Car batteries are the strong, silent member of the automotive team. They do their job regardless of heat, cold weather and the drivers who demand so much of them. While a battery that allows a car start at the first turn of the key is a joyful thing, it doesn't last forever.
In fact, depending on where you live and how you drive, the condition of your charging system, and a number of other factors, a battery lasts about four years on average. And when it does give out, there's generally no sign of trouble -- your car just dies.
While the lead-acid car battery hasn't changed much in the last 100 years, it's still a difficult part of the car to check during routine maintenance. Simple battery testers can't, at this time, muddle through the chemical complexity of what goes on in a battery. Instead, they provide a sort of snapshot of the battery at the time it's being tested -- without the context of the battery's chemical composition before or after the test.
So the rule of thumb is simple for battery replacement: You have approximately four years before the battery will theoretically begin its slide from chemical powerhouse to chemical paperweight. At the four year mark, start watching, and hope your mechanic will detect a problem before it's too late.
But due to the nature of the chemical cocktail inside any battery, it may give out before the four year mark, or maybe it will last for several more years. So you have to ask yourself, "Do you feel lucky?"