Hi All,
I would like some general feedback on an ongoing issue I am currently dealing with.

An employee was using a battery operated drill (14.4v) when the battery pack decided to explode, literally. This caused the batteries within to fly off in various directions at speed, a few of which hit the employee causing soft issue damage (severe bruising and swelling, requiring hospital treatment).

Could this have been prevented/detected?

Background.
Tool used infrequently in a retail environment to build flat pack furniture.
Tool c.3 years old.
Subject to user checks (recorded).
No defects were reported.
Drill worked perfectly on the last use which happened to be the day before.
Tools are not subject to rough handling or any exposure to adverse conditions.

Any tips on prevention would be greatful as I am at a loss due to the fact that the battery pack is a sealed unit so can't be properly inspected.

Your post suggests that it was the battery compartment that exploded. Or was it the batteries themselves?

Batteries can explode if short-circuited; its a bit of a party trick among physics students.

If it was the compartment then my first thought turns to hydrogen - had it been charged just before being used?

We may be barking up the wrong tree, but that is my thought. I am also wondering whether the weeping battery has something to do with it.

Certainly, overcharging many battery types will lead to hydrogen being emitted.

The very infrequent use may be a factor (although I wouldn't rule out a deliberate act - call it horseplay if you will). Is it conceivable that something was dropped, fell or was placed into the battery recess of the drill in the intervening period which caused a short circuit. A small piece of wire or copper strip would be enough to cause the short-cicuit, but pliable enough to allow the battery pack to engage (and so would a mischeivously inserted small piece of aluminium foil). These little things would probably not be spotted or linked to the incident in the immediate aftermath or clean-up.

Do your checks include pre-use and what is actually to be checked?
Is the drill kept reasonably secure and protected (in a case) and under lock and key when not in use?