[zygomatic]

Was the battery on a charger / tender over the winter?

[Tedzilla]

From before the brine/salt gets spread until the roads are clean again it's on a Porsche Charge-O-Mat. It's started every 2 or 3 weeks. Never a problem... until now.
The battery compartment cover was destroyed by the explosion and there were shards of the battery case blocking the drains in the bottom of the compartment.

[Dandelion]

The failure is usually caused by hydrogen gas build-up in the cells (e.g. by electrolysis during charging). Flooded cell batteries have vent tubes which are supposed to prevent the buildup from becoming an issue; maybe your vent tube got clogged?

I've change everything which doesn't already have one to AGM batteries; much less susceptible to this (and have no vents).

Here's a paper from the Internet about it (at the beginning):
https://www.sbsbattery.com/PDFs/SBS_...Detector_8.pdf

ed

[Vicegrip]

They like to explode when they first boil out most of the water in the electrolyte. This provides the space for the air/fuel mix. Charger cooking a battery is a common precondition for battery bombs/

[HoodPin]

This is a reason I like using AGM/Gel type batteries, like Optima.
Reminds me of an old race car story (not me) where they tried re-wiring the charging system so that the alternator was only working when the car was under braking. I guess they got it wrong (or used the brakes too much...) because the battery exploded DURING the race.