Mercury Mariner 25HP 4-Stroke Outboard Service Manual 1997, Page 74Get this manual
Battery Precautions
When charging batteries, an explosive gas mixture
Specific Gravity Readings
Use hydrometer (a) to measure specific gravity of electrolyte in each cell
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 pre-