Comment by arghwhat

Battery fire suppression systems often use water, and firefighters will also certainly apply water. Just don't walk up to the cell with a bucket of water.

Water can and is used to cool batteries during battery fires, and more importantly, to cool the surroundings so that the fire cannot spread to, say, neighboring banks/cells or construction materials. Modern EV fire suppression systems for parking garages use high-pressure water mists to avoid the fire spreading to neighboring vehicles for example.

The recommendation about oil and water focuses on a larger container of liquid fuel that is on fire at the surface and heated far past the boiling point of water, such that dumping a large volume of water onto it all at once causes it to immediately boil and explode, spreading large amounts of oil as a mist in the air, which both spreads the fire and causes a much more violent combustion. A water-based fire suppression system (not a guy with a bucket of water), or a firefighter with a water hose, can absolutely extinguish such a fire.

Hacking together a sand container of acrylic may well do nothing to limit the fire while simultanously giving it more fuel (acrylic) and pathways to spread to (whatever the acrylic is near).

The point is: Don't hack together a fire suppression system, leave that to an expert please.