Battery Charging
When the batteries sit without charging the voltage drops below the chargers start up range. You did good charging the batteries individually.
Now the one that is failing to charge, check the voltage with a meter and than check the cells with a hydrometer. Those tests should prove that the battery is not usable.
Base your option of battery replacement on the readings of both test. If the specific gravity is weak on most of the batteries after four years your batteries have run there course and will fail one at a time in the near future.
Contrary to SOME opinions it is not prudent to replace one bad battery with five four year old batteries because after the charge cycle completes and the charger shuts down the new battery will drain down to the level of the groupe.
This is the science of it, not a wives tail.
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