I believe you will be looking for an animal called an isolation amplifier. You will have to google it. Analog devices has some good ones.
Another method is to take the voltages at a series of points, all referencing a common ground, within your battery string and use a resistor network voltage divider to put them in the range you can handle.
The voltage of the sub-string closest ground is itself. The next sub-string is itself minus the first sub-string's. The next is that voltage minus the sum of the previous strings', etc.
The problem you have to get around is that all the voltages have to be relative to a common ground -- this is what the isolation amplifier can fix -- and as you work your way up the battery chain the whole voltage to common ground is quite large -- and decidedly unsafe. So the accuracy will get dodgy as you work your way up the string. Perhaps you can work in strings of zener diodes to lop off a chunks of voltage offset so that your divider will give you some more accuracy to work with.