I bought a 100-month Kirkland Group 35 auto battery (item # 12865) about 4 years ago. It apparently doesn't hold a charge any longer, but it may be that my alternator (or other part of the charging system) is not charging the battery. I had the battery tested at Sam's Club since Costco told me they won't test auto batteries.
The Sam's test said the battery was 49 months old and the average life in my area was 51 months and that the battery was delivering only 403 CCA's versus its rated capacity of 750 CCA's. So, if the battery is actually defective (rather than just discharged) I may have gotten out of the battery about what should have been expected.
I called my local Costco and asked what the process was for returning the battery. They said to simply take it to the return counter inside the store -- not to the auto department, and they would calculate the proration there. Then I take that receipt to the battery department for a new battery.
Interesting process; they don't even check a returned battery to see if it's actually defective. Obviously the proration schedule is such that they make money when you return a battery -- defective or not, but then I think that's how all battery proration schedules probably work.
I'm trickle charging the battery overnight to see if it will fully charge, and will take it to another place to have it tested -- just to be sure that it is the battery that is not charging properly and is, indeed, why my car is having a hard time starting. It would be really nice if Costco offered that testing service . . . .