It's really annoying having a starting battery with a bad (shorted internal) cell and you know more about the batteries than the person doing the testing and making the warranty replacement decision.
Battery is fully charged and tested fine. A battery with a bad cell will do that. But let it sit a few hours and it's dead. It's more rare with AGM batteries (although those crappy Optima spiral AGMs it's pretty common), but it happens. Had this starting battery only 16 months. The last couple of weeks it's been balking at starting the van. It's like, "I dunno. Well, OK, grumble, grumble, grumble, I'll start it." But the last 3 days, nope. Had to jump it.
"Oh, your alternator isn't charging the battery."
"Oh, but it is. It's fully charged. See? It didn't get fully charged by itself."
<tests the alternator>
"Alternator is fine. Must be the starter."
"It starts fine with a fully charged battery. See? Probably not the starter."
"Well it can't be the battery because the battery tests OK. You have a parasitic drain."
<connected a clamp amp meter, no parasitic drain - like I hadn't done that already>
So, I disconnect the battery from the negative cable, eliminating the vehicle from the equation, let it sit all night, and it's dead.
So, I go to a AAA place that specializes in batteries and they put it on their super duper battery diagnostic tool. Cell 3 is bad.
Go back to the Zone. Told ya. And they replace the battery.
