My car was sitting and had not been started in a month when I returned home I had AAA jump the battery. Car would turn over but would only stay on for a few seconds. Tried this twice. Also key fab would not let me unlock doors. Is this a battery issue or possibly an alternator issue?

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    You should properly charge the battery with an external charger, then re-test it. – Solar Mike Sep 6 '18 at 18:49
  • I think that is what AAA did. – Che Sep 6 '18 at 18:49
  • Alternator was replaced 3 years ago – Che Sep 6 '18 at 18:52
  • AAA attempted to jumpstart the car, they did not charge the battery. Please do as suggested by @SolarMike or failing that if you so prefer, get a new battery – ElectronicsNewbie Sep 6 '18 at 19:53
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    When I say "properly charge the battery" then it means overnight so it gets a proper charge, not 5 minutes quick boost... – Solar Mike Sep 6 '18 at 19:54

Modern cars have electronics that are always on and use up battery power, for instance alarms and the systems that listen for your keyfob. Lead acid batteries also lose charge over time, the combination of the two means that if you leave your car sitting long enough eventually you won't be able to start it.

How long you can leave a car sit depends on your battery's condition and how much drain there is from your car's electronics. Batteries lose capacity as they age (and due to other factors), so an older battery which may work when driven often may lose enough charge to not start after sitting for a couple of weeks. A battery in good condition should be able to start a car after sitting 4 weeks in normal conditions.

Jump-starting a car doesn't charge the battery, it supplies enough power to get the engine running, then your battery will be charged by the alternator. It takes time for this to happen, if you do a short drive and stop the engine the battery will not have enough charge to start the car again. You either need to drive the car for awhile or the battery needs to be put on a charger for several hours. If the battery won't hold a charge it needs to be replaced.

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