No fool like an April fool.

OK , I goofed- made duplicate post

Apologies- new to this forum- couldn't find post so assumed had lost it.

Mind is getting low on charge!

You're in good company here then, those of us that haven't lost the plot were barking long before anyway.

In this case I'd think the lack of venting might be a factor, though if the hydrogen ignites I suppose any lead-acid battery could explode.

Any lead-acid battery given a significant charge will generate gas (hydrogen) which will need to vent somehow. I would think it very unlikely that the vent could block solidly enough to explode the casing, so a spark-initiated explosion is the likely cause. That would be outside the battery itself, so the contents would not travel far, though the shock wave might well cause the damage.

An external hydrogen explosion doesnt seem to fit this description very well.

Hydrogen in the open will disperse fairly rapidly, being lighter than air, so without containment a big blast, with fragmentation, doesn't seem all that likely.

If the battery contents were an explosive hydrogen-air mixture, so the battery itself exploded, it seems more plausible, but one would think evolving hydrogen would displace the air.

Clearly batteries do explode, but I'm not sure exactly how they go about it.

In this case (fnar fnar, geddit?) IF the vent was blocked and the battery shorted when starting current was demanded, I'd speculate that could cause enough thermal expansion to rupture the casing, with a subsequent hydrogen explosion as a dramatic sequel..