Here is an example of a differential multiplexer that can accept up to a 72V input. You'd need one for every 4 cells assuming that your battery voltage is within that limit. The output of the mux would go to an instrumentation amplifier that can also take a 72V input. This amp will take the difference between the two inputs, which will be the cell voltage, and safely feed that to the arduino input.
Not exactly a beginner project, but it is definitely doable if you take care and learn along the way.