Mercury 35/40HP 2 Cylinder Outboards Service Manual PN 90-42794--1, Page 26Get this manual
Battery Precautions
When charging batteries, an explosive gas mixture forms in each cellPart 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 charged Sparks orflames 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 broken Always 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