Ok, if you want to know the voltages, you'll have to either use the 4051 multiplexer or use an Arduino Mega or compatible (which has 16 analog inputs.. might be easier) Example:http://goo.gl/cOe3B
So your voltages will all be referenced to the Arduino/Battery ground. You need to scale the 30 volts down to 0..5 volts (or less) with resistor voltage dividers. Since you'll handle this in software, the ratio does not have to be anything special. Something like 8 to 1 would be good and safe. You need a separate voltage divider of 2 resistors for the "top" of each cell.
You want fairly high value resistors so the current drain is not a problem. If you used a 470K resistor on the top and a 68K resistor on the bottom of each voltage divider, the ratio would be 0.145 so the 30 volt "top" would give you about 4.3 volts into the Arduino analog input. So the actual voltage reading done in software would be like 4.3/.145 = 29.7 volts etc.
You would read the "top" of every cell, and calculate each cell's voltage.
You can get 1% tolerance resistors cheap now. Here are 20
...( Ooops.. Short Break for an Earthquake here in Italy )... ok.. Back from the street to the house..
Like I said, 20 resistors each of 20 values of 1% resistors for $3.75 here (and many other suppliers): http://goo.gl/EW055
For your purposes, even 5% resistors would give you 10% accuracy worst case..
Let us know how it goes and what you end up doing.. others need to do this too..
DISCLAIMER: Mentioned stuff from my own shop...