The acid heats up and can even boil. This could result in a burst battery or even an explosion due to the hydrogen gas released by the boiling acid.
The newer sealed batteries are more likely to fail catastrophically as there is no where for the gas to go and so builds up until the pressure forces the casing to fail.
The newer sealed batteries are more likely to fail catastrophically as there is no where for the gas to go and so builds up until the pressure forces the casing to fail.