Great thread! You're right you are seeing normal voltage with the engine running. That's alternator voltage. If you have a multimeter, measure the voltage at the battery with the engine off after it has "sat" for about 8 hours. If the battery is weak, the voltage will probably be under 12.4 volts.
I keep recommending Costco's 100 month/36 month free replacement batteries. They have higher capacity than the OEM battery and are reasonably priced.
- Jack
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