Most car batteries will only stand being pulled down to 0 volts about 3 times in their life of several years. Try to stay above 50% charge, they should have an amp hour rating.
Ideally you'd want to have a thread on a computer monitoring fulltime and always have a number like a bank balance that's the result of monitoring charging and discharging. You'll probably be able to convert volts to digital, you'll need a series or shunt resistor to change amps to volts. That can be something like a paper clip but it has to be stable, if it rusts after being overheated that will change the reading. A piece of nichrome wire from a toaster or electric clothes dryer might be a good start. Remember Ohm's law and you'll probably want to pick up a cheap DMM for calibrating. Not to mention a package of alligator clip leads for temporary connections.