You should have your car's battery tested by a repair shop as part of an annual safety inspection, particularly if it's more than three years old. The test takes two minutes and should cost about $15. Replace a battery that fails.
When shopping for a new battery, start by checking with your car's owner's manual (or a battery retailer) to identify the group size that will properly fit your vehicle. The wrong size battery might not provide enough current.
Also consider the performance you'll need in a battery. One indicator is cold cranking amps, or CCA. This is the main gauge of how the battery will perform when starting and is a measure of battery current available at 0 F.
You should use the manufacturers' claimed CCA as a rough guide to actual performance. We found in our own tests of 35 batteries in four group sizes that only a small minority of batteries met manufacturers' claims, although some came close.
Other considerations when shopping for a replacement battery:
Buy the newest battery available, as determined by the shipping-date code stamped into its case or on its side label. The code usually includes a letter for the month (A for January, B for February, and so on) and a number for the year (4 for 2004). Look for a date no more than six months old, since car batteries tend to degrade while sitting on the shelf.
Be aware that battery warranties involve two numbers - 2 4/8 4, for example - that signify the free-replacement period and the total warranty coverage period, in months. The first number is more important because coverage after the free period is prorated and reimbursement drops significantly.
Our tests of batteries focused on cold-cranking-amp performance, as well as reserve capacity (our measure of how long a battery can supply power if your car's charging system fails or you leave your lights on) and life-test performance (a battery's ability to endure repeated charge and discharge cycles at engine-compartment temperatures). All our test batteries will start and run the vehicles they fit. The best are likely to last the longest.
Group 65. These fit most large Ford, Lincoln and Mercury cars, pickups and SUVs. The top-scoring ProStart Premium 65-750 ($60, at Pep Boys) is a very good choice for cold or warm climates, and scored high in reserve capacity. Its low price qualifies it as a CR Best Buy.
Group 75. The group size for Daimler Chrysler midsized cars, recent General Motors subcompact and compact cars, and most pre-1995 GM vehicles. Top scoring in this group was the Interstate Mega-Tron MT-75 ($75), although the very good Advance Autocraft Silver 75-2 ($55, from Advance Auto) is a CR Best Buy.
Group 24 (Similar: 24F). This fits post-1998 Honda Accords, all Honda Odysseys, 1992-97 Nissan pickups, Acura CLs and TL, and most Toyota Siennas. The top-scoring Advance Autocraft Silver 24-5 ($50, from Advance Auto) is a very good performer in cold or warm climates, with high reserve capacity and excellent marks for life-test performance.
Group 3 4/7 8. Group 34 fits many large Daimler Chrysler vehicles; Group 78 fits most 1996-2000 GM midsized and large sedans, and most GM trucks and SUVs. An excellent all-around choice, the Advance Autocraft Titanium 3 4/7 8-4 ($75, from Advance Auto) is also a CR Best Buy. If reserve power matters most to you, also consider the very good DieHard SUV, Truck and Van 39890 ($90 from Sears, but available only in the North).
Northern batteries, such as the DieHard 39890, emphasize CCA and typically contain more conductive plates than a southern battery (such as the very good DieHard 39990 in group size 3 4/7 8), which offer added heat resistance.
By the editors of Consumer Reports at www.consumerreports.org.