How do I know when to change a car battery?

When it doesn't hold a charge for long. If you have trouble starting, then have it tested at shop, Autozone may do it free. Sometimes it's just the alternator not charging, but much rarer than bad battery.How do I know when to change a car battery?when your car doesnt start
How do I know when to change a car battery?
Well, you could use a battery tester ... but why go to the expense ... put one wire on the positive terminal and one on the negative terminal, use at least 12 guage wire aprox 12 inches long each, and stick both wires to your tongue. If you feel a little buzz the battery needs replaced. If you don't feel a little buzz you hooked the wires up wrong, try again.