"First understand there are two sets of circumstances. One is a gassing lead acid battery, which on a modern car since around 1995 using lead calcium batteries is rare if the electrical system is in balance. This type of battery will only gas when it is charged vigorously such as by an overcharging alternator or a bench charge at the end of its charge. It would not gas in normal use. Additionally, they are mostly fitted with a spark arrester, which stops the flash getting into the box.



The second circumstance is a spark to ignite the gas residue. In this case the likely source is likely to be either a loose terminal that sparked or a welded dry joint internally that gave way when the key was turned. If you put a voltmeter across the battery poles which are usually still Intact and you get a reading usually 12.5 volts or indeed any it means the internal circuit is intact and it then is almost certainly an external spark.



Underuse of typical ‘weekend’ cars (like many Porsches) causes a battery to sulphate if the voltage gets low and can in certain circumstances cause gassing to occur."


Perhaps of interest.

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61k miles Ph.1 Diablo '6 Project Thread