OK I know it is lead acid battery
BUT what configuration?
Gel cell, or other sealed types
OR
wet cell battery.
IF wet cell and you have electrolyte covering the plates (that is all you need) then should not smell. Even on overcharge it should not smell UNLESS you are overheating. Check temperature (do not touch plates with metal thermometer) and check voltage 15 is good value for maximum voltage.
IF sealed unit some of them have 14.4 max voltage. The "hissing" makes me think this is what you have.
Blowing up - if you overcharge a sealed unit then pressure builds up inside. If relief valves cannot keep up with pressure change the case gets overpressurized and can rupture.
Dan Bentler
