I am far from a solar
guru but I have read repeated mentions over on the NW Arizona solar forum that the quality dedicated battery
monitor are much more accurate than the "integrated" monitoring functions in most charge controllers.
As I understand it, the shunt that is used with the BM allows much more detailed monitoring of current
flow in and out of the battery.
That makes a certain sense to me. The charge controller cannot tell if that current
flow is going back into the battery or to power a load. The BM can.