I am hoping someone can explain what I should be looking for:

1. 2020 Jeep Cherokee 4x4, 38,000 miles, purchased 6 months ago. I live in the desert of Phoenix with the summer heat.

2. 5 days ago noticed battery seemed a little sluggish when first starting, otherwise no problems.

3 I waited a few days and I took the car to my trusted mechanic. They were busy but did look at the battery.
Discovered negative battery cable loose. Attempted to tighten it but bolt was stripped.They fixed all of it, tested the battery and said you should be fine.
Run it and let I build up again.

4. Did not notice much of a difference. Two days later went to start the car - won't crank, battery drained.
Jump started it and it was very dead requiring almost all of the portable battery that was used to start it.

5. Drove to Autozone. Tested the battery, checked out OK. Checked voltage, checked out OK. Voltage display in car is always 14.

6. Drove the car for errands and at 4 PM I went hiking and finished at about 7PM. Car started fine, no problems.
drove home with the lights on, a/c fan on and the radio. I did notice alternator display was 13.7 and not the 14.

6. Next day went to start the car at about 4 pm ad DEAD BATTERY!

I need some good advice what should I expect the mechanic to be looking for. Car is just out of warranty so I'm praying this is not a $1000 problem!
Could it be a bad battery, alternator or as I'm reading some kind of electrical problem draining the battery, or something as simple as a fuse?

Thank you in advance.

 

To find out if it is a battery or a draw killing it , you my need to disconnect the battery and if it stops going dead then you have a draw and will need to track it down .

 

Do you know how old your batt is?
In case you go draw hunting:

 

Thank you! Had it tested, battery was only cranking 300 vs 750, so did replace battery (stock battery 2020 but 38000 miles so out of warranty!) seems to be running better but start/stop still says battery charging. Will check the info on battery draw.

 

Thank you! New battery and it is working good, will see if start/stop works again, and will check into battery draw.

 

Did you ever get a resolution to this? Mine is doing the exact same thing... I am on my fourth battery.. the dealership just had it for 10 days and claim voltage was fine... the car should always run at 14v and is now running at 12.8 and start/stop system not working.. voltage increases to high 13's while coasting and braking then returns to 12.8... and the voltage is slowly dropping on the battery but they claimed the alternator was good

 

Things are not right there

Step 1) Measure voltage at battery terminal without engine running. It should never be below 12.3V

2) with engine running, at battery terminals, measure voltage. It should be above 13V, preferably closer to 14V

3) Measure voltage at alternator terminal, it should be same as above

If any of the above is not true, you have a failing alternator, wiring, or voltage regulation system (PCM controls this)

4) Measure parasitic current draw by disconnecting battery terminal, measure between battery post and terminal, current should be less than 50mA, preferably <30mA

If not you have parasitic current draw flattening your battery

Finally, you may have a wiring problem in your large wires. There is a way to test for voltage drop on these, check Youtube, its a bit hard to explain

 

The car continues to work fine after the new battery. I have not had any problems. Yes, the voltage seems low but from what I understand this is normal for this type of battery. I am concerned that your start/stop is not working. Are you under warranty with the dealership? The start/stop should be working.