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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?

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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...

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  • If the battery is bad and the alternator is healthy. Maybe The multimeter will show lower the voltage again? – Armin Jul 17 '19 at 20:23
  • My OBDII tester (INNOVA 3160d) has the vehicle voltage on the front screen. Used it just earlier this week to diagnose a bad battery in a car. Said it was at 7.44vdc with the ignition on, engine stopped. Once we jump started it and was running, the alternator read 13.6vdc ... told them to replace the battery as we knew the alternator was charging just fine. Fun times. – Pᴀᴜʟsᴛᴇʀ2 Jul 18 '19 at 17:32
  • @Pᴀᴜʟsᴛᴇʀ2 good job then. And must improve my typing skills... like that’s going to happen... – Solar Mike Jul 18 '19 at 17:36
  • No worries. We're all in this together ... and I'd suggest your typing skills are probably better than mine ... might just be my proofreading skills are working good today, lol. – Pᴀᴜʟsᴛᴇʀ2 Jul 18 '19 at 17:45

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