We've discussed in the past.
My idea was to have a string of relays and a battery powered arduino acting as a voltmeter/relay controller. One relay would close at a time connecting the +/- of each battery to the arduino ADC input, thus the arduino only sees one battery voltage at a time, and the arduino "gnd" floats and is the same level as the battery -.
The relay control circuit can be a series of TOIC6B595 shift register that is easily driven and can sink decent current.
Altho, the relays can likely be simple 5V telecon relays and likely driven from arduino outputs directly.
For example, here's 15 being controlled by a '1284
Those weren't DPDT. These are, and need just 11mA at 3.83v to turn on.http://www.digikey.com/product-detail/en/IM22TS/PB1168-ND/1828440