Here is a basic BMS/Ah counter (fuel gauge) schematic - done with 5 analog inputs.
-The battbridge mentioned above can be accomplished with the Arduino using an input at half pack
-The weakest cell is monitored for charger cutoff
-Includes warning LED (optional opto to turn on the vehicle's low oil pressure light or whatever as well) and charge LED
-Contrast ON/OFF is what turns the LCD on when the keyswitch is clicked to the first position
-Reset button kills power to the Arduino for a full hard reboot
-Voltage divider on pack inputs is 20:1
-Fuel gauge is for my car - your motorcycle likely does not have a fuel gauge