Call Conan the Electrocutor...
How long does a 50 month battery really last?
TOM: That's one of the great unanswered questions of our time, Jean. It's up there with "what is the meaning of life?" and "how many Corinthians were sacrificed for each crushed-Corinthian Chrysler Cordoba interior?"
RAY: The answer is that nobody really knows. The length of the warranty is simply an estimate based on the strength of the battery. So don't select a battery on the basis of months (you'll probably sell the car before then anyway). Compare the "cold cranking power"--a better measure of the battery's strength. These numbers are usually related--but check! You'll also find that a battery's strength tends to be proportional to it's size and weight; the more plates inside the battery, the more power it has.
TOM: It's also important to remember that you can't start your car with a warranty. Getting back a pro-rated portion of your battery's cost won't make you feel any better after your car left you stranded some frigid morning. If you live in a part of the country where it gets cold (i.e. if you own a pair of long underwear), you should always get the biggest, strongest battery that will fit under your hood.
RAY: You should also stick with brand names with many locations that will honor the warranty. If you buy "Fred and Ethyl's Homemade 36 Month Battery," you may have to return it to Fred and Ethyl's Gas Station and Charm School in East Treetrunk where you bought it in order to get your $10 back.
TOM: Buying a battery is a perfect time to apply one of the famous Click and Clack Great Unyielding Truths: "It's the stingy person who spends the most." The difference in price between Fred and Ethyl's 9 volt and a top-of-the-line "Conan the Electrocutor" battery is usually $20 or $30 bucks. Get the best battery you can, you'll be glad you did.