The average battery life is 3-4 years. If you leave a battery in a car too long, it can harm the alternator. An alternator is designed to keep a battery charged, not to charge a dead battery. Slowly over a period of time, your alternator will have to run longer and harder until it gets to the point that whenever the vehicle is on, the alternator is running at maximum voltage just to keep the battery charged. Replacing an alternator is a lot more expensive than replacing a battery.
Batteries are temporary. There’s a chemical reaction that occurs between the plates in the battery and the fluid within the battery. As that chemical reaction occurs, the battery slowly dissolves itself from the inside out and the amperage is reduced.
When we test a battery, we check what cold-cranking amps are available, at what voltage the charging system is charging, what the starter is drawing. The starter motor can begin to go bad and will draw more amperage than the battery is capable of providing. A battery has to have the cold-cranking reserve so it is able to give the starter that initial “shot” to get the starter turning and to get everything working.
We recommend having your battery tested and the terminals and connectors cleaned not only with regular maintenance, but also with the change of each season.