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Alternator or battery

My wife’s 2007 Buick Lucerne wouldn’t start the other day after sitting maybe 20 minutes with the headlights on. She had to call AAA for a jump start. It starts fine now. I took it to an autoparts store, the battery shows 90% life left. Brought it home, charged in 10 minutes using my car charger. My question is, what could cause the car not to start so easily? I am skeptical the battery was the culprit. How can I check the alternator, etc? Any other thoughts?

Would it not crank at all – that rrr rrr rrr sound? Or would it crank ok, but not catch and run?

If you could charge the battery to 100% with only 10 minutes of charging, that battery was nearly fully charged. A battery at 50-75% takes me at least 6 hours to recharge to 100% at the 2 amp setting.

If it the original battery you are overdue for a new one. You can check the volatge to the battery while running, should be around 14 volts. Then check the battery voltage with the engine off, less than 12 volts bad battery. Of course connections ie terminals and ground connection can go bad, let us know.

Barky, I think you mean less than 12 volts ??

Thought I edited that to 12 volts, re edited to 12, Right you are @BillRussell can edit it now so please forgive typo.

Do you have a multi-meter? If yes, set it to check voltage in the range of 20 volts. When the car isn’t running the battery voltage should be around 12,5 volts. Now start the car. At a normal idle rpm you should get a reading of 13 to 13.5 volts. At about 1500 to 2000 rpm the voltage should read about 14.0 volts. If you get these reading the alternator is putting out a charge sufficient to keep the battery charged. If the reading is just 12 volts, the alternator isn’t charging the battery.

If your car isn’t getting a 1 hour trip once a week then it is possible the battery isn’t getting fully charged.

Since the car is now running I suggest you take it into a good shop that can test the entire charging system for you. Tests that require special test equipment you are not likely to have on hand. If something needs attention they will let you know. The cost for doing that shouldn’t be much. Some places may do it for free.

Our Acura battery was 3 1/2 years old and the wife listened to the radio at the gas station while I went inside for less than 5 minutes. The battery was dead. If its more than a few years old, I’d replace it. How else can it drain and then come back to 90% in ten minutes of charging? I suspect the battery test was faulty although 20 minutes with the lights on is not an insignificant amount of time.

Headlights (and taillights) can be 10-20 amps. times 1/3 hour, that is 4-6 amp hours.

A new battery is about 40-100 amp-hours capacity. The battery would have to be pretty dead for it to discharge in that time.

Uncle Turbo and Cougar… good suggestions. Will do such!