After your last post I am not so sure I'm right. Try two things. First with the engine running take a voltage reading between the alternator case and the body. This will check how good the ground is. Second with the engine running take a voltage reading between the output stud of the alternator and the battery post on the starter relay (I'm assuming that's where you terminated your remote battery lead). This will check for voltage drop on the positive side. Problem may be on the positive side and not the negative (ground) side.