So a couple of mornings ago, the car started fine. I drove to work, and then on my break, I sat in my car for 30 minutes, 10 of which I had the engine on. 3 hours later, once I finished work, I came back to a dead battery. I got it jumped, and headed home to test it with my multimeter. With the engine running, it was at 14v+, and after leaving it switched off for an hour, it was at 12.78v. It started fine, so I drove to Advance Auto Parts to have them test it, and they said that everything was passing. (they did the full test with switching the car off).
Next morning, the car started fine, and then went to test it on my break at work to make sure it was not dead, and it was again fine. But then 3 hours later it was dead like the previous day. I have no idea what is going on, but I can only assume it is the battery. The starter and alternator seem to be fine, at least from what I have seen. But it doesn't make sense that is the battery because it is fine when left overnight, and it is fine when left at work in the mornings before my break. Could it be a bad battery and the Florida heat is causing it to discharge?
The battery is 1.5 years old and the car is a 2015 Any advise? Thanks!

 

12.78 with car off?

 

I suspect a bad voltage regulator. Here's how I would test.

Measure open-circuit voltage at the battery terminals with the engine off. Should be roughly 12.6 V. Looks like you did this.

Now start the engine and rev to 1500 rpm and measure. You want a solid 2 V gain. Looks like you did this too, although I wonder if you got a full 2 V more, as I don't know what 14+ means. Low 14+ is trouble whereas high 14+ isn't.

Now the third step: Rev to 2000 rpm and turn on every high draw accessory you have: headlights, A/C, defogger, etc. - and measure a third time.

This time you want a solid 0.5 V gain. If you don't get that, it means your voltage regulator can't keep up with increasing current demands. If you've been using the A/C more lately, that would certainly qualify.

 

Anyway, 12.78v with engine off is good as stated. No measurements when battery was supposedly dead. Battery tested good. Maybe the starter is starting to short out internally? It seems to be common on these cars including mine. One time have a problem starting then fine for a while...then problem,,, then fine etc If that's the case it will just keep getting worse, ie more frequent and harder to get started.

 

Well you may have this sorted out by now, but what is your symptom that you "think" the battery is dead? engine will not turn over? probably the all emcompassing starter failure, seems like a dead battery, solenoid clicks but starter does not turn. 12.8V is expected on a fully charged battery at rest, 14v is normal out of an alternator. If it is/was the battery after a start attempt the voltage will crash way down below 12.8v, a single start attempt either with or without the starter turning should have negligible affect on battery voltage, maybe a 0.2V drop each attempt. If a single attempt takes it down below 11V then the battery is gone.