It is fine to leave the CB outputs un-connected or open.
It is not recommended to tie them directly to ground or Vdd in case the CB outputs get activated accidentally in software.
it would be interesting to see what the idd consumption of the AD7280A is in the condition you mention.
If the cells are discharging through the device then you should see an equivalent current.
Can you put the device into normal mode / sleep mode and measure the idd ?
In normal Master mode the idd should be 7.3mA with 5uA in the powerdown state.
thanks,
Mike