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There is a quick easy test for this, with the use of a voltage meter. With the engine running, take a voltage reading at the battery terminals. If the voltage is not between 13.2-14.4v, or fluctuates in and out of this area, the Alternator will need to be replaced. If the Alternator passes this test, turn the engine off and wait 15min. Take another reading at the battery terminals, this time with the engine off. If the voltage is anything less than 12v, the battery may have a "bad cell" and is not accepting the alternator's charge. If you do not have access to a voltage meter, most local "big chain" parts stores, like AutoZone, will run these tests for Free right in their parking lot. Make sure to also check the battery cables to make sure they are tight, making a good connection and are free of any corrosion at this time. If you found this answer helpful, please help me by leaving a rating using the "stars" at the top of the page or click 'accept'.Rating or accepting my answer does not cost you extra. It allows me to be credited for this Q&A.
If there were any other questions you had on this, or would like the procedures to replace either the battery or the alternator, please feel free to reply and ask!
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