Hey all confusing situation here.
2007 Lexus GS 350.
Battery year 2017.
Stock alternator.
Put in a aftermarket subwoofer and 1000 watt rms amp with 2 fared capacitor running off stock deck. I'm running the amp at 4ohms so its only putting out 500 watts. (I never play the stereo with car off) A few days later my dash battery light came on and my steering tightened up. Drove across the street to auto zone and they said battery was dead and they couldn't check my alt with a dead battery. I unplugged the capacitor and unplugged my amp. Also replaced battery and they checked alt and said the alt was bad. (Voltage Regulator Failed). Took to 3 different auto zones to have them check the alt and one O'Rileys and all said bad voltage regulator. So I bought a Denso Alt and went to a shop to have them replace as I replaced one on my 2006 IS 350 and it was a PAIN IN THE A$$. They called me back a few hrs later and said that they checked the stock alt while in the car before replacing and the Alt is good, no need to replace. I said are you sure and they said yes, no need to replace. So I picked the car up and went to another Auto Zone to have them check and they said it FAILED. (Voltage Reg) So I took the car home and checked with a volt meter and below are my readings.
Car off battery 12.72
Car on battery 13.36
Car on (at idle)with AC, stock stereo and lights on 12.45.
Car on (engine at 2k RPM) with AC, stock stereo and lights on 13.40.

Regarding my above readings, The car under load at idle dips and stays at 12.45. This to me is low.
Now the same load at 2k RPM jumps up to 13.40 and holds. This to me is almost acceptable. Its almost as if the alternator is doin its job while the rpms are up but not doing its job at idle?
Suggestions?
Alt good or bad?

 

Sounds like your voltage is to low while running, I would replace the alternator, but before that I would clean and inspect the battery terminals.

Call back the mechanic and tell him, when you checked the voltage that you were getting 12.45 at idle

 

New battery, clean terminals.
No corrosion.
Gonna check again when I get out of work today and see what the readings are then call him back if they are not higher.