RE: battery exploded
Can you describe how it exploded? Was it a "BOOM" and pieces of plastic and battery were scattered all over the compartment or was it where the case cracked and the acid leaked out?
Batteries generate hydrogen gas, even the sealed ones. If during charging (vehicle running), or worse over-charging, there is hydrogen gas liberated. It there was a spark or arc anywhere near it ther battery could detonate.
Two other types or battery failure are;
The battery is chronically overcharged, it will overheat, swell and split, also making a big mess. I have seen batteries that ended up shaped like cantelopes because the plastic melted, the case swelled and it cooled into the new shape.
The battery suffers some type of internal failure. eg, plates short out causing a massive current surge that will blow the battery "down", through the bottom or sides of the battery.
If the top is missing I would suspect a hydrogen initiated explosion, if through the bottom or sides I would suspect an overcurrent event.