Why does my battery keep dying? (I dont think it's the alternator)

[MrPaco]

Bought a new battery on January '19 (Duralast from Autozone), worked fine until June '20 then died all of the sudden. Took it to Autozone to get tested, the test said "bad battery", got a new one covered by the warranty.
That one worked fine until late March '21 when it also died all of the sudden, went back to Autozone and same thing: "bad battery" and got another one under warranty.
That one worked fine until today, also dead out of nowhere. Went to Autozone, and you guessed it: "bad battery". This time they also tested the alternator to make sure it wasnt that, the test on the alternator said "working fine". So they gave me another new battery, but said that this was the last one, the limited warranty has a limit of three replacements I guess.

What I'd like to know if it's just bad luck that I got three bad batteries in a row (or maybe Duralast is crap in general?), or if maybe there's something going on with the truck's electrical system that is damaging the batteries. I wouldn't be surprised if these are just crappy batteries (why else would the manufacturer limit the number of times they would replace them if they are meant to last past the warranty period), but since the last one only lasted a couple weeks I'm thinking that maybe there's something else going on.

Just to clarify- the batteries are not just dead or uncharged, the store's battery tester is diagnosing it as a BAD battery, not UNCHARGED. Also, other than the battery dying out of nowhere there's nothing unusual about the truck's electrical system's behavior, when the engine is running the voltmeter on the dash hovers on the 14 amp mark, suggesting that the alternator is working fine (I think).

Thanks

[camoit]

First is to check voltage at the back of the alternator 14.2 to 14.4 max.

And as for the battery. Here is AutoZone making new ones.

[xboxrox]

Marty ~ I needed that guy to install my new rebuilt steering gearbox, no doubt he could make it fit Now I know why those Redtop batteries are so expensive coz of all the rubber gloves they use in their handmade operations..!