When large currents flow through the battery power cables, a voltage drop results due to resistance in the cables. Since the controller regulates the batteries according to battery voltage, it is important that the controller measure the voltage directly at the battery. The Battery Sense connection allows battery voltage measurement directly at the terminals because the sense lines carry very little current (hence, small-gauge wire) enabling accurate regulation.   Without the sense lines, the controller must use the battery power connections, which may be different than actual battery voltage. The Battery Sense connection is optional and is only needed when significant voltage drops exist on the battery power cables.