Correct. Read the battery voltage, engine off, should be around 13V if fully
charged. If your alternator is really putting out 20V, you've probably fried
something.

Take the alternator out NOW and get it checked / replaced. Don't even try to
start the car until you do, unless you want to fry more expensive stuff  eg
engine and transmission computers.

Stewart DIBBS
www.pixcl.com/lancerproject.htm

You might see 20 volts if the battery wasn't installed, and the voltage
regulator was toast.

20 volts is definitely bad.  Did you check that voltage at the battery?

What happened, to cause you, to start working on a 4 year old car.

If the car runs with out the instrument cluster, then at least you
didn't toast the ECU.

I'd get that voltage under control first (or make sure your meter is
working correctly)

Once your sure your meter is working, measure the battery voltage (with
the car NOT running), at the battery.  It can never be greater than 14
volts, by the laws of chemistry.  To check your meter, measure a small
flash lite battery, to see if the voltage reading is correct.
Start the car and measure the voltage again.  If it reads greater than
18 volts, stop the car immediately,  You have at least two problems. 1,
is probably a faulty battery, or a not correctly connected battery , 2,
is probably a faulty regulator. 3. a wiring error or fault.

The regulator senses the battery voltage through a separate lead,
attached to the "S" terminal on the alternator.  If this lead is open,
the alternator will output maximum current on a different lead, to
charge the battery.  It still should not put out 20 volts though (unless
it is malfunctioning).

The instrument cluster circuit board needs to be replaced. I've seen a few
of them fail on a Mirage.