i agree with what the guys said . a standard battery charger left on these little batteries for a extended period of time will boil the water/acid out of them and sulfate the cells . and with the combination of vibration and the stator trying to charge and faulty battery , it will swell and in cases explode.
a dry or sulfated battery confuses the voltage regulator in to thinking that the battery is low and thus putting a full amp charge into a battery that has no room to store it . kind like 10 lbs in a 5 lb bag ,
best charger for extended periods of time is a automatic style , low amp. 5 amp or less . it willl sense and maintain battery voltage ., Battery Tender is a very good unit .