This uses a single board per cell, each board measures the cell voltage, has a 1/2A shunt that can be turned on or off and a thermistor to monitor the cell temperature. The modules communicate in a daisy-chain using an optocoupler with a 2-wire link. Parts cost per board under $5.
There is a master control unit that handles all the comms, raises alarms if a cell is too low or too high in voltage, switches shunts on or off if balancing is being done, displays cell voltages (highest and lowest), displays cell temperatures, has outputs that can cut the charger back to 1/2A or turn it off entirely.
The cell modules are done and I am working on the master with the intention of modifying the Cougar LCD unit to handle these functions. It has more than enough spare capacity and also a nice LCD. I can get enough i/o pins to make it happen by deleting some of the existing functions, such as the "mode" switch, the CAN bus, one of the LEDs and so on.
My first iteration is a stand alone micro/LCD though, with the merging with Freyguy's design a bit later.
Here's a cell module circuit diagram:
