It's normal for the voltage to drop as accessories are turned on, so if you see 13.5V with the heater fan on full blast and the headlights on, it's still no cause for concern.
This is from a
site concerning replacement of the 87-93 Ford Mustang's 75-amp alternator with the 130-amp one very similar to the one in our Contours:
How to Test Your Alternator and Voltage Regulator
You can check the functioning of the alternator with a voltmeter. Put the voltmeter across the battery terminals. With engine off, fully charged battery is 12.5-12.7 VDC. With engine on, all accessories off, it should be around 14.6 VDC when engine and alternator are cold. As you start turning on accessories (lights, A/C, A/C blower) voltage will drop. With stock 2G, 75A alternator, voltage will drop below 12.7 VDC. With 3G, 130A alternator, voltage will not drop below 13.0 VDC.
NOTE: There are 2 reasons for the drop: 1) as the temperature of the alternator increases, the regulated output voltage decreases. This is due to a negative temperature coefficient of the voltage regulator (approx. -10mv/ºC.). If the alternator has a temperature rise of 100ºC (68ºF to 248ºF), the regulated voltage will decrease by 1.0V, 2) as the alternator load increases, the regulated output voltage will decrease. The 14.6V regulator output is at 0% load. At 6000rpm at 90% load, the voltage will decrease 0.3V. After the alternator reaches operating temperature (which is hot!) and you put a heavy load on it, you will see a voltage ouput of about 13.0V. As long as the alternator output voltage is slightly higher than the full charge of the battery (12.5-12.7V), the alternator (not the battery) will be supplying the current to the vehicle, and therefore, you are neither charging nor discharging the battery.
