When replacing your car's battery, obtain the size of battery that your owner's manual indicates is compatible with the car. If you no longer have the owner's manual, use an in-store or online fit guide to make sure that you buy the right size for your make and model.
Another way to make sure that you get the right battery size is to take the old one out and measure it. In addition to size, other factors to consider include life-testing performance, cold-cranking amps and reserve capacity. The age of the battery is also important.
Check websites such as Consumer Reports to see which batteries have the longest battery life, particularly if you live in a warmer part of the world. Higher temperatures are harder on batteries and accelerate the corrosion of the plates. People who make mostly short car trips need longer battery life, as the brevity of the drives doesn't allow the battery to recharge itself.
Look at the shipping code on the battery case. Some use a number for the year, such as 1 for 2011, and a letter for the month, such as A for January, while others use the numeric date. Don't buy a battery that's more than six months old, as even sitting in a warehouse drains battery power.