I have a 92 caravan doing the same thing, high charge, new alternator . Every time I scan it with a drb it gives a target voltage and it is exactly where it should be, like 14.5 on a 60 degree day. I have tried other batteries, cleaned underhood grounds, the gauge reads just below 18 volts, it is not boiling water out of batteries and drove it like this for 2 years. It must be the pcm but why does the DRB scan tool say it is perfect?