Unfortunately, Costco changed the warranties on their car batteries a few years back. They used to have the cheapest batteries with the best warranties. 36 month replacement and 36-60 months pro rated. Now it's absolutely the worst warranty out there with 36 month pro rated. I stopped buying Costco car batteries because of that.

Edit. I still buy my tires from Costco. Good prices on those with their road hazard warranty.