How can I tell if my 12v battery is losing power, need replacement?

As with many things, condition and warranty depend on where you purchase -- authorized dealer or gray market/outlet.

 

that might be correct Patrick. I noticed not too long ago the Irvine Costco no longer sells Optima, & Costco insists their vendors maintain a certain level of quality.
.

 

FWIW I'm at least 6 years into my Optima and no issues, even with the Phoenix heat and it's reputation for killing lead acid batteries. Can't speak to their recent quality, but generally they have an excellent reputation. Vendors hawking old stale batteries can and does happen with any brand. You should be able to get a date or lot code from a reputable vendor to see how fresh they are.

 

In your signature you mention Optima Battery Kit for your '05. Does that mean that you changed the cable ends to fit a larger post? Or to phrase it a different way, does the Optima battery you have now have larger posts than the original battery you replaced?