it doesn't have to be charging over 14-15V to be over charging, what's needed is for the regulator [internal or external] to be bad so it's constantly feeding max charging current even tho the battery is/was fully charged, that's why plug-in battery chargers work in stages and taper off the charging current as the battery slows the rate it's taking amps ... any type battery that's overcharged will be getting hot, and any battery that's 'hissing' is gassing off hydrogen [explosive], wetcell, AGM or gel - also any overcharged battery will be damaged to some extent by sulphation from overcharging and should be checked properly by a shop that can evaluate it's load capacity
Bill
