We have a machine that tests electrical systems, and we have given it a nickname. We call it the "CAN'T BE" machine. That's because so many people bring in cars with electrical problems, like won't crank, or battery dies overnight, or alternator light is on, or cranks very slowly, or lights are getting dimmer and a multitude of other problems, and they say that the lights and horn work, so "It can't be the battery."
So we drag out the trusty "can't be" machine and in many cases find that it is indeed the battery. "How can that be", they protest, "the lights and horn work, so doesn't that prove that the battery isn't dead?"
Dead, no, weak, yes. You see, it only takes five or ten amps to operate the lights and horn, but it takes hundreds of amps to crank an engine. Think of it like a stress test on humans. You might be able to walk slowly on a treadmill, proving that you're not dead, but what if you can't sprint uphill like a teenager? If you're a human you get an easier job or put out to pasture, if you don't stroke out. If you're a battery, you're dead. They're going to yank you out of the system and replace you with something younger. It doesn't seem fair, after all those years of faithful service, but batteries don't have retirement plans. As they age, they lose their ability to provide big bursts of energy (just like people do), to start up an engine. They may develop small leaks and seepages (just like people do) and only be good for small jobs for short periods, but they can't crank an engine. The average life span of a car battery is less than three years, although some have made it five or even ten years. When they get old, their internal plates get brittle, (like osteoporosis) and even though they have been operating fine all along, if a sudden trauma occurs, like leaving headlights on until the battery dies, the resulting rapid discharge and recharge can break those delicate internal connections, and catastrophic failure results.
Dead batteries give no warning, no hints that something was going bad, just sudden, total failure. Like scaring grandpa to death. Years ago he would have chased you down and taught you a lesson, but the ticker is weaker now, and you might do him harm. So if you leave the headlights on when you go into the restaurant for grandpa's surprise birthday party where a girl jumps out of a cake, you might have a bad evening ahead.
Sometimes people go out and buy a battery because theirs went dead. That could be money wasted because two suspects could have caused the battery to die. Not enough charging from the alternator, or a drain on the battery when the key is off. If either of these culprits is at large, the new battery will also go dead. It takes a "can't be" machine to check the charging system, which can have anything from a slipping belt to bad diodes causing the problem, and the alternator light might not work. Other testing devises can check for an unauthorized current leak when the car is off. It could be the rear reading lights on, visor vanity lights on, glove box or trunk lights on, or other things pulling the battery down.
The moral is: Get it checked before spending money on parts. Sometimes a "can't be" machine will show that it "can be," or it could be that it "can't be," I think.