Alternator AC Voltage
 

Wife's 2010 Hyundai Sonata with 100,515 miles. New battery. When car sits more than 8 hours battery goes Dead. When running the Alternator is charging at 14.4 Volts (DC) and when car is first shut off battery shows 12.95 to 13.05 volts. This morning I hooked up my DVM to do a parasitic voltage test and it show an 11.95 voltage drain with everything off except theft deterrent . I pulled all fuse one at a time and nothing affected voltage drain. I then started the car and hook up my DVM set to AC volts and it show 30 volts AC from alternator. I thought my DVM was bad so I got my old analog voltmeter and it showed the same thing 30 volts AC.
Can the diodes on the alternator leak that badly? If they can, I think that may be the problem with the battery going dead.

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First, doing a parasitic drain test is not with a volt meter rather with an ammeter.
An ammeter is connected in series with the battery with all things turned off.
The reading on the meter will tell you if there is a drain.
Zero amps is ideal. And anything less than 50 milliamps (0.050 amps) is suppose to be good
Less than 25 milliamps (0.025 amps) is preferred.

Where are you reading the 30 volts ac?
Across the alternator or across the battery terminals?
I would look across the battery terminals .. cause that is where it matters.
There should not be that much ac voltage at the battery. Check your scales?
Cause it might mean the internal voltage regulator of the alternator is not functioning correctly.

Okay with everything turned off, one some times must wait a little while.
There are BCM (body control modules) that wake up when one opens the door or trunk.
The may be a few other things not known.
But wait and check the reading/drain.
Then take one fuse out a time and look at the current (amps) reading.

Good luck.

Thank you for your reply. I disconnected the battery and let the car sit overnight to make sure that everything had a chance to cycle off. First thing I did was to hook my DVM in series with the negative terminal (cable off with one lead to terminal and one lead to cable) as I have always done. I set the scale to 10A. It showed 0.30. I also use my old analog multimeter to verify the 0.30 just to make sure the DVM was reading correctly. Amperage did not change at all after pulling fuses. After the amp test I disconnected one of the leads, set the scale to 20 volts DC, reattached the lead and it still showed a voltage draw of 12.25 volts with everything still off and all doors closed except the hood. Not sure how many volts the hood switch draws (anti theft deterrent system).
I did the alternator AC volt reading twice. The first was across the battery terminals I thought it seemed awful high so I shut off the car, attached the +lead directly to alternator post and the -lead to clean body ground. I got the same 30 volt AC read both times. When I tested AC voltage across the battery I used both my DVM set to 200 volts AC and my analog multimeter set to 50 volts AC. Both showed 30 volts.
In case you are wondering the DVM is a Fluke and the analog is old Simpson 260 not some Walmart Specials.

There is no need to disconnect the battery to give everything a chance to cycle off.
There may be things such as the BCM (body control module) that wakes up when a door is opened.
May take a few minutes for it to go back to sleep mode.

0.300 amps or 300 milliamps is way to high a current draw for being off.
It seems it may be your alternator which is faulty.
Some auto parts shops will take them out and bench test them for you.
They want your business.

Anyway, I believe you have a good idea of what is wrong.
Check this video out on how to do a parasitic drain test.
It may help you?


Good Luck