Have a Chevy blazer that charges battery one time you drive it the next it might not. Could it be something besides a bad alternater?
You may have a bad voltage regulator. Or your battery could be too old and not holding a charge. You can put a volt meter on the output side of your alternator and let the vehicle run. Watch the screen and see if the volts cut out or drop. You may also have a bad ground or severely corroded battery cable. Not just the end on the battery. Sometimes the corrosion will creep up the cable underneath the coating. Doesn't hurt to check the other end to the starter either.
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