big volts

Find where the 5 V drop from ALT stud to BATT+ occurs. Should be <0.5 V. First, measure across the battery post, i.e. from post to cable clamp. Corrosion there is common and needs a yearly cleaning in MO. If across the bulkhead connector (2 terminals: in & out), then your bulkhead will soon melt since P=(5V)*(40A) = 200 W = hot! That is also common.