The main problem with measuring 2 volt cells with a common ground is each cell will rise 2 volts on average so that causes a major problem using ADC on a pic. I have been talking with a mate over this issue and we came up will a possible solution that uses a RFpic say a RF12f675. Now by doing it this way each pic can measure 2 cells and have a IR sensor to be signalled when a packet needs to be sent. Each pic can be powered by 2 cells so power won't be a problem.
This is one complicated idea but if any guy's have any easier way chime in as this is one project I want to do.
Any modern PIC of say 18 pins is going to have 12 adc (e.g. 16f1827) and up from there to 30 channels on a 40 pin device (e.g. 18f46k22). Hook them up, and just subtract the previous reading from the next one up the ladder of 2V battery cells.
EDIT: Of course you would have to use a voltage divider to cover the range of the overall battery pack