1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 32Get this manual
Battery Precautions
When charging batteries, an explosive gas mixture forms in each cellA portion of this gas escapes thru holes in vent plugs and may form an explosive atmosphere around battery if ventilation is poorThis explosive gas may remain in or around battery for several hours after it has been chargedSparks or flames can ignite this gas and cause an internal explosion which may shatter the batteryThe following precautions should be observed to prevent an explosion1DO NOT smoke near batteries being charged or which have been charged very recently2DO NOT break live circuits at terminals of batteries because spark usually occurs at the point where live circuit is brokenAlways be careful when connecting or disconnecting cable clamps on chargersPoor connections are common cause of electrical arcs which cause explosions3DO NOT reverse polarity of battery cables on battery terminals