For vehicles with two batteries, these devices electrically isolate the batteries to prevent the battery with the higher charge from draining into that with the lower charge. In addition, both batteries can be charged simultaneously from one source. The amp rating of each item is the maximum alternator rating. Most alternators on new vehicles have an integral electronic voltage regulator that requires the use of the 4-stud battery isolator. The small 4th stud is for connection to a circuit switched by the ignition switch. The newer alternators were first introduced by Delco and are therefore sometimes called 'Delcotron-type alternators'. A 4-stud battery isolator can be used with older pattern alternators (in this case the 4th stud will remain unconnected), but a 3-stud battery isolator cannot be used with the Delcotron-type alternator. Per Leg Ratings Battery isolators have a 'per leg' rating which indicates the maximum current it can deliver to either battery. All Littelfuse Cole Hersee brand battery isolators have a per leg rating equal to half the alternator rating. For example, the 48051 with a rating of 75A has a per leg rating of 37.5A per leg.