A decade ago, a battery's free-replacement period was as little as 3 months, and the prorated period (which allows only partial reimbursement) might have been just 50 months. Today, a 36-month free-replacement period is typical, and the prorated period can stretch to 108 months.
But even with today's longer warranties, it's still important to choose a battery with the longest free-replacement period you can get.
A battery warranty code of 24/84, for instance, indicates a free-replacement period of 24 months and a prorated warranty of 84 months. But the amount you'll be reimbursed usually drops off pretty quickly once you're into the prorated period.
For example, the Duralast Gold 34DT-DLG carries a 96-month prorated battery warranty. Using the formula listed on the manufacturer's Web site, this means that after four years, or half the warranty period, you would be credited with $40 of the $80 original price if the battery failed. After five years, you would get back $30.
Signs of neglect, such as low-water levels and improper installation, can void a warranty. So can use in heavy-duty applications such as high-end car audio and marine use if the battery is not recommended for them.