14 cells with a total of about 50volt suggests a 14S LiPo stack.
You must already have a BMS (battery management system) in place to charge/equalise them.
Some of them already have some sort of cell voltage measuring output.
Like this one, with bluetooth out.
There are special (expensive/rare) chips available for cell/stack measurements.
A DIY solution could be a bunch of relays and 'flying caps', driven by TPIC shift registers.
A DPDT connects a small capacitor to a cell, most of the time.
And disconnects the whole cap from the cell, to connect it to the A/D for a fraction of a second.
The cap is returned to the cell after it's voltage has been measured.
This could take less than 20ms per cell when small relays are used.