I have a 2003 Sonata with about 97K miles. Have started to see the battery charge warning light "flicker" occasionally. Battery test good as does the alternator. Alternator and battery were both replaced about 1 year ago.
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2003 Hyundai Sonata Question: Battery Warning Light
Answer #1patrick mannion from Greg Solow's Engine Room, July 26, 2009, 12:22Master
Flashing alternator light is a sign of a failing alternator. Put a voltmeter across the battery engine off, the voltage of a fully charged battery should be 12.6v with the engine running the voltage should read 14.6 volts if the alternator is OK.
It is possible that you have poor connections causing a voltage drop. An oscilloscope is needed to check the charging pattern properly. Perhaps the alternator was a poor quality rebuild ,and maybe covered under a warentee.