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the problem is the charger
the regulator/float is not turning off and/or regulating the charge
causing it to over charge the batteries
click the link to buy a charger
you may also want to have the batteries load tested,just to make sure the batteries are sound...if you don't have a load tester,auto zone and most parts stores may load test the batteries for free if you take them to the store
there may be a technician out there somewhere that can repair it....but,i don't know of one,,,may be just as cost effective to buy one
the voltage isn't really high..it is just not regulating the voltage,,the charger isn't shutting off when the batteries are fully charged