The voltage of my car battery is not fixed when AC going to active. In fact, I saw that the voltage dropped when the compressor clutch turns on. I suspect alternator and battery. Now I want to test the alternator of my car. I have an ODB2. How can I do it?
The voltage can and does vary... you need a multimeter as a minimum.
With the engine at about 2000rpm you should see somewhere between 13.7 and 14.8 (can be up to 15.3) volts depending on the type of alternator. However, if the battery is low on charge you may be around the lower end of the voltage.
If you want to test the current then you need to be careful, as either you need a contactless current meter or you need to make a proper connection between battery and alternator.
The meter used must have a range greater than the output of the alternator or use a shunt with a multimeter. Most alternators now can produce 80A or more...