You MUST check(or have checked) the voltage regulator for the battery charger.
In the mean time, you can use a "sacrifice" post, best is some pennies, placed close to the battery posts; they will attract the free negative electrons in the acid cloud that is forming over your overcharging batteries, which are 'cooking' and leaving an oxide on everything the cloud touches.
You will see corrosion forming on all the metal surfaces in proximity; Vaseline will protect them short term, but not after the corrosion has started.
They will require cleaning and sealing,like the tray.
