1 ) In Florida, have the battery checked if it is 2 years old or older, preferably by a shop that actually has a big carbon pile resistor load tester that actually places a high current load on the battery that is selected to match the battery. Alternator rebuild shops that have been in existence for a long time are more likely to have this type of tester, compared to your average mechanic, or shops that have not been around for a long time.
2) Those same shops should be able to test the charging system.
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Alternators usually completely fail, not intermittently like you are describing. If the vehicles Voltage can not stay high enough when the RPMs are low, it usually is that the alternator can not keep up with the demand at those low RPMs because the battery is not doing its share of the work, and even may be draining too much from the alternator.
Battery life varies greatly. "In areas with hot climates like Florida, Arizona, Texas, and other VERY HOT AREAS, THEY CAN AND OFTEN DO, WEAR OUT IN AS LITLE AS 2 YEARS." In areas where the average ambient temperature is much cooler they can last a lot longer. in other words the average ambient temperature the battery sees over its life, greatly determines how long it will last. In general chemical reactions happen twice as fast if the temperature is raised by 10 degree C ( 18 degree F ) , and happen half as fast if lowered by 10 degree C ( 18 degree F ). An average ambient temperature change of only 10 C / 18 F over the life of a battery can drastically change how long the battery last.
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Another possibility with an older vehicle it is that the belt tensioner is not supplying the proper tension. And if that is the case the alternator may not be turning fast enough because both: the drive pulley from the engine that make the pulley move can slip some, and the pulley on the alternator can slip some. Combine both of those and the alternator ends up not turning fast enough to do its job even though there is nothing wrong with it.