Alternator stops charging running AC for 30 minutes
 

I just bought a nice 2016 370z roadster. When I bought it the the guy said I don't know if the AC works I never tried it. The next day I was driving and turned on the AC and all is fine for a while. Then it happened. It quit charging and battery went dead. From 14v to 12v after running AC 30 minutes or so. Got new Battery. It's like something is overheating the alternator. Too much draw from the AC?

it is the alternator if it is not charging.

It charges fine till I start running the AC. Then after 30 minutes or so the meter on dash goes from 14 to 12. Then after the car cools down it is 14 again. Could the AC draw too much? Thanks

Red Flag: I've never used the AC.

Location: Florida; "the guy said I don't know if the AC works I never tried it."
That's not a red flag thats alarm bells and flashing red lights.

Unfortunately there really isn't any lemon law that covers used cars private party.

Seems like it could be some sort of short circuit with the HVAC blower circuitry. Find out which circuit is pulling too many amps when the AC is running is the start point then check that circuits wiring. These kinda problems are a pain to trace down and that's probably why he was selling it.

1.Did you check your Accessory belt tension?

2.The condition of the belt itself?

and most important to both 1 and 2 above: Is your tensioner working? (ie is it within range marks stamped on tensioner)

It's the easiest thing to check, that is common to both AC clutch and Alternator.

DRIVE BELT : Checking INFOID:0000000011736876
WARNING:
Be sure to perform this step when engine is stopped.
• Check that the indicator (C) (notch on fixed side) of drive belt auto-tensioner is within the possible use range
(A).
NOTE:
• Check the drive belt auto-tensioner indication when the engine is cold.
• When new drive belt is installed, the indicator (notch on fixed side) should be within the range (B) in the fig-
ure.
• Visually check the entire drive belt for wear, damage or crack.
• If the indicator (notch on fixed side) is out of the possible use range or belt is damaged, replace drive belt.
DRIVE BELT : Tension Adjustment

other than that, looking at FSM, your IPDM and compressor clutch can be at fault. See if you have a code (that would be real handy)
Next easy thing to check and see if your AC clutch and pulley are working and not siezed.

Thanks all, The car has 16k miles but I will check the belt and all that. The AC works great. No noise,ice cold,30 minutes later Dash volt meter drops to 12v which means I better be close to home. Something is overheating. It's damn hot here in Florida right now. I'm going to pull the alternator and take it to a local shop that specializes in these. Thanks for all the input. Appreciate it. Also going to Napa for a code read.

Just because your meter is showing low voltage or dropping off suddenly, it doesn't mean you have a charging problem. It could be the gauge or where it's getting information from. The triple meter gets info from combination meter. Check those connectors and wiring under the dash. The voltmeter also requires power form ignition switch.

Before you pull out the alternator, (napa can do this or with a simple multimeter) make sure you are actually charging, or not, by checking on alternator output directly.

Checking charging directly on battery might give you false readings as the ECM controls when to charge the battery.

If it fluctuates between 14.4-14.6 volts and then drops to battery voltage, say 12.6-12.8, then its working.