Mazda CX-9 2016 Owner's Manual page 451

Keep all fl ames, including cigarettes, and sparks away from open battery cells:
Flames and sparks near open battery cells are dangerous. Hydrogen gas, produced
during normal battery operation, could ignite and cause the battery to explode. An
exploding battery can cause serious burns and injuries.
NOTE
Before performing battery maintenance, remove the battery cover.
Maintenance and Care
Owner Maintenance
6–39