[shorod]

If the battery was run down to completely dead due to the key being left on, that would have made the battery more susceptible to freezing. However, if the battery didn't freeze and was allowed to charge up completely the next day, it probably didn't damage anything. However, if it was just jumpstarted and you drove the Explorer right away, that would have put a pretty good load on the alternator. That could have accelerated wear on the alternator.

I'd have a hard time condemning the battery based on the checks you've performed. Sounds like the alternator or the wiring of the alternator is a bit screwy. The fact that your voltage goes as high as 15 volts when the engine is running and believing that your battery is fine would start to point to the voltage regulator. I believe on your '97 the regulator is part of the alternator.

-Rod