Bad alternator or battery

Truck info: 2019 5.7L V8 original battery. 76k on odometer.
I have never had trouble with my truck starting ever. Today I turned it off to throw a couch in the back and went to leave and it wouldn't start. Nothing. No clicks even. Just a battery light flicker. Used my Noco jumper and started it. Had the same thing happen three times in a row. Checked volts while truck is running and I get 13.4. Turn off the truck and check it and the volts are 9. Vip said a bad voltage regulator on the alternator, but that was with alot of corrosion on the posts. Cleaned them, but still the same results voltage and starting wise. Truck won't start even if I just turned it off for a second. Slight fluctuations occasionally on voltage reg on dash board while running. Thoughts?

 

My bet is the battery is shot.

 

I concur, battery is not holding a charge. Alt output is a bit low but as long as it puts out 12.6+ volts, it is charging the battery.

 

Battery. Easy check is to read current across batt terminals when running. If it slowly falls it is your alternator. OEM batteries are notorious for failing with no warning. I just swapped my 2019's battery due to perceived sluggishness at startup.

 

I’d find a battery with more than 9v before doing anything else.

 

If you get 4 years out of a battery these days your doing good. Most probably the battery. Take it for a free test to check CCA, dead cell, etc. Also typically alternator is putting out over 14 volts when checking across the battery.

 

Yeah battery, modern vehicles are power hogs and Toyotas are right up there, wife’s Camery, and a Lexus she had needed batteries every 3 years almost like clock work, and usually gave no warning when they went out

 

My Tundra goes through a battery almost every 2 years. Don’t know what’s wrong but the warranty covered me this time so it comes out a two for one deal. I’ll see if they cover the next one.

 

My 2019 sat in a driveway for about 3 weeks waiting for repair and when I turned the key into pos II without starting the engine I saw voltage gauge at 9V. Measured voltage across battery terminals, it was 6.9! Attempted to charge the battery, it was cranking at 25 Amps and in a few minutes stopped charging, like it is full. So it is a battery sulfation. Most of new/better charger can run desulfation, that may help.