I can't tell if my regulator of alternator is bad. The problem is that there is no charge (about 11 volts) for about the first 25 miles of driving, then the charge jumps up to 14.5 volts. When I shut the car off, it starts all over again. Do I have an alternator problem or a voltage regulator problem?

What year? I would start by checking for loose connections & put your VOM on the alt output stud, brush terminals & VR terminals & see what you have for readings when you have no charge. & VR continuity to ground. part time "open" somewhere.

What year model are you working on?

The most probable causes are:

Stuck/ worn brushes not making contact on the slip rings.

If you have an old "points" (69 and earlier) regulator, the points/ regulator could be corroded/ gummed up inside.

It's possible you have a loose wire connection between the charging lead and the battery, you'll just have to follow the path from the alternator/ firewall connector/ ammeter/ back through the firewall/ to the battery

Doubtful that the ign feed to the regulator is the problem, because that's pretty much connected to the ignition

It's a mongrel. It's a 1980 truck with a street rod wire harness.

I'm using a 1990 Dakota mini alternator with a 1980 solid state regulator.

The output wire goes directly to the battery cable side of a Ford style starter solenoid along with the positive battery cable. I'm getting good readings from the battery post to the alternator post. I've also checked and am getting good readings to the regulator plug from the alternator.

It only started in the past few times I've driven it.

The simple check to isolate it to the regulator or alternator is with the engine running and no output take the field winding that goes to the regulator off the alternator and ground the alternator terminal.

If the regulator is bad you will see a dramatic increase in the battery voltage depending on rpm it could be over 20 so don't let it run like that long. If you don't see a change you need to check a couple of more things on the alternator end to isolate it to the alternator.

I would then check to see that the other field winding has battery voltage. If it doesn't then there is likely a wiring issue/ bad connection and it may not be the alternator. put 12 volts on this terminal and see what the output does.

If none of these gets the system charging the alternator is bad..